Sweetpotato:                     Major Pests,                     Diseases, and                     Nutritional           ...
Sweetpotato: MajorPests, Diseases, andNutritional DisordersT. Ames, N.E.J.M. Smit, A.R. Braun, J.N. O’Sullivan, and L.G. S...
C      O        N       T         E    N       T         SThe International Potato Center (CIP) is ascientific, nonprofit ...
Page                                                                 PageLeaf Folders (Brachmia convolvuli, Herpetogramma ...
Foreword                                                  Page       This field guide presents information on common pests...
Acknowledgments                                                   Introduction    The information on nutritional disorders...
Insect Pests of Sweetpotato                                                                                  and Their Man...
Storage Root FeedersSweetpotato WeevilsCylas spp. (Coleoptera: Curculionidae)    Description. Three species of the genus C...
Distribution and importance. Cylas weevils                      At 27°C, C. puncticollis has a total development timeare s...
Treatment of planting material. Dipping plantingmaterial in a solution of Beauveria bassiana or in an in-secticide (such a...
West Indian Sweetpotato WeevilEuscepes postfasciatus(Coleoptera: Curculionidae)    Description. Adult weevils are reddish ...
Rough Sweetpotato WeevilBlosyrus sp. (Coleoptera: Curculionidae)    Description and biology. Adult weevils areblackish or ...
Clearwing Moth                                                  White GrubsSynanthedon spp. (Lepidoptera: Sesiidae)       ...
Stemborers and FeedersClearwing MothSynanthedon spp.(Lepidoptera: Sesiidae)    Description and biology. Adults lay batches...
Sweetpotato StemborerOmphisia anastomasalis(Lepidoptera: Pyralidae)    Description and biology. Most eggs are laidindividu...
Distribution and importance. The stemboreris one of the most destructive pests of sweetpotato introp-ical and subtropical ...
Striped Sweetpotato WeevilAlcidodes dentipes and A. erroneus(Coleoptera: Curculionidae)    Description and biology. Adult ...
Sweetpotato Weevils                                         Minor Stemborers and Feeders    Cylas spp. larvae can do consi...
Sweetpotato BugPhysomerus grossipes (Hemiptera: Coreidae)    Description and biology. The sweetpotato buglays groups of eg...
Foliage FeedersSweetpotato ButterflyAcraea acerata (Lepidoptera: Nymphalidae)    Description and biology. Pale yellow eggs...
Tortoiseshell BeetlesAspidomorpha spp. and others(Coleoptera: Chrysomelidae)     Description and biology. Eggs are laid on...
Control. Control is rarely warranted. Removal ofconvolvulaceous weeds in the surrounding area may reducetheir numbers. Sev...
Sweetpotato HornwormAgrius convolvuli (Lepidoptera: Sphingidae)   Description and biology. The small, shiny eggsare laid s...
ArmywormsSpodoptera eridania, S. exigua, S. litura(Lepidoptera: Noctuidae)    Description. Adult female moths of S. eridan...
consume the parenchymal leaf tissue, leaving only theveins (Fig. 41). The late instars of S. litura are very voraciousand ...
Leaf FoldersBrachmia convolvuli (Lepidoptera:Gelechiidae), Herpetogramma hipponalis(Lepidoptera: Pyralidae), and others   ...
enemy action is not disrupted by pesticide use, control             Strobiderus Beetleis rarely needed. The use of uninfes...
Minor Leaf FeedersGrasshoppers and LocustsZonocerous variegatus(Orthoptera: Pyrgomoriphidae),the variegated grasshopper an...
Virus TransmittersAphidsAphis gossypii and others(Homoptera: Aphididae)    Description and biology. Aphids are soft-bodied...
WhitefliesBemisia tabaci (Homoptera: Aleyrodidae)    Description and biology. The female of B.tabaci lays eggs on the unde...
MitesErinose caused by Eriophyid mites, Aceria sp.(Acari: Eriophyidae)    Description. Vines and leaves become excessively...
Natural Enemies                                                                Earwigs    The natural enemies of sweetpota...
Pheidole and Other Predacious Ants    Cuban farmers practice augmentation of Pheidolemegacephala (Myrmicinae) and Tetramor...
Beetles    Carabids are one of the most important families ofpredatory beetles. The majority of carabids in agriculturalsy...
Flies and Parasitic Wasps    Hover flies (Syrphidae) are brightly colored and fre-quently visit flowers to feed on pollen ...
The wasp Telenomus spodopterae (Scelionidae) is a            Virusescommon egg parasite of S. litura. The encyrtids Anasta...
Fungal PathogensMetarrhizium anisopliae andBeauveria bassiana    Sweetpotato weevils are among the insect speciesattacked ...
Diseases and Pathogens ofSweetpotatoand Their Management  A number of pathogenic organisms affect sweetpotato.Most appear ...
Viral DiseasesSweetpotato Feathery Mottle Virus(SPFMV)Aphid-transmitted potyvirus    Symptoms. Symptoms of SPFMV on the fo...
Sweetpotato Sunken Vein Virus                                  Sweetpotato Virus Disease (SPVD)(SPSVV)Whitefly-transmitted...
Other Viral DiseasesSweetpotato Mild Mottle Virus                                                                        O...
Bacterial DiseasesBacterial Stem and Root RotErwinia chrysanthemi    Symptoms. Aerial symptoms are water-soakedbrown to bl...
Bacterial WiltPseudomonas solanacearum    Symptoms. Infected stands usually contain somewilted plants (Fig. 64A). The dise...
Soil RotStreptomyces ipomoea   Symptoms. The first indication of the disease isan extensive chlorosis and bronzing of the ...
Foliar and Stem DiseasesCaused by FungiLeaf and Stem ScabElsinoe batatas, Sphaceloma batatas    Symptoms. Brown to tan rai...
Alternariosis, Anthracnose, BlightAlternaria bataticola    Symptoms. Brown lesions with a typical bull’s-eyeappearance of ...
Phomopsis Leaf Spot(Phyllosticta Leaf Spot)Phomopsis ipomoea-batatas(Phyllosticta batatas)    Symptoms. Whitish to tan to ...
Minor Leaf Spot Fungi    Other fungi cause leaf spots and can be identifiedby inspecting spores with a microscope. These f...
Chlorotic Leaf DistortionFusarium lateritium     Symptoms. The first noticeable sign or symptomis a white, waxy (crusty) m...
Fusarium WiltFusarium oxysporum f. sp. batatas    Symptoms. The first indication of this diseaseis a dullness and yellowin...
Violet Root RotHelicobasidium mompa     Symptoms. Affected plants become chlorotic andmay defoliate. Fibrous roots rot and...
Sclerotial Blight and Circular SpotSclerotium rolfsii   Symptoms. Sclerotial blight and circular spot aretwo diseases caus...
Black RotCeratocystis fimbriata    Symptoms. Dark to black sunken cankers in thelower part of the stem are the most distin...
Storage Root andPostharvest DiseasesFoot RotPlenodomus destruens    Symptoms. Brown lesions form on the stem ator below th...
Java Black RotLasiodiplodia theobromae(Diplodia gossypina)    Symptoms. This rot is firm and moist initially,but storage r...
Charcoal RotMacrophomina phaseolina    Symptoms. This disease is found only on fleshyroots during storage. The fungus does...
Soft Rot(Rhizopus stolonifer, Mucor sp.)    Symptoms. Soft rotting occurs after harvest. Storageroots become soft, wet, an...
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  1. 1. Sweetpotato: Major Pests, Diseases, and Nutritional Disorders T. Ames, N.E.J.M. Smit, A.R. Braun, J.N. O’Sullivan, and L.G. SkoglundISBN 92-9060-187-6
  2. 2. Sweetpotato: MajorPests, Diseases, andNutritional DisordersT. Ames, N.E.J.M. Smit, A.R. Braun, J.N. O’Sullivan, and L.G. Skoglund International Potato Center (CIP)
  3. 3. C O N T E N T SThe International Potato Center (CIP) is ascientific, nonprofit institution dedicated to theincreased and more sustainable use of potato, Pagesweetpotato, and other roots and tubers in the Foreword viideveloping world, and to the improvedmanagement of agricultural resources in the Acknowledgments viiiAndes and other mountain areas. CIP is partof the global agricultural research networkknown as the Consultative Group on Introduction 1International Agricultural Research (CGIAR). CGIAR Insect Pests of Sweetpotato and Their Management 3International Potato CenterApartado 1558 Storage Root Feeders 4Lima 12, Peru Sweetpotato Weevils (Cylas spp.) 4 West Indian Sweetpotato Weevil (EuscepesISBN 92-9060-187-6 postfasciatus) 10Press run: 1000 Rough Sweetpotato Weevil (Blosyrus sp.) 12Printed in Lima, PeruAugust, 1997 Clearwing Moth (Synanthedon spp.) 14 Peloropus Weevil (Peloropus batatae) 14Cover: Photo of chlorotic spots with and without purple margins induced White Grubs 15 by SPFMV (taken by S. Fuentes). Stemborers and Feeders 16T. Ames, N.E.J.M. Smit, A.R. Braun, J.N. O’Sullivan, and L.G. Skoglund. Clearwing Moth (Synanthedon spp.) 16 1996. Sweetpotato: Major Pests, Diseases, and Nutritional Disorders. International Potato Center (CIP). Lima, Peru. 152 p. Sweetpotato Stemborer (Omphisia anastomasalis) 18 Striped Sweetpotato Weevil (Alcidodes dentipes and 1. Sweetpotato – Insect pests. 2. Sweetpotato – Diseases. A. erroneus) 223. Sweetpotato – Nutritional disorders. 4. Sweetpotato – Integrated manage- Sweetpotato Weevils (Cylas spp.) 24ment. I. International Potato Center. Minor Stemborers and Feeders 25 Peloropus Weevil (Peloropus batatae) 25 Sweetpotato Bug (Physomerus grossipes) 26 Long-Horned Beetle 26 Foliage Feeders 28 Sweetpotato Butterfly (Acraea acerata) 28 Tortoiseshell Beetles (Aspidomorpha spp. and others) 30 Sweetpotato Hornworm (Agrius convolvuli) 34 Armyworms (Spodoptera eridania, S. exigua, S. litura) 36 iii
  4. 4. Page PageLeaf Folders (Brachmia convolvuli, Herpetogramma Sweetpotato Sunken Vein Virus (SPSVV) hipponalis, and others) 40 (Whitefly-transmitted closterovirus) 68Strobiderus Beetle (Strobiderus aequatorialis) 43 Sweetpotato Virus Disease (SPVD) 69Rough Weevil (Blosyrus sp.) 43 Sweetpotato Mild Mottle Virus (SPMMV)Sweetpotato Weevils (Cylas spp.) 43 (Whitefly-transmitted potyvirus) 70Minor Leaf Feeders 44 Other Viral Diseases 71Grasshoppers and Locusts (Zonocerous variegatus, the variegated grasshopper and Attractomorpha Bacterial Diseases 72 psitticina, the slant-faced grasshopper, Bacterial Stem and Root Rot (Erwinia chrysanthemi)72 and others) 44 Bacterial Wilt (Pseudomonas solanacearum) 74 Soil Rot (Streptomyces ipomoea) 76Virus Transmitters 46Aphids (Aphis gossypii and others) 46 Foliar and Stem Diseases CausedWhiteflies (Bemisia tabaci) 48 by Fungi 78 Leaf and Stem Scab (Elsinoe batatas, SphacelomaMites 50 batatas) 78Erinose caused by Eriophyid mites, Aceria sp. 50 Alternariosis, Anthracnose, Blight (Alternaria bataticola) 80Eriophyes gastrotrichus 50 Phomopsis Leaf Spot (Phyllosticta Leaf Spot) (Phomopsis ipomoea-batatasNatural Enemies 52 (Phyllosticta batatas)) 82Earwigs 53 Minor Leaf Spot Fungi 84Spiders 53 Chlorotic Leaf Distortion (Fusarium lateritium) 86Pheidole and Other Predacious Ants 54 Fusarium Wilt (Fusarium oxysporum f. sp. batatas) 88Beetles 56 Violet Root Rot (Helicobasidium mompa) 90Flies and Parasitic Wasps 58 Sclerotial Blight and Circular Spot (Sclerotium rolfsii) 92Viruses 61 Black Rot (Ceratocystis fimbriata) 94Fungal Pathogens (Metarrhizium anisopliae and Beauveria bassiana) 62 Storage Root and Postharvest Diseases 96Diseases and Pathogens of Sweet- Foot Rot (Plenodomus destruens) 96potato and Their Management 65 Java Black Rot (Lasiodiplodia theobromae (Diplodia gossypina)) 98Viral Diseases 66 Charcoal Rot (Macrophomina phaseolina) 100Sweetpotato Feathery Mottle Virus (SPFMV) Soft Rot (Rhizopus stolonifer, Mucor sp.) 102 (Aphid-transmitted potyvirus) 66 iv v
  5. 5. Foreword Page This field guide presents information on common pests,Diseases Caused by Nematodes 104 diseases and nutritional disorders of sweetpotato. It isRoot-Knot Nematode (Meloidogyne spp.) 104 intended primarily as a tool for correct identification ofBrown Ring (Ditylenchus destructor, D. dipsaci) 106 these problems and ailments, as an essential first stepReniform Nematode (Rotylenchulus reniformis) 108Lesion Nematode (Pratylenchus spp.) 110 in their control.Disorders of Unknown Origin 112 The principal entries in the guide are accompaniedFasciation 112 by photographs or illustrations, and pinpoint where specific problems occur. Additional information is provided onNutritional Disorders and Their symptoms and recommended control practices, withManagement 115 emphasis on integrated crop management.Causes of Nutritional Disorders 116 We believe that researchers, extension agents, stu-Diagnosing Nutritional Disorders 117Correcting Nutritional Disorders 118 dents and farmers alike will find this guide useful, asNutrient Requirements of Sweetpotato 120 it can assist them in controlling pests and diseases whileNitrogen Deficiency 122 safeguarding the natural environment needed for sus-Phosphorus Deficiency 124 tainable agriculture.Potassium Deficiency 130Magnesium Deficiency 132 Wanda CollinsBoron Deficiency 134 Deputy Director General for ResearchIron Deficiency 136 International Potato CenterAcid Soils and Aluminum Toxicity 138Salinity 140Literature Consulted 142About the Authors 146Photo Credits 147Index 148 vi vii
  6. 6. Acknowledgments Introduction The information on nutritional disorders of sweetpotato The purpose of this field guide is to aid researcherspresented in this field guide was derived from a research and extensionists throughout the world in the identificationproject of the Australian Centre for International Agricultural of common pests, diseases, and nutritional disorders ofResearch (ACIAR). The author of the section on nutritional sweetpotato (Ipomoea batatas). This guide is based ondisorders, J.N. O’Sullivan, is a member of the project based the authors’ experiences with the crop in various regionsat the University of Queensland, Australia. of the world since 1990. We would like to thank virologist Richard W. Gibson In contrast to most other major staple food crops,of the Natural Resources Institute for contributing to the sweetpotato is able to produce a comparatively high yieldsection on virus diseases. under relatively adverse conditions; however, a number of pests, diseases, and nutritional disorders affect the crop. Among the pest and disease constraints, sweetpotato weevils (Cylas spp.) and virus diseases probably contribute the most to yield losses, although leaf-feeding insects, such as the sweetpotato butterfly (Acraea acerata), can cause significant losses during outbreaks. Nutritional disorders can cause slight to complete yield loss, and are the main factor limiting most unfertilized crops. They may also reduce tolerance of pests and diseases. The basis for successful management of sweetpotato pests, diseases, and nutritional disorders is integrated crop management. This implies prevention of insect infestation and infection by pathogens through the use of adequate cultural practices, and the conservation of natural enemies. Adequate cultural practices include the selection of healthy planting material from well-adapted varieties, rotation, good field sanitation, and maintenance of soil fertility. Conser- vation of natural enemies involves avoidance of pesticide use, enhancement of natural enemy action through favorable cultivation practices, and the introduction of natural enemies if necessary. viii 1
  7. 7. Insect Pests of Sweetpotato and Their Management Integrated crop management for sweetpotato is cov- Many insect species attack sweetpotato and the im-ered comprehensively in the Farmer Field School Guide portance of different species varies between agroecologicalfor Sweetpotato Integrated Crop Management. This pub- zones. Within a zone, the importance of a species dependslication can be obtained from the CIP Regional Office for on the season; many insect pests are a problem mainlyEast and Southeast Asia and the Pacific or from CIP in dry periods.headquarters in Lima.1 Nutritional disorders are coveredcomprehensively in the publication Nutritional Disorders In this guide, we divide pest species into three groupsof Sweet Potato,2 available from the Australian Centre for according to whether the damage is caused to leaves,International Agricultural Research. stems, or roots. Defoliation reduces yield depending on the severity of infestation and the growth stage of the The photographs in this publication were assembled sweetpotato crop in which it occurs. In some areas,from the collections of the authors and their colleagues, sweetpotato is grown for its foliage and leaf feeders canand are acknowledged as such. be a problem. In addition to feeding, certain insects, such as aphids and whiteflies, transmit viruses. Extensive stem damage can result in wilting or even in death of the plant. Damage to the vascular system caused by insect feeding and tunneling and pathogen invasion of the injured tissue can reduce the size and number of storage roots. Damage to storage roots, the plant part generally consumed by humans, is of two kinds. External damage results in a loss of quality. Although roots with external damage may bring a lower market price or be unsaleable, they can often still be consumed in the farm household. Internal damage often causes complete loss.1 To order copies of the Farmer Field School Guide for Sweetpotato IntegratedCrop Management, contact the CIP-ESEAP Regional Office, Fax: 62-251-316264;email: cip-bogor@cgnet.com or CIP headquarters, Fax: 51-1-435-1570; email:cip@cgnet.com .2 Nutritional Disorders of Sweet Potato is available from the Australian Centrefor International Agricultural Research (ACIAR), GPO Box 1571, Canberra 2601,Australia, Fax: 61-6-2170501, email: aciar@aciar.gov.au . 2 3
  8. 8. Storage Root FeedersSweetpotato WeevilsCylas spp. (Coleoptera: Curculionidae) Description. Three species of the genus Cylas arepests of sweetpotato; they are commonly called sweetpotato 1 2weevils. All three species—Cylas formicarius, C. puncticollis,and C. brunneus—are found in Africa. C. formicarius ispresent in Asia and in parts of the Caribbean. The elongatedant-like adults of the three species can be distinguishedfrom each other. Cylas puncticollis is the easiest to distinguish becausethe adult is all black and larger than the other two (Fig. 3 41). C. formicarius has a bluish black abdomen and a reddishbrown thorax. C. brunneus adults are small and not uniformin coloring. The most common type can easily be confusedwith C. formicarius. In all species, the eggs are shiny and round (Fig. 2).The legless larvae (Fig. 3) are white and curved, and thepupae are white (Fig. 4). 6 Damage. Damage symptoms are similar for all threespecies. Adult sweetpotato weevils feed on the epidermisof vines and leaves. Adults also feed on the external surfacesof storage roots, causing round feeding punctures, whichcan be distinguished from oviposition sites by their greaterdepth and the absence of a fecal plug (Fig. 5). The developinglarvae of the weevil tunnel in the vines and storage roots,causing significant damage. Frass is deposited in the 5tunnels. In response to damage, storage roots producetoxic terpenes, which render storage roots inedible evenat low concentrations and low levels of physical damage.Feeding inside the vines causes malformation, thickening,and cracking of the affected vine (Fig. 6). 4 5
  9. 9. Distribution and importance. Cylas weevils At 27°C, C. puncticollis has a total development timeare serious pests of sweetpotato worldwide, especially of about 32 days, whereas C. brunneus takes 44 days.in drier agroecological zones. They are often the most Adults of the first species live an average of 100 days,significant sweetpotato pest. whereas the latter dies after about 2 months. C. puncticollis females lay 90–140 eggs in their lifetime, whereas C. Distribution. C. formicarius is an important pest brunneus females lay 80–115.in India and Southeast Asia, Oceania, the United States,and the Caribbean. In Africa, it has been found only in Control. When sweetpotato weevil populations areNatal, South Africa, and at one location in coastal Kenya. high, no single control method provides adequate pro-C. puncticollis and C. brunneus are confined to Africa. tection. The integration of different techniques, with emphasis on the prevention of infestation, provides Biology. All sweetpotato weevil species have a sustainable protection.similar life history. The adult female lays eggs singly incavities excavated in vines or in storage roots, preferring Cultural control. Cultural practices have provedthe latter. The egg cavity is sealed with a protective, gray to be effective against the sweetpotato weevil and shouldfecal plug. The developing larvae tunnel in the vine or be the main basis of control. Cultural practices include:storage root. Pupation takes place within the larval tunnels.A few days after eclosion, the adult emerges from the •Use of uninfested planting material, especially vine tips.vine or storage root. Because the female weevil cannot •Crop rotation.dig, she finds storage roots in which to lay her eggs by •Removal of volunteer plants and crop debrisentering through soil cracks. Alternate hosts of sweetpotato (sanitation).weevils are Ipomoea spp. weeds. •Flooding the field for 24 hours after completing a harvest. Adults of all species may be conveniently sexed by •Timely planting and prompt harvesting to avoid a drythe shape of the distal antennal segment, which is filiform period.(thread-like, cylindrical) in males and club-like in females. •Removal of alternate, wild hosts.The males have larger eye facets than the females. •Planting away from weevil-infested fields. •Hilling-up of soil around the base of plants and filling At optimal temperatures of 27–30°C, C. formicarius in of soil cracks.completes development (from egg to egg) in about •Applying sufficient irrigation to prevent or reduce soil33 days. Adult longevity is 2 1/2 to 3 1/2 months and cracking.females lay between 100 and 250 eggs in this period.At suboptimal temperatures, development takes longer. 6 7
  10. 10. Treatment of planting material. Dipping plantingmaterial in a solution of Beauveria bassiana or in an in-secticide (such as carbofuran or diazinon) for 30 minutesprior to planting (Fig. 7) can control sweetpotato weevilsfor the first few months of the growing season. Less-susceptible varieties. Varieties with im-munity or a high level of resistance are not available.Some varieties have low to moderate levels of resistance.Others escape weevil damage because their storage rootsare produced deep in the soil or because they maturequickly and can be harvested early. Sex pheromones. The species-specific pheromonesof all three Cylas species that are released by female 7weevils and attract males have been identified. Pheromonelures for C. formicarius are commercially available. Phero-mone traps are used as monitoring, training, and man-agement tools. Many effective traps have been designedby farmers using locally available materials (Fig. 8). Trapsare so sensitive that their failure to catch weevils is agood indication that the pest is not present. Microbial control Promising biological control control.agents for sweetpotato weevils appear to be the fungiB. bassiana and Metarrhizium anisopliae and the nema-todes Heterorhabditis spp. and Steinernema spp.The fungiattack and kill adult weevils, whereas the nematodes killthe larvae. Predators. Ants, spiders, carabids, and earwigs are 8important generalist predators that attack weevils.Theyare described more fully in the section on "Natural Enemies." 8 9
  11. 11. West Indian Sweetpotato WeevilEuscepes postfasciatus(Coleoptera: Curculionidae) Description. Adult weevils are reddish brownto blackish gray, and are covered with short, stiff, erectbristles and scales (Fig. 9). Eggs are grayish yellow toyellow. Larvae are white. Pupae are whitish and sedentary. Distribution. E. postfasciatus is widespread in theCaribbean and South America, and is usually present insweetpotato fields and storage. Damage. Adults feed on sweetpotato stems andstorage roots, and emerge by chewing exit holes (Fig. 910). Larvae feed deep in the plant tissues. Internally,flesh and stem tissues are severely damaged. Affectedroots are not edible by humans or animals. Control. Integrated pest management includesremoval of infested sweetpotato vines and storageroots from the field after harvest, removal of alternatehosts, and use of uninfested planting material. Biologicalcontrol with B. bassiana and the use of early-maturingvarieties also reduces damage. 10 10 11
  12. 12. Rough Sweetpotato WeevilBlosyrus sp. (Coleoptera: Curculionidae) Description and biology. Adult weevils areblackish or brownish and the surface of the elytra is ridged(Fig. 11). This makes them look like a lump of soil. Larvaeare whitish and C-shaped. Adult weevils lay eggs under-neath fallen leaves (Fig. 12). The larvae develop in thesoil and pupate there. Adult weevils are found on theground underneath foliage during the day. Damage. Adult weevils feed on foliage, but the larvaecause greater damage. While feeding under the soil surface,they gouge shallow channels on the enlarging storageroots (Fig. 13). These "grooves" reduce marketability. When 11 12extensively damaged, the skin of the storage root hasto be thickly peeled before eating, because the flesh discolorsjust under the grooves. Distribution and importance. This weevil isa common pest of sweetpotato in East Africa, and causesserious problems in some localities. Control. Some of the cultural control measures usedto control Cylas should be effective in reducing incidenceof this pest, especially rotation and sanitation. The pos-sibility of biological control is under investigation. 13 12 13
  13. 13. Clearwing Moth White GrubsSynanthedon spp. (Lepidoptera: Sesiidae) White grubs, the larvae of various species of scarabidThe larvae can tunnel through the vine into the storage beetles, live in the soil. In the larval stage, they are largeroots. Usually, only the tip of the storage root is affected. and fleshy with swollen abdomens, well-developed headThis pest is discussed in detail in the section on "Stem- capsules, and large jaws and thoracic legs (Fig. 14). Theyborers and Feeders." usually adopt a C-shaped position. When they feed, white grubs gouge out broad, shallow depressions in sweet- potato roots. Most species attack a wide range of host plants. Control is not usually necessary. Handpicking ofPeloropus Weevil exposed grubs during land preparation and weeding is useful. Light trapping can be used to control white grubsPeloropus batatae when they become a chronic problem in a localized area.(Coleoptera: Curculionidae)The larvae of the Peloropus weevil can tunnel down thevines to the storage roots. This species is discussed inthe section on "Minor Stemborers and Feeders." 14 14 15
  14. 14. Stemborers and FeedersClearwing MothSynanthedon spp.(Lepidoptera: Sesiidae) Description and biology. Adults lay batches of70–100 yellowish eggs on vines and leaf stalks. The larvaeburrow into the vines soon after they hatch and tunneldownward. They are whitish, with transparent skin anda brown head capsule. They can grow up to 2.5 cm long.Pupation occurs in the main stem just above ground leveland the grayish brown pupal case (Fig. 15) can be seenprotruding from the swollen stem. The adult moth hasdistinctive transparent wings (Fig. 16). 15 Damage. The larvae burrow into the vines and some-times into the storage roots. The vine base is charac-teristically swollen and traversed with feeding galleries.With heavy infestation, the vine breaks off easily at thebase. Distribution and importance. Three closelyrelated species of Synanthedon are regularly found in sweet-potato, but they are prominent pests only in some local-ities in East Africa. Control. Frequent earthing-up around the plant basereduces the incidence of this pest. Other cultural controlmeasures, such as those practiced for sweetpotato weevilspecies, also help to control clearwing moths. 16 16 17
  15. 15. Sweetpotato StemborerOmphisia anastomasalis(Lepidoptera: Pyralidae) Description and biology. Most eggs are laidindividually along the underside of the leaves, along theleaf margins. Some are laid on the stem. The egg, larval,and pupal stages last an average of 55–65 days. Thereare six larval instars. A newly emerged larva has a brownhead and a reddish or pinkish body. After a few days,it turns creamy with black markings. Full-grown larvaeare 30 mm long (Fig. 17). Infested plants usually havea pile of brownish frass around their base. Before pupating,the larva makes an exit hole that is covered with theepidermis of the stem. Pupation lasts about two weeks 17and takes place in a web-covered cocoon within thetunnel (Fig. 18). The adults emerge by breaking throughthe dry papery covering of the exit hole. They live 5–10 days and the females lay an average of 150–300 eggs.The moths are 15 mm long and have reddish brown headsand bodies, and light brown wings (Fig. 19). Damage. The larva bores into the main stem shortlyafter hatching and sometimes penetrates the neck of thestorage root. Larval feeding results in enlargement andlignification of the stems at the base of the plant andin the formation of hollow cavities filled with frass. Plantsmay wilt and die. Attack during the early stages of plantgrowth may inhibit the formation of storage roots. 18 18 19
  16. 16. Distribution and importance. The stemboreris one of the most destructive pests of sweetpotato introp-ical and subtropical Asia and the Pacific. It is widespreadin the Philippines, Indonesia, India, Sri Lanka, Malaysia,Taiwan, Hawaii, and Vietnam, where it is a severe pestin the central region of the country. It also occurs in China,Japan, Cambodia, Laos, Burma (Myanmar), and Thailand.Infestation during the establishment phase of the cropcan result in yield losses of 30–50% or more. Control. Using planting material infested withstemborer eggs or planting a new field next to an infestedone are often the main means of disseminating this pest.Treatment of planting material and crop rotation are valuablemeans of control. Hilling-up, often practiced to reduce 19damage from sweetpotato weevil, also contributes to thecontainment of a stemborer infestation. Hilling-up is effec-tive when the holes, made to provide the adults witha means of exiting the stems, are covered with soil. Earwigsand ants may attack the larvae developing withinsweetpotato vines. Sources of resistance have beenidentified by the Asian Vegetable Research and Devel-opment Center, Taiwan. 20 21
  17. 17. Striped Sweetpotato WeevilAlcidodes dentipes and A. erroneus(Coleoptera: Curculionidae) Description and biology. Adult A. dentipesis about 1.4 cm long and has conspicuous white stripeslongitu-dinally along the elytra (Fig. 20). Adult A.erroneusis bigger than the former, and is brownish black withan irregular yellowish patch on each elytron. Larvae ofboth species are white, with an orange-brown head capsule,and are C-shaped. Larvae and pupae (Fig. 21) are foundinside the sweetpotato vine, most often at the base. Theadults eat their way out of the vine. The life cycle isvery similar to that of sweetpotato weevils. Larvae andpupae also resemble those of immature Cylas weevils, 20but the Alcidodes later instars are much bigger. Damage. The larvae bore into the vines and sometimesinto the storage roots. The vine base swells up. Adultweevils girdle the vines, causing wilting. Distribution and importance. Alcidodes spp.are minor pests throughout most of East and Central Africa. Control. Control is not usually required. Frequentearthing-up around the plant base reduces the incidenceof this pest. Other cultural control measures, such as theones practiced for sweetpotato weevil species, also helpto control Alcidodes spp. 21 22 23
  18. 18. Sweetpotato Weevils Minor Stemborers and Feeders Cylas spp. larvae can do considerable damage tosweetpotato vine bases by tunneling. In East Africa, this Peloropus Weevildamage can prevent a crop from establishing. This species Peloropus batatae (Coleoptera:is discussed in greater detail in the section on "Storage Curculionidae)Root Feeders." This reddish brown, compact, 3–4 mm weevil has been found inside stems and storage roots at some locations in East and Central Africa. The last instar of the white larva is longer than one would expect considering the small size of the adult. The larva makes long tunnels in the stem and can go down to the storage root via the storage root "neck." Pupae and adults are found at the end of the tunnels. The life cycle is long compared with that of other sweetpotato weevils—2 months or more. Because the larva enters via the storage root neck, storage roots that seem undamaged on the outside could be inedible because of several tunnels on the inside. Control is not usually required. 24 25
  19. 19. Sweetpotato BugPhysomerus grossipes (Hemiptera: Coreidae) Description and biology. The sweetpotato buglays groups of eggs on the undersides of leaves or onthe stem. The mother bug guards her eggs (Fig. 22A,B)and the young gregarious nymphs (Fig. 23). The egg stagelasts about 15 days. There are 5 nymphal stages andtotal development takes about 85 days for males and88 days for females. The adult is 20 mm long. Damage. The nymphs and adults pierce the stemsand petioles of the sweetpotato and suck the plant sap,thus causing wilting and stunting. 22A 22B Distribution and importance. The sweetpotatobug is found in Southeast Asia, where it is a minor pest. Control. Large numbers of bugs are usually foundfeeding together, making handpicking of the bugs or re-moval of the infested plants a feasible means of control.Long-Horned Beetle A species of long-horned beetle (Coleoptera:Cerambycidae) has been found to attack stem bases insome localities in East Africa. The larvae are large, withbig heads, and they are found inside the stem base. Theycause severe swelling. Control is seldom necessary. 23 26 27
  20. 20. Foliage FeedersSweetpotato ButterflyAcraea acerata (Lepidoptera: Nymphalidae) Description and biology. Pale yellow eggs (Fig.24) are laid in batches of 100–400 on both surfaces ofthe leaves. The caterpillars are greenish black and covered 24with branching spines. These larvae are concentrated ina protective webbing during the first 2 weeks after hatch-ing. They then become solitary and hide from thesunlight on the ground during the day (Fig. 25). The pupaeare yellowish and hang singly on the underside of leavesor on another support. The attractive adult butterfly hasorange wings with brown margins (Fig. 26). The life cycletakes 27–50 days depending on temperature. Damage. The caterpillars feed on the leaves. Young 25caterpillars feed on the upper leaf surface, whereas olderlarvae eat the whole leaf except for the primary midribs.Complete defoliation may result from severe attacks (Fig. 27). Distribution and importance. The sweetpotatobutterfly is a pest in East and Central Africa. It is an importantproduction constraint in some localities. Outbreaks aresporadic and seasonal and usually occur at the beginningof the dry season. 26 Control. Sweetpotato fields should be observed forsweetpotato butterfly adults and damage early in the dryseason. Webs containing young caterpillars should becollected and destroyed weekly. Early planting andharvesting enable the crop to escape severe attacks. Incase of severe outbreaks, chemical control can be carriedout with carbaryl, pyrethrum, etc.; however, effects onnatural enemies should be considered. Various speciesof tachinid, braconid, and ichneumonid parasites attackthe larval stage. Beauveria bassiana infects the sweet- 27potato butterfly. 28 29
  21. 21. Tortoiseshell BeetlesAspidomorpha spp. and others(Coleoptera: Chrysomelidae) Description and biology. Eggs are laid on theun- derside of the sweetpotato leaves or otherConvolvulaceae in batches cemented to the leaves. Theeggs of some species are concealed in a papery oothecum(Fig. 28). Larvae are characteristically flattened and spiny. 28In some species, the tail is held up over the back (Fig.29A,B) and the larva may carry excreta and previous castskins (Fig. 30). The pupa is less spiny than the larva, andis fixed inert to the leaf. The adults are broadly oval andmay be bright and patterned (Fig. 31). Larvae, pupae, andadults are found on both sides of the foliage. Developmentfrom egg to adult takes 3-6 weeks depending on thespecies. 29A 29B Damage. Both adults and larvae eat large round holesin the leaves. Attacks are sometimes sufficiently severeto completely skeletonize the leaves and peel the stems. Distribution and importance. Four species ofAspidomorpha and eight other Chrysomelidae havebeen recorded in Kenya on sweetpotato. Several speciesoccur in Southeast Asia including Cassia circumdata andC. obtusata, the green tortoiseshells; A. miliaris, the spotted 30tortoiseshell; A. elevata, the golden tortoiseshell (Fig. 32A,B), and A. amabilis, with reddish brown elytra. Tortoiseshellbeetles are widely distributed and often common. Althoughtheir damage is quite conspicuous, they seldom if evercause yield losses. 31 30 31
  22. 22. Control. Control is rarely warranted. Removal ofconvolvulaceous weeds in the surrounding area may reducetheir numbers. Several natural enemies including egg andlarval parasites (Tetrastichus sp., Eulophidae; Chalcidae) andpredators (Stalilia sp., Mantidae) have been reported. 32A 32B 32 33
  23. 23. Sweetpotato HornwormAgrius convolvuli (Lepidoptera: Sphingidae) Description and biology. The small, shiny eggsare laid singly on any part of the plant. The larvae havea conspicuous posterior "horn." They vary in color fromgreen to brown and are marked with distinct striped patterns.The last instar caterpillars reach 9.5 cm in length (Fig. 33). Hornworms are found mainly on young shoots. Thelarval period lasts 3–4 weeks. Pupation takes place inthe soil and takes 5–26 days, depending on the tem-perature. The large, reddish brown pupa (Fig. 34)ischaracterized by a prominent proboscis, which is curved 33downward. Adults are large, gray hawkmoths with blacklines on the wings. Wingspan is 8–12 cm. Damage. Yield losses can occur if heavy defoliationtakes place when the crop is young. A large caterpillarcan defoliate a plant and a large population of late instarlarvae can defoliate a field overnight. The larvae feed onthe leaf blades, causing irregular holes, and may eat theentire blade, leaving only the petiole. Distribution and importance. A. convolvulioccurs worldwide. It is not usually a serious pest, althoughsevere outbreaks have been reported in Vietnam. Control. Handpicking the larvae from the leaves isusually sufficient. Plowing the land between crops exposes 34the pupae. Light trapping can be used to monitor thepopulation of adults. When a large increase in adult numbersoccurs, manual removal of small larvae can prevent thebuildup of an outbreak population of the voracious lateinstar larvae. Pesticide use disrupts the action of the eggand larval parasites of the hornworm. 34 35
  24. 24. ArmywormsSpodoptera eridania, S. exigua, S. litura(Lepidoptera: Noctuidae) Description. Adult female moths of S. eridania arelight brown with dark spots on their front wings (Fig.35). Males are smaller with a black spot or a bar on thecenter of the front wings. Larvae in their first stages aregregarious and black and velvety with lateral yellow lines.In later stages, larvae are gray to olive green with twoparallel dorsal lines (Fig. 36) and they disperse all overthe plant. The white eggs of S. exigua are laid in round or ovalclusters and are covered with a layer of a felt-like substance 35 36(Fig. 37). The larvae are initially a grass-green color; theythen become green or dark brown with yellowish stripesin later instars (Fig. 38). Pupation occurs in the soil anddevelopment from egg to adult takes about 23 days. S.exigua adults can lay 1000 eggs. The eggs of S. litura are laid in clusters containingas many as 350 eggs. These are of variable shape andsize and are covered with “felt.” The caterpillars hatchafter 3–5 days and take about 2 weeks to reach the pupalstage. The larvae (Fig. 39) possess two characteristic blackcrescents on the fourth and tenth abdominal segments,bordered by yellow lateral and dorsal stripes. The larvaeprefer moist sites and may hide in the soil during theday, attacking plants at night. Pupation occurs in the soil.Female moths mate several times and produce a sex 37 38pheromone. The male moths are very sensitive to thepheromone on the fourth day after emergence. The femaleadults (Fig. 40) can lay as many as 2000–3000 eggs. Damage. Early instar larvae feed by scraping andscarifying the leaf surface. From the third instar on, they 36 37
  25. 25. consume the parenchymal leaf tissue, leaving only theveins (Fig. 41). The late instars of S. litura are very voraciousand may bore into sweetpotato roots when these areexposed. Distribution and importance. Armyworms arewidespread and feed on many host plants. S. litura isconfined to Asia, the Pacific, and Australia. Control. Weedy hosts should be eliminated. Ipo-moea reptans (kankung) and several weeds (Amaranthus 39sp., Passiflora foetida, Ageratum sp.) are common alternatehosts in Asia. Collection of egg clusters or leaves infestedwith gregarious young larvae can be an effective meansof control. Spot applications of insecticide or Bacillusthuringiensis can be made in the early larval stage whenlarvae are still gregarious. Formulations of nuclear poly-hedrosis viruses are available. The green muscardinefungus, Nomuraea rileyi, is highly pathogenic to S. litura,and the virus Borrelinavirus litura can cause mortalityafter an incubation period of 4–7 days. Predatory bugs, 40carabid beetles, vespid wasps, and spiders attack thelarvae, and more than 40 species of scelionid, braconid,ichneumonid, and tachinid parasites are known. 41 38 39
  26. 26. Leaf FoldersBrachmia convolvuli (Lepidoptera:Gelechiidae), Herpetogramma hipponalis(Lepidoptera: Pyralidae), and others Description and biology. The larvae of the blackleaf folder, B. convolvuli (Fig. 42), and the green leaf folder,H. hipponalis (Fig. 43), feed inside a folded leaf, leavingthe lower epidermis intact. In most cases, only one larvais found per leaf fold. The black leaf folder lays yellowishwhite oval eggs singly on the leaves. The eggs hatchafter 3–5 days and the five larval instars last 2–5 dayseach. The average total larval period is 11 days. The larvahas prominent bland and white markings on the thorax 42and abdomen. The pupal period is 4–7 days. The femalemoth lives an average of 5 days. The green leaf folderlays eggs in groups on the upper surface of the leaf nearthe midrib. The eggs are shiny green, oblong, and coveredwith a scale-like gelatinous material. The eggs hatch after3–5 days and there are five larval instars. The larva is greenishyellow with sparse brown setae and a dark brown headand prothoracic plate. The pupal period lasts 4–8 days.The adult is a yellowish brown moth with dark brown markingson its wings. The female moth lives about 3 days. Damage. The leaf margin is folded once by B. con-volvuli. H. hipponalis folds the leaf margin twice and prod-uces some webbing. Leaf folder feeding results in a lace-like appearance of the leaf, with the main leaf veins leftintact. 43 Distribution and importance. Leaf folders arewidespread throughout Asia and Africa. Control. A high rate of parasitism by braconid waspsis common. Earwigs and other generalist predators arealso important in maintaining natural control. If natural 40 41
  27. 27. enemy action is not disrupted by pesticide use, control Strobiderus Beetleis rarely needed. The use of uninfested planting material Strobiderus aequatorialisis an effective means of reducing the incidence of leaffolders. (Coleoptera: Chrysomelidae) This is a small, yellowish beetle, 5–7 mm long ocurring The brown leaf folder, Ochyrotica concursa (Lepidoptera: in East Africa. The adults perforate the leaves and causePyralidae), and the pink-striped leaf folder, Anticrota ornatalis damage similar to that of tortoiseshell beetles. Control(Lepidoptera: Pyralidae), have been reported in the Phil- measures are not usually necessary.ippines. The green larvae of the pyralid leafroller, Tabidiaaculealis, scarify the mesophyll from the inner side ofthe rolled leaf. They prefer full-grown leaves and the pupae Rough Weevilare found in leaf cases. Reported in Indonesia, theyoccasionally cause serious damage. Blosyrus sp. (Coleoptera: Curculionidae) The adults feed on the leaves of sweetpotato in Africa, but their important pest stage is the larva, which affects the storage roots (see the section on "Storage Root Feed- ers"). Sweetpotato Weevils Cylas spp. (Coleoptera Curculionidae) The adults feed on the leaves of sweetpotato, but are much more important as pests of the storage roots and stems (see the section on "Storage Root Feeders"). 42 43
  28. 28. Minor Leaf FeedersGrasshoppers and LocustsZonocerous variegatus(Orthoptera: Pyrgomoriphidae),the variegated grasshopper andAttractomorpha psitticina(Orthoptera: Acrididae),the slant-faced grasshopper, and others In Africa, both adults and nymphs of Z. variegatus candefoliate sweetpotato (Fig. 44). Outbreaks seldom occurand control is rarely needed. An Asian species, the polyphagous slant-faced grass-hopper is bright green and characterized by a pointedconical head and short antennae. It measures 30–40 mmin length. The female taro grasshopper, Gesonula zonocenamundata (Orthoptera: Acrididae), is found in SoutheastAsia. It bores through the petiole of the host plant tolay its eggs. These are covered with a reddish browngummy substance. The pale brown to green adult is 30mm long and has black stripes running from its eyesto the tips of its wings. The hind legs are black and the 44tibia are bluish with white-tipped spines. Taro and waterhyacinth are also hosts. 44 45
  29. 29. Virus TransmittersAphidsAphis gossypii and others(Homoptera: Aphididae) Description and biology. Aphids are soft-bodiedinsects, 1–2 mm long, yellowish green to black, with orwithout wings (Fig. 45). Aphids can multiply asexually,resulting in fast population buildup. Several generationsoccur per year. Damage. Aphids damage plants by sucking sap fromgrowing shoots. Symptoms of aphid attack are wrinkling,cupping, and downward curling of young leaves. Duringheavy infestation, plant vigor is greatly reduced. As aphids feed and move from plant to plant in thefield, they transmit viruses. The most important aphid-transmitted virus is sweetpotato feathery mottle virus.Winged forms may travel long distances and introduceviruses into new areas. A. gossypii has a wide host range,including cotton, cucur-bits, and many legumes. Distribution and importance. Aphids are cos-mopolitan. Their main impact in sweetpotato is as vectors 45of virus diseases. Control. Control is rarely necessary. Predators suchas ladybird beetles, lacewings (Chrysoperla sp.), and syr-phids naturally reduce aphid populations. In case of heavyoutbreaks, farmers tend to apply insecticides; however,these should be used with great caution since they reducenatural enemy populations and can contribute to furtheraphid outbreaks. 46 47
  30. 30. WhitefliesBemisia tabaci (Homoptera: Aleyrodidae) Description and biology. The female of B.tabaci lays eggs on the undersides of leaves. All thenymphal stages (Fig. 46) are greenish white, oval inoutline, scale-like, and somewhat spiny. The adult(Fig. 47) is minute and covered with a white, waxybloom. Development of one generation takes 3–4weeks. Damage. High whitefly populations may causeyellowing and necrosis of infested leaves. The pest ismore important as a transmitter of viruses, especiallysweetpotato mild mottle virus. B. tabaci has a wide host 46range, including cotton, tomato, tobacco, and cassava. Distribution and importance. The main impactof B. tabaci, a cosmopolitan species, on sweetpotato isas a vector of virus diseases. Control. Control measures are not usually needed.Controlling whiteflies is not usually an effective meansof limiting the incidence of the viruses they transmit. 47 48 49
  31. 31. MitesErinose caused by Eriophyid mites, Aceria sp.(Acari: Eriophyidae) Description. Vines and leaves become excessivelyhairy, beginning at the shoot tip (Fig. 48). Biology. Erinose is present in East Africa and theUnited States. The problem is more pronounced at loweraltitudes where the climate is hot and dry. Research suggeststhat yields may be reduced. Control. Control is through selection of mite-free 48planting material and good field sanitation. This mightnot be effective enough, however, because mite popu-lations can build up rapidly.Eriophyes gastrotrichus (Acari: Eriophyidae) Description. Infested vines present pocket-like gallson leaves, petioles, and stems (Fig. 49). Initially, galls arelight green, but they become brown afterwards. Severalmites in every stage of development live together insideeach gall. When heavy infestations occur, leaves becomecrinkled and lose their shape. Biology. Erinose caused by the gall mite is present 49in the Philippines and Papua New Guinea. Control. Mite-free planting material should be usedtogether with good field sanitation and destruction ofweeds that can act as hosts. 50 51
  32. 32. Natural Enemies Earwigs The natural enemies of sweetpotato pests have re-ceived limited research attention. What little is known Earwigs (Dermaptera: Forficuliidae) have a character-of their biology and ecology relates mostly to generalist istic hind pair of forceps-like pincers that are used forpredators, such as ants, and to fungal pathogens of the defense. Adults can live for several months and are mostsweetpotato weevils. active at night. They enter stemborer tunnels in search of larvae. Occasionally they climb the foliage to prey on Generalist predators are often the most important leaf folder larvae. They can consume 20–30 prey daily.group of biological control organisms in agricultural systemsbecause they can switch from one prey type to another.Their flexible food habits allow them to respond as one Spiderspest re-places another in terms of relative abundance.Herbivorous species that cause little or no economic damage The importance of spiders as predators has been clear-play an important ecological role in agricultural systems ly demonstrated for rice, but their role has not been studiedby providing food to maintain populations of beneficial adequately in many other crops and little is known aboutspecies at levels that can prevent damaging pest outbreaks. their contribution to biological control of sweetpotato pests. In Asia, where sweetpotato is often grown in rotationwith rice, many of the generalist predators commonly The lynx spider Oxyopes sp. and the wolf spider Lycosafound in rice fields persist in the sweetpotato crop that sp. are abundant in sweetpotato fields. These do notfollows (for more information on these, see Shepard et spin webs but rather hunt prey directly. Web-spinningal. 1987). spiders are also common. 52 53
  33. 33. Pheidole and Other Predacious Ants Cuban farmers practice augmentation of Pheidolemegacephala (Myrmicinae) and Tetramorium guinensis(Myrmicinae) to control the sweetpotato weevil, C.formicarius. Several ants—Pheidole sp. (Fig. 50),Iridomyrmex anceps (Dolichoderinae), and Anoplolepislongipes (Formicinae)—have been confirmed as predatorsof the sweetpotato weevil in Indonesia. Although theirrole as predators of other sweetpotato pests is unknown,Pheidole ants are known to attack small prey stages suchas eggs and early instar insect larvae. A number of other ant species are reported fromsweetpotato fields in Indonesia (Tetramorium sp., 50Monomorium sp., Odontoponera transversa, Dolichero-derus thoracicus, Polyrachis sp., Camponotus maculatus,Diacamma sp., Myrmicaria sp., Leptogenys sp., Pachy-dondylasp., and Odontomachus simillimus), Uganda (Myr-micariasp.), and Vietnam (Solenopsis geminata and Paratrechinasp., found in tunnels made by Omphisia anas-tomasalis).Although these ants have not been confirmed as predatorsof sweetpotato pests, the literature contains referencessuggesting a predacious behavior for several species. Anexample is Myrmicaria sp., often found attack-ing caterpillarsin agricultural fields and dragging them to their nests. 54 55
  34. 34. Beetles Carabids are one of the most important families ofpredatory beetles. The majority of carabids in agriculturalsystems are ground-dwelling species that feed on otherinsects that live or pupate in or on the soil. An exampleof a predatory carabid identified from sweetpotato fieldsin Indonesia is Pheropsopus sp. A few carabids climbfoliage and can be found within the chambers made byleaf folders. The staphylids are another common family of scav-engers and generalist predators. Of these, Paederus spp.(Fig. 51) are common in many crop environments, including 51sweetpotato. P. fusciceps occurs in Indonesia. The Coccinellidae, or ladybird beetles, are a large family,nearly all of which are predacious. Their main prey areaphids, mealybugs, and scale insects, but they also feedon insect eggs or on the slow-moving early instars ofsome Lepidoptera. Both the larvae (Fig. 52) and the adultsare predacious. Adult females deposit distinctive yellowcigar-shaped eggs in groups of 5–30 on plants, mostlynear colonies or groups of prey. Ladybird larvae forageon the plant and pupate on the foliage. 52 56 57
  35. 35. Flies and Parasitic Wasps Hover flies (Syrphidae) are brightly colored and fre-quently visit flowers to feed on pollen and nectar. Theadult flies (Fig. 53) lay their elongated white eggs singlyon plants, often near aphid colonies, which serve as preyfor their white, maggot-like offspring. Syrphid larvae areactive mostly at night and feed mainly on aphids. Syrphids,cocinellids, and parasitic wasps form a natural enemycomplex that plays an important role in regulating aphidpopulations. Tachinid flies (Fig. 54) mostly attack medium-to-lateinstar caterpillars. Some species deposit their eggs directlyon a host. Others lay eggs on the plant close to where 53the host is feeding. The eggs are consumed by the hostbut remain intact and develop within the host’s body.Other species lay incubated, fully developed eggs on theleaves; the newly hatched larvae search for a host andpenetrate its cuticle. Caterpillars parasitized by tachinidscan be recognized by the presence of dark spots on thecuticle, or by discoloration and deformation. Adult tachinidsvisit flowers for nectar and survive for several weeks. The tachinid Zygobothria ciliata is a larval parasiteof the sweetpotato hornworm. Cuphocera varia, alarviparous species, and Blepharella lateralis arecommon parasites of armyworms, and are widespreadthroughout Southeast Asia. The larvae of C. varia aredeposited on leaves near the feeding sites of theirhosts and rapidly bore into the side of a host between 54the segments. The host caterpillar attempts to dis-lodge the attached parasite by twisting and turningviolently. B. lateralis lays it eggs on the leaves andthe flies often emerge from the host pupa. Carceliakockiana, a highly polyphagous species, parasitizesarmyworms in Indonesia. Carcelia normula parasitizesthe sweetpotato butterfly in Uganda. 58 59
  36. 36. The wasp Telenomus spodopterae (Scelionidae) is a Virusescommon egg parasite of S. litura. The encyrtids Anastatusdasyni and Oencyrtus malayensis are egg parasites of Nuclear polyhedrosis viruses are common on army-Physomerus grossipes. Development of A. dasyni takes worms. The larvae become infected by eating virus-con-16 -18 days and the females live for about one month. taminated foliage. As infection develops in a larva, itAs many as 8 adults of O. malayensis emerge from an becomes sluggish and stops feeding. Later the larva turnsegg of P. grossipes. The wasps are able to attack eggs whitish and then black and hangs from the foliage bywithin one day of emergence, and live for about one its prolegs. The fluid oozing from its body contaminatesmonth. The eulophid Tetrastichus sp. has been reported foliage and continues the disease cycle.as a pupal parasite of the green tortoiseshell beetle.Brachymeria sp., a chalcid, parasitizes the green leaf folder. Granulosis viruses attack lepidopteran larvae. Horn-Trichogramma minutum (Trichogrammatidae) is an egg par- worm species are often affected. Host larvae that eatasite of the sweetpotato hornworm. Charops sp. contaminated foliage move sluggishly and then stop(Ichneumonidae) has been reported from the sweetpotato feeding. After 1-2 weeks, the body becomes constricted,butterfly. The adult is slender with a long, stalked abdomen, giving a segmented appearance. Infected larvae turn yellow,which is flattened laterally. The adult also has a charac- pink, or black and become soft.teristic stalked and barrel-shaped pupal cocoon. Many of the host insects of braconid parasites livein protected sites such as tunnels, mines, and foldedleaves (Fig. 55), or under webbing. Some examples areMacrocentrus sp., a parasite of the black leaf folder;Microbracon cylasovorus and Bassus cylasovorus, parasi-tes of C. formicarius; and Meteorus sp., a parasite ofsweetpotato butterfly larvae. 55 60 61
  37. 37. Fungal PathogensMetarrhizium anisopliae andBeauveria bassiana Sweetpotato weevils are among the insect speciesattacked by M. anisopliae. Spores germinate on the bodyof a host insect under conditions of prolonged high hu-midity. The fungus penetrates the insect and uses itsinternal body contents as a substrate for proliferation. Whenthe host dies, the fungus emerges through joints in theinsect exoskeleton, appearing first as a white growth.When spores are formed, the fungus turns green. Sporesemerging from the dead host are spread to new hostsby wind or water. 56 Beauveria bassiana (Fig. 56) attacks stemborers, leaffolders, and bugs among others, and is a confirmed patho-gen of sweetpotato weevils and the sweetpotato butterfly.Like other fungal diseases, it requires conditions of pro-longed high humidity for the air-or waterborne sporesto germinate. The fungus invades the soft tissues andbody fluids of the host and grows out of the body tosporulate. Affected insects appear to be covered witha powdery, white substance. Other pathogens that may play a role in the biologicalcontrol of pests in sweetpotato fields include the fungiHirsutella spp. and Nomuraea rileyi, and the nematodesHeterorhabditis spp. and Steinernema spp. 62 63
  38. 38. Diseases and Pathogens ofSweetpotatoand Their Management A number of pathogenic organisms affect sweetpotato.Most appear to be widespread, but damage levels vary. In this guide, we include viral, fungal, and bacterialdiseases, as well as those caused by nematodes. Eachgroup of pathogens is covered in a separate section. Patho-genic bacteria, although not very common, are responsiblefor important economic losses. They affect vascular tissueas well as storage and fibrous roots, thus causing vinewilting and rots. Fungal pathogens are classified accordingto the type of disease they cause, such as foliar, stem,storage root, and postharvest diseases. In general, foliarand stem diseases are mild and cause little damage, exceptfor scab, which is a very important disease in SoutheastAsia. These diseases contribute to lower yields by reducingphotosynthetic area and transport of nutrients and productsto the storage roots. In some countries, storage rots donot cause much damage because sweetpotatoes areconsumed shortly after harvest. Tuber-rot pathogens,however, are present in the field and can cause significantlosses. Plant parasitic nematodes are included as the causeof serious damage to storage roots both in the field andduring storage. Although the symptoms of virus diseases appear inthe foliage, these have been accorded a separate sectionbecause of their importance. Of all the sweetpotato patho-gens, viruses appear to contribute the most to yield losses. 65
  39. 39. Viral DiseasesSweetpotato Feathery Mottle Virus(SPFMV)Aphid-transmitted potyvirus Symptoms. Symptoms of SPFMV on the foliageof sweetpotato are generally slight or absent. If present,they appear as faint, irregular chlorotic spots occasionallybordered by purplish pigment. Chlorosis (feathering) alongmidribs (Fig. 57) and faint-to-distinct chlorotic spots withor without purple margins occur in some cultivars (Fig.58). Symptom visibility on foliage is influenced by cultivar sus- 57ceptibility, degree of stress, growth stage, and strain viru-lence. Increased stress can lead to symptom expression,whereas rapid growth may result in symptom remission.Symptoms on storage roots depend on the strain of SPFMVand the sweetpotato variety. The common strain causesno symptom on any variety, but the "russet crack" straincauses external necrotic lesions or internal corking oncertain varieties (Fig. 59). SPFMV can be latent in vines. Biology. SPFMV is transmitted by a wide range ofaphid species in the nonpersistent manner through brieffeeds of only 20–30 seconds. Both colonizing speciesof aphids and winged aphids of noncolonizing species 58may transmit the disease. It is also perpetuated betweencropping cycles in infected cuttings, but the lack of symp-toms in the foliage makes it difficult for farmers to selectSPFMV-free cuttings. In Uganda, symptom-free cuttingswere mostly virus-free. SPFMV is found with SPSVV (seenext section) in some countries; the combination results in asevere disease known as sweetpotato virus disease (SPVD). Distribution and importance. Occurs worldwide. Control. Aphid control is not economically feasible.The main controls are avoidance of use of diseased plantsfor cutting material, sanitation, and use of resistant varieties. 59 66 67
  40. 40. Sweetpotato Sunken Vein Virus Sweetpotato Virus Disease (SPVD)(SPSVV)Whitefly-transmitted closterovirus Symptoms. Diseased plants become severely stunted and the leaves become small and narrow (straplike), Symptoms. The symptoms reported for SPSVV vary often with a distorted edge. Puckering, vein-clearing, andgeographically; in East Africa, the disease may cause stunt- mottling may occur. The mottling is often pale so thating and color change in leaves (reddening or yellowing the whole plant may appear chlorotic.usually) depending on the variety. Elsewhere, symptomsinclude mild vein yellowing, some sunken seconday veins Biology. This disease seems to be caused by aon adaxial leaf surfaces, and swollen veins on abaxial synergistic combination of SPFMV and SPSVV; it is unclearsurfaces. The disease may also cause no symptoms. whether other virus combinations are involved. Biology. SPSVV is transmitted by the whitefly B. Distribution and importance. SPVD is commontabaci in the semipersistent manner, needing feeds of in Africa, where it is the main virus disease of sweetpotatoseveral hours to acquire or transmit efficiently. It may in Nigeria, Cameroon, Ghana, and Uganda. It causes virtuallyalso be perpetuated through cropping cycles via infected total yield loss in affected plants. It may be identical tocut-tings. SPSVV is generally identified in combination with a severe disease reported in the Americas. SPVD hasSPFMV, causing the severe disease SPVD (see next section). also been reported in Argentina, Brazil, Peru, Kenya, the United States, and Taiwan. Distribution and importance. By itself, SPSVVmay cause only small yield losses, but combined infection Control. The main controls are avoidance of diseasedwith SPFMV causes SPVD, a severe disease associated plants as sources of planting material and use of resistantwith almost complete loss of yield. SPSVV occurs in Kenya, varieties. Farmers usually avoid diseased planting materialUganda, and Nigeria, and has been reported in Asia, Ar- because symptoms are so severe.gentina, Brazil, Peru, and the United States. Control. The main controls are avoidance of diseasedplants as sources of planting material and use of resistantvarieties. 68 69
  41. 41. Other Viral DiseasesSweetpotato Mild Mottle Virus Other viruses have been identified through serological(SPMMV) identification techniques, such as sweetpotato latent virusWhitefly-transmitted potyvirus (SPLV), reported in Taiwan, Japan, Kenya, China, and Israel; sweetpotato chlorotic fleck virus (SPCFV), present in south- Symptoms. The predominant symptoms associated east Africa, Indonesia, the Philippines, China, Japan, andwith SPMMV are leaf mottling and stunting (Fig. 60). Vein Central and South America; sweetpotato caulimo-like virusclearing and distortion may also occur. None of these (SPCV), found in Puerto Rico, Madeira, New Zealand, Papuasymptoms is easily diagnosed in the field and the virus New Guinea, the Solomon Islands, and Kenya; sweetpotatocan be latent. ring spot virus (SPRSV), reported in Papua New Guinea and Kenya; and cucumber mosaic virus (CMV), found only Biology. SPMMV is transmitted nonpersistently by in Israel, Kenya, and the United States. Sweetpotato chloroticthe whitefly B. tabaci. It is also carried in infected cuttings. stunt virus (SPCSV) (Fig. 61) is found in Kenya and theThere is some evidence that SPMMV forms a complex Caribbean.with SPFMV, but this is unclear. Distribution. It has been identified in Kenya, Uganda,Tanzania, and Indonesia, but yield effects are unknown. Control. Some sweetpotato varieties appear to beimmune and others are tolerant. Sanitation and selection byfarmers of symptomless planting material also help achievecontrol. 61 60 70 71
  42. 42. Bacterial DiseasesBacterial Stem and Root RotErwinia chrysanthemi Symptoms. Aerial symptoms are water-soakedbrown to black lesions on stems and petioles. One ortwo branches may wilt, and eventually the entire plantcollapses (Fig. 62). Localized lesions on fibrous roots mayalso be present. On fleshy roots, localized lesionswith black margins can be observed on the surface,but more frequently the rotting is internal, with no evidenceoutside (Fig. 63). 62 Biology. The pathogen has several other hosts inwarm, humid areas of the world, where it remains inthe soil on plant debris and weeds. Infection occurs throughwounds. Distribution and importance. This disease isfound worldwide. Losses can be economically important. Control. Cuttings for transplanting should be takenabove the soil line. Using less-susceptible cultivars andtaking care to avoid wounding can reduce disease in-cidence. 63 72 73
  43. 43. Bacterial WiltPseudomonas solanacearum Symptoms. Infected stands usually contain somewilted plants (Fig. 64A). The disease starts at the baseof the stem as yellowish water-soaked lesions that soonturn brown. The vascular bundles of affected stems andsprouts are discolored (Fig. 64B). In storage roots, vasculardiscoloration is also present, but mainly longitudinal brownstreaks appear as well as brown water-soaked lesionson the surface (Fig. 64C). Slightly affected fleshy roots,when stored, can rot completely and develop a distinctiveodor (Fig. 64D). Biology. The bacterium is soil-borne, but it is usuallycarried with the propagative material. Once the soil isinfested, the bacterium can persist from one to threeyears. Dissemination in the field can also occur via irrigationwater. Distribution and importance. The disease isimportant in some areas of southern China when sus-ceptible varieties are grown. 64 Control. The use of less-susceptible varieties anddisease-free planting material reduces disease incidence.When the bacterium is already present in the soil, floodingand crop rotation with graminaceous hosts are recom-mended. 74 75
  44. 44. Soil RotStreptomyces ipomoea Symptoms. The first indication of the disease isan extensive chlorosis and bronzing of the foliage as aconsequence of the destruction of fibrous roots (Fig. 65).On storage roots, besides dark brown necrotic lesions,we frequently find cracks radiating from the center andmalformations such as dumbbell-shaped roots (Fig. 66). Biology. Soil rot causes more damage in dry alkalinesoils. The pathogen can survive in soil for long periods. Distribution and importance. This disease re-duces yield and can be destructive in some areas of the 65United States and Japan. Control. Planting material should come from areaswhere the disease is not present. Maintaining soil moisturehelps reduce disease incidence. Using sulfur to reducesoil pH is another alternative, but large amounts of thiselement are required. 66 76 77
  45. 45. Foliar and Stem DiseasesCaused by FungiLeaf and Stem ScabElsinoe batatas, Sphaceloma batatas Symptoms. Brown to tan raised corky lesions, withpurple to brown centers, appear along the stems (Fig.67). Coalescing tiny lesions cover the leaf veins, thus makingthem shrink and causing the leaves to curl (Fig. 68). Distribution and importance. The disease isimportant in Southeast Asia and the South Pacific Islands,where the pathogen causes serious losses from poor for-mation of fleshy roots. The disease is also present inBrazil. 67 Little is known about the biology of the pathogen.Humid weather, however, favors the disease. Control. Good levels of varietal resistance are avail-able. Pathogen-free planting material of the most resistantvarieties and good sanitation practices should be used.The resistance of native and introduced material is beingevaluated in Southeast Asia and the Pacific. 68 78 79
  46. 46. Alternariosis, Anthracnose, BlightAlternaria bataticola Symptoms. Brown lesions with a typical bull’s-eyeappearance of concentric rings occur on leaves, especiallyolder leaves. Black lesions appear on petioles and stems(Fig. 69). Bases and middle sections are more affectedthan the vine terminals. Death of vines can occur. Theground under affected vines is often carpeted with black-ened leaf debris (Fig. 70). Biology. Disease and lesion size increase with alti-tude. High relative humidity or free water is necessaryfor infection and sporulation. The fungus survives in debris,and the spores are spread through infected planting material,wind, splashing rain, and water. 69 Although Alternaria spp. can be found infectingsweetpotato in all agroecological zones, the form knownas alternariosis or anthracnose occurs at mid to high el-evations. Distribution and importance. Published infor-mation and experience point to Alternaria blight as themost important fungal disease in East Africa and Brazil. Control. Susceptibility to the pathogen varies amongvarieties. Pathogen-free planting material of the more re-sistant varieties and good sanitation practices will helpcontrol the disease. 70 80 81
  47. 47. Phomopsis Leaf Spot(Phyllosticta Leaf Spot)Phomopsis ipomoea-batatas(Phyllosticta batatas) Symptoms. Whitish to tan to brown lesions, usuallyless than 10 mm in diameter, form on the upper andlower surfaces of leaves. The lesions usually have a darkbrown or purple margin (Fig. 71). Pycnidia are visible inthe center of the lesions (Fig. 72). Biology. The fungus survives in debris and is notknown to have other hosts. Spores spread through in-fected planting material, wind, splashing water, and possibly 71insects. Distribution and importance. The disease iswidespread and occurs in all agroecological zones. It isnot known to depress yield, but it can reduce the qualityof vines for planting material and fodder. Control. No control measures are known. Controlis not normally necessary. 72 82 83
  48. 48. Minor Leaf Spot Fungi Other fungi cause leaf spots and can be identifiedby inspecting spores with a microscope. These fungiare Alternaria spp., Cercospora sp. (Fig. 73), Septoriasp., Ascochyta sp., Curvularia sp., Colletotrichum sp.,and Pestalotia batatae. Control. No control measures are known. Controlis not usually needed. 73 84 85
  49. 49. Chlorotic Leaf DistortionFusarium lateritium Symptoms. The first noticeable sign or symptomis a white, waxy (crusty) mucilaginous layer, which containsmycelium and sporodochia, that covers newly expandedleaves (Fig. 74). Microscopic observation reveals it on apicalmeristems and axillary buds. As the leaves age, the waxycovering spreads along the leaf margin and eventuallydisappears. In some cultivars and environments, leavesbecome chlorotic. Occasionally, leaves become distortedand plants are stunted (Fig. 75). Biology. The pathogen may be present on the entiresurface of the aerial part of the plant and it can be transmittedthrough true seed. It cannot be eliminated by surface 74disinfection of the seed. Symptoms are more severe inhot, dry weather. There is a long latent period (3–6 weeks)from infection to symptom expression. Distribution and importance. The disease hasbeen found in Peru, a few areas of East and Central Africa—primarily at low altitudes where it is hot and dry—andin the United States. It is not known to cause economicdamage in its less-virulent form. When stunting and distortionoccur, losses should be expected. Control. Pathogen-free planting material is essential.Varietal differences in susceptibility are observed. Usevegetative planting material from plants free of symptoms.Do not harvest true seed from diseased plants, especiallyif the seed is to be shipped to areas where chloroticleaf distortion is not present. No chemical control is known. 75 86 87
  50. 50. Fusarium WiltFusarium oxysporum f. sp. batatas Symptoms. The first indication of this diseaseis a dullness and yellowing of the leaves, followed bywilting and death of the vine. Affected vines show thevascular discoloration typical of this disease (Fig. 76). Biology. The fungus is soil-borne and specific tosweetpotato and a few close relatives, and barley andflue-cured tobacco. It can survive in the soil and in debrisfor several years. Though tip cuttings are usually pathogen-free, roots and cuttings from the base of the vine canbe infected. Movement of infested soil on tools and byanimals can lead to outbreaks in new areas. The diseaseoccurs under a variety of environmental conditions. Yieldreduction depends on the stage of plant growth whendisease occurs. Distribution and importance. This disease isfound in most areas where sweetpotato is grown andis more important in temperate areas than in the tropics. Control. Good sanitation will help reduce the impactof the disease and limit its spread. Some varietal resistance 76has been observed, and breeding programs in some coun-tries have released resistant varieties. 88 89
  51. 51. Violet Root RotHelicobasidium mompa Symptoms. Affected plants become chlorotic andmay defoliate. Fibrous roots rot and become covered withthick whitish threads of mycelium that soon become pinkand finally violet (Fig. 77). Storage roots start rotting apicallyand then they completely decay and are covered by thesame mycelial mat as the fibrous roots. At the same time,flat black sclerotia are formed. This violet mat of coarsemycelium and sclerotia may be found on the ground inplaces where plants have rotted. Rotted storage rootshave a characteristic smell of alcohol. Biology. The fungus has a wide host range besidessweetpotato. It can survive in the soil for at least 4 yearsas mycelium or as sclerotia. Infected transplants and irrigationwater can disseminate the fungus. Temperature is nota limiting factor for disease development, but considerablemoisture in the soil favors the disease. Distribution and importance. The disease ispresent in several areas of Asia and the Americas. It cancause serious losses in Asia. Control. Planting material should come from healthyplants. Early maturing varieties can escape the disease. 77Rotation with cereals can also help prevent the disease. 90 91
  52. 52. Sclerotial Blight and Circular SpotSclerotium rolfsii Symptoms. Sclerotial blight and circular spot aretwo diseases caused by the same pathogen. Blight symptoms start in both seedbeds and newlyplanted stands. Shoots emerging from the mother rootsuddenly collapse and die. Affected shoots are easilypulled and separated from the rest of the plant. A matof white mycelium and numerous round brown sclerotiaresembling rapeseed are found at the base of affectedplants (Fig. 78). Circular spots are observed only in fleshyroots. Symmetric brown sunken lesions that sometimesshow crackings are present (Fig. 79). 78 Biology. The fungus attacks several plant species.It is soil-borne and survives for long periods as sclerotia.Moisture and organic matter in the soil favor attack. Distribution and importance. This disease iscommon in tropical and subtropical regions of the world.Losses are not usually serious. Control. Disease incidence can be reduced by avoid-ing growing sweetpotato in infected soils and using dis-ease-free planting material. The use of good sanitationand less-susceptible cultivars also helps to reduce thedisease. 79 92 93
  53. 53. Black RotCeratocystis fimbriata Symptoms. Dark to black sunken cankers in thelower part of the stem are the most distinctive symptom.In severe infections, yellowing, wilting, and plant deathcan occur. Affected storage roots develop black to graysunken areas (Fig. 80) on which black spine-like structuresof the fungus can be seen protruding from the surfaceof roots. A smell of alcohol resembling that of fermentingsugar is frequent. Biology. The use of infected cuttings for plantingperpetuates the disease. Transmission occurs throughwounds made by sweetpotato weevils (such as Cylas spp.), 80wireworms, crickets, and mice. The fungus is a soil in-habitant that can remain 1–2 years in crop debris. Moisturedoes not affect disease development. Distribution and importance. The disease isparticularly important in Southeast Asia and Oceania whereit reduces yield and quality of fleshy roots. Control. Cuttings for transplants should come frompathogen-free planting material. In places where it is difficultto find healthy mother plants, cuttings should be made2 cm above the soil line to avoid infected portions ofthe plant. Rotate with nonhost plants for at least 2 yearsand use good sanitation practices. Cure during the 5 daysfollowing harvest at 30–35°C and 85–90% relative humidity. 94 95
  54. 54. Storage Root andPostharvest DiseasesFoot RotPlenodomus destruens Symptoms. Brown lesions form on the stem ator below the soil line. Wilting and death occur in severecases. Black pycnidia can be seen (Fig. 81). A canker ex-tends down the stem and affects the proximal end ofthe storage root (Fig. 82). This decay is dark brown, firm,and dry. 81 Biology. The fungus does not survive well in thesoil except in infected roots and stems. It is spread byinfected cuttings, especially those from the base of thevine, and by contact with spores from infected roots instorage. Other hosts include members of theConvolvulaceae. Diseased roots should not be stored. Distribution and importance. This disease isfound in Peru, Brazil, and Argentina. Storage damage canreduce marketability. Control. Sanitation and the use of healthy vine tipsfor planting are the best means of control in the field. 82 96 97
  55. 55. Java Black RotLasiodiplodia theobromae(Diplodia gossypina) Symptoms. This rot is firm and moist initially,but storage roots soon become totally blackened andmummified. Rot starts at either or both ends of the storageroot and is initially brown, before turning black. Eruptiveblack stromatic masses that bear pycnidia are a diagnosticfeature (Fig. 83). Biology. Java black rot is spread by infested soil,infected storage roots, and contaminated storage boxes,baskets, or tools. Infection occurs via wounds, especially 83the cut stem end. Though the pathogen can infect stems,it grows very little and is seldom a problem. Yields canbe reduced in the field or through storage losses. Distribution and importance. This disease isdistributed worldwide. It is one of the most importantstorage diseases of sweetpotato. Control. Timely harvesting can reduce losses.Good sanitation and care in handling to reducewounding are important. 98 99
  56. 56. Charcoal RotMacrophomina phaseolina Symptoms. This disease is found only on fleshyroots during storage. The fungus does not attack otherplant parts. Infection starts on the surface of the rootand progresses through the vascular ring toward the pith. Three distinct zones are found in a cross section ofan infected root: an unblemished periderm, an inner zoneabout 6 mm wide of reddish brown tissue where a crustylayer of sclerotia is found, and the inner part of the rootwith light tan tissue. Sometimes the center of the pithsplits and the entire root becomes mummified (Fig. 84). 84 Biology. The fungus is distributed worldwide andattacks several plant species. It is soil-borne and can survivesaprophytically on plant debris or freely as sclerotia. Distribution and importance. This disease isfound in tropical and subtropical areas of the world. Lossesare seldom serious. Control. No control measures are known. 100 101
  57. 57. Soft Rot(Rhizopus stolonifer, Mucor sp.) Symptoms. Soft rotting occurs after harvest. Storageroots become soft, wet, and stringy, often starting at oneend. A strong alcohol-like odor is produced. These fungiare commonly seen sporulating on the surface of rottingstorage roots (Fig. 85). Biology. The disease is spread by infested soil orair-borne spores that enter wounds. Optimum relative hu-midity and temperature for progress of infection and dis-ease vary by variety, but are usually high. Soft rot candestroy harvested roots in 48 hours if they are left 85unprotected under sunlight. Distribution and importance. This disease isfound worldwide in sweetpotato and other crops. It attacksthe fleshy organs of plants that are rich in sugar or starch. Control. Washing storage roots is especially con-ducive to rot. Care in handling and proper curing can reducedisease incidence. So far, no resistance has been found,but some varieties rot faster than others because theyare more susceptible. Curing is accomplished by storingafter harvest at 29–32°C and 95–100% relative humidityfor 5–7 days with adequate ventilation (at least 8 cubicfeet of air per ton per day). Subsequent storage is bestat around 13°C and 95% relative humidity. 102 103

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