Seedling die-back: Symptoms:This disease is more severe on one to four months old seedlings.Infection may start from the tip of the stem or from cotyledonary stalkor from the collar region.The disease appears as dark brown to black, water soaked, linear lesions.This lesions extend to the leaves through petiole result in wilting andsubsequent defoliation of the seedlings.In advanced stage die back of the seedlings.
Etiology: Phytopthora palmivoraThe mycelium is septate and 7 µm in dia., sporangiophores are simple orbranched.Sporangia are inverted pear shaped, terminal and measure 38 to 72 µm ×33 to 42 µm.Zoospores are 8 to 10 µm in dia. Oospores are spherical and measure 33to 45 µm in dia.
Management:All the infected seedlings in the nursery should be removed and destroyed.The disease can be controlled effectively by providing adequate drainage.Soil drenching with Bordeaux mixture 1.0 per cent or copper oxychloride 0.25 per cent.A combination of seed dressing and soil drenching with Kocide at a concentration of 0.91 kg in 45 litres of water effectively controls pre- and post-emergence seedling death.
White thread blight Symptoms:Severe incidence of white thread blight in India was noticed in 6 months old seedlings in the nursery in Karnataka during 1990.The young branches of the affected plants contain white mycelial threads of the fungus which spread longitudinally and irregularly along the surface of the stem. The growth of the fungus is very rapid on the stem under favourable conditions of high humidity and entered the leaf at the nodes along the petioles. The fungus invades the cortical tissues which eventually turn dark brown to black. The diseased leaves also turn dark brown. The dead leaves in a branch eventually get detached from the stem but are found suspended by the mycelial thread in a row. The extensive death of the young branches and suspended leaves in rows are the common field symptoms of white thread blight.
Causal agent: Marasmius scadens Mode of spread and survival: The disease spreads from plant to plant and to different branches of the same plant through the mycelium. The dead leaves with the mycelial mat can be easily carried by wind on to the leaves and stems of the healthy plants and initiate the disease. Epidemiology: High humidity, less aeration and sunlight due to thick shade are the predisposing factors for the occurrence of white thread blight disease. Management: Damage can be reduced by removal of the dead materials and pruning of affected parts. Shade reduction and some structural pruning of branches are necessary to reduce the humidity in the canopy and the disease.
Black pod diseaseSymptoms:Cocoa pods may be attacked at any stage of their development. Infectionof the pod may be proximal (stalk end), distal (tip) or lateral (sides).First sign of the disease is appearance of brown spot on the pod.The brown discolouration rapidly spreads in all directions. Usually, thereis a line of demarcation of diseased and healthy tissues. Thediscolouration spreads over the whole pod. Under humid conditions, awhite mould appear on the surface of the pod.Internal tissues of diseased pods become brown. Infected beans arediscoloured.Causal agent: Phytopthora palmivora, P. megakarya, P. capsici
Mode of spread and survival:The fungus is disseminated by wind-borne spores which are produced oninfected pods. The spread is also by splashing rain, rodents and bycontact between healthy and diseased pods. Ants have been found tospread the disease. The fungus persists as mycelium in infected cushionsand pods and as chlamydospores during dry weather.Epidemiology:The fungus infects chillies, citrus, coconut, cotton and rubber.
Epidemiology:Pods of all ages are susceptible. Closer spacing between trees, damplocality and cool damp weather favour the rapid spread of the disease.The disease spreads rapidly under conditions of high rainfall and longperiods of high humidity. Pods and other tissues damaged by insects, rats,man etc. are highly susceptible to infection. Bark damage is necessary forwood infection and canker development.
Management:Important control measures are regular removal and destruction ofinfected pods at weekly intervals, during the rainy season pruning ofshade trees.Proper pruning of cocoa trees is also very essential to minimize the shade.These operations reduce inoculum as well as the disease.Spraying just before the onset of monsoon with Bordeaux mixture 1.0 percent or copper oxychloride 0.25 per cent and then at intervals of twoweeks during the peak period of incidence along with weekly removal ofinfected parts give better control of the disease. Spraying with captafolor fentin acetate or metalaxyl or aluminium ethylphosphonate is alsoeffective. Spray should be directed at the pods and bearing branches.Resistant varieties if available can be used. Lafi No.7 and Sic.28 areresistant clones reported from Samoa and Brazil respectively.
Charcoal pod rotCharcoal pod rot is found throughout the year. But the disease is severeduring summer months.Pods of all ages are susceptible.Symptoms: The disease occurs on wounded pods or pods which are"under stress. The infection takes place through wounds generally causedby insects and rodents. The infection appears as dark brown to blackspot on any place on the pod surface and spreads rapidly. The wholemass of the pod including the beans becomes black in colour. If theinfection occurs in the early stages of pod development, the beans maynot develop fully and get mummified.
Fungus: Botryodiplodia theobromae [Diplodia theobromae] Pycnidia areupto 5mm in dia. Conidiogenous cells are 5 to 15 µm. Conidia are hyalineand thin walled, becoming thick walled, dark brown and single septate.Management:Spraying with Bordeaux mixture 1.0 per cent is recommended forcontrolling this disease.Since injury on the pods caused by insect attack is a predisposingfactor, a combination spray is given by mixing with insecticide.Rodent control is also necessary to reduce the disease.
Witches’ broom disease: Marasmius perniciosusSymptoms:Infection of buds results in systemic infection of young shoots whichstimulates the growth of lateral buds in the leaf axils to produce thebroom effect - a cluster of closely formed stems.Diseased stems are thicker. Only few leaves are produced. The broomsdies after several weeks but remain attached to the tree. During wetweather small, pinkish mushroom - like fruiting bodies are produced ondead brooms.The fungus infects flower cushions and produce parthenocarpids andfloral brooms. Pods are infected at an early stage and they are distorted.The internal tissues are destroyed. External necrosis of fruits occur justbefore ripening.
Mode of spread and survival: Basidiospores which cause infection arereleased during night and they are spread through wind.Management: Young brooms before the production of sporophore shouldbe removed. Varieties like:Scavina 6 and hybrids of this variety are resistant.
Swollen shoot It was first reported in cocoa from Ghana by Posnette (1940). In 1936,peculiarly swellings on cocoa branches were noticed in the eastern regionof Ghana and the phenomenon named swollen shoot.
Symptoms:•Swellings develop on nodes, internodes and tips of the quick growingshoots. Swellings on tap root is noticed. Necrosis may be observed onfibre roots.•Red vein banding appears in the early stages and produce a network overthe lamina. Later, the pigment is restricted to the midrib and lateralveins and portions of the finer veins adjacent to them and produce thered feathering stage. Reddening usually disappears as the leaves turngreen and harden. It may change to a green vein banding which sometimestemporarily retains a reddish tint. As the leaf enlarges, the red patternis joined by chlorotic or transparent lesions. It is usually associated withthe veins. Later, chlorosis takes the form of blotching or spotting andcoalesce into bands or blocks. These patterns, unlike reddening, disappearas the leaf matures, but may undergo changes until hardening has reachedthe final stage. They seem to result from disorder of the tissues,preventing it from developing properly. The mesophyll remainsundifferentiated, lacking intercellullar spaces. The chloroplasts remainsmall and flattened.
• Young unripe pods develop a light and dark green mottling. Then this is overlaid by dark marbling or blotching. The surface of the pods becomes smoother and its shape rounder. Infected plants gradually die-back beginning with the drying up of twigtips. Though fruit production is slightly reduced in the year of infection,significant yield losses is noticed after a few years.Causal agent: Cocoa swollen shoot virus (CSSV), Theobroma virus- 1or Cocoa mottle leaf virus. The virus particles are bacilliform, notenveloped and 28 x 130 nm in size. The thermal inactivation point of thevirus is 55 to 60°C, longevity in vitro is 28 to 85 days.
Mode of spread and survival:The virus spreads systemically throughout the plant.It is transmitted by insect vectors, especially mealy bugs or mechanicallyor by grafting.Main vectors(mealy bug) are Dysmicoccus brevipes, Ferrisia virgata,Planococcus celtis, P. citri, P. kenyae, Pseudococcus longispinus.Transmitted in a semi-persistent manner.Passively they are transmitted by wind, harvested fruit or plantingmaterial and ants.
Management:Remove and burn infected parts and severely infected plants.In addition, all wild plants serving as host for the virus or vectors like wildTheobroma, Adansonia digitata, Cola chlamydantha, Cola cordifolia,Hibiscus spp., should be eradicated within and near cocoa plantation.In old cocoa plantations, the control of the mealy bug vectors with contactinsecticides is difficult because the insects find a number of hiding placesand are protected by ants so that the chemicals do not reach them. Removalof all dead twigs and ant nests especially in younger plantations, control ofthe protectants, i.e. the ants, with DDT, aldrin and dieldrin and directcontrol of the mealy bug vector with parathion and systemic insecticides inyoung plantations have been proved successful. Proper spacing of individualtrees within plantations helps to reduce the spread of the pathogen byvector.
PESTS:A large number of insects feed on cocoa. The adaptation of localinsects takes 20-30 years when cocoa is introduced into any area.However, although over 1500 different insects are recorded on cocoa,only about 2 per cent has economic importance.The occurrence of insect species is characteristic to countries. Theprimary pests of cocoa are mirids (Capsids), pod borers and bollworms.Others like mealy bugs are important as they act as vectors of viruses.Another serious pests are the rodents, which cause considerable lossto pods (Entwistle, 1985).
MiridsThe most important mirids (Capsid) that attack cocoa are Helopeltisantonii, Sahlebergella singularis, Distantiella theohroma and Monalonionspp. These are found widespread in South America, West Africa and SriLanka (Entwistle. 1972; Gibbs et al .. 1968).Helopeltis is reported to infest in Indian states (Abraham and Remamony,1979; Daniel, 1994; Sundararaju and Babu, 1999).Mirids feed by sucking the juices from plant tissues. The nymphs andadults infect cherelles, pod stalks, chupons and fan branches. This resultsin water soaked areas of tissue forming lesions, which later turn black.The Helopeltis is especially severe in cherelles causing wilting.
Management:Lindane has been used to control the pest.A few parasitoids and many predators have been recorded, but attemptsfor biological control have been unsuccessful.The chemical control of mirids is not easy, as it requires high volumespraying. The chlorinated hydrocarbons and gamma-HCH were usedearlier in West Africa to control mirids. Later low volume sprayingmethods have been developed. However, if the pest attack is limited, it isbetter to avoid any chemicals and allow biological control to take place innature.
Mealy bugsThe mealy bugs are small sap sucking insects characterized by sedentaryflightless, juvenile and adult female stages covered by wax sectionsdorsally.Population dynamics of mealy bugs vary among cocoa progenies and variablesusceptibility has been observed (Campbell, 1990).Cocoa mealy bug, Planococcus lilacinus was reported to be a serious pest inseventies in India (Radhakrishnan Nair, 1979).Other mealy bugs recorded on cocoa are P. citri and P . Njalensis.Ants for their sweet excretory substance generally attack these mealybugs. Mealy bugs cause damage to all parts of cocoa plants especially thetender portions.
Management:The control of mealy bugs has been found to be difficult. There are threemain approaches for effective control viz., biological control, control ofants and pesticidal control.Natural enemies of mealy bug like ladybird beetles and other predatorshave been reported. But it has not given appreciable control over thepests.Use of pesticides to control mealy bugs indirectly by controlling antpopulation is with high risk of persistence of pesticides and is generallynot recommended. Spraying of systemic insecticides was effective incontrolling the mealy bugs.
ThripsThe most abundant species of thrip that attack cocoa is Selenothripsrubrocinctus.They infect lower leaf surfaces. The sap sucking results in leafshriveling. The population is found to increase with soil stress conditions.The use of drought tolerant types can control the thrips.However, chemical control with spray is also possible.
Aphids The aphid species, Toxoptera alurantii that is dark brown to black in colour, affects tender leaves, cushions and cherelles. The other minor aphid found on young shoots and flowers is Aphis gossypii. The aphids are not considered as serious pests, but results in leaf curling and flower wilt.
Ring bark borersPhassus hosei and P. sericeus are ring bark borers.They damage the bark around the stem. They attack both young and oldtrees.Drenching 1 per cent dieldrin into the holes and sealing them can controlthe pest.Red borer: (Zeuzera coffeae ) have been recorded in severalcountries (Daniel. 1994; Kalshoven, 1919b). These damage the plants bymaking tunnels in the stem. The damage is considerable and when youngstems are attacked, the plants die. Control is achieved by pruningaffected branches and application of insecticides.
Cocoa Moth (Cocoa Pod Borer)Conopomorphs crameralla is a small moth which does much damage topods in Java and the Philippines. It also occurs in Papua New Guinea andthe Celebes.Eggs are laid in the epidermis of pods, usually in furrows.The larvae go through the husk and bore around the beans for l5-18days before leaving to form a cocoon.Damage cannot be seen until the pod is opened and found to be full offrass; the beans are useless.
BollwormThe spiny bollworm of cotton, Earias biplaga, attacks cocoa from CotedIvoire to the Congo. It mainly attacks unshaded plants up to 3 yearsold.It is therefore a greater problem on cocoa planted on clear-felled land,particularly when the early shade is inadequate. They bore into thepericarps of unripe pods.The control can be achieved by providing adequate shade during earlyplanting.A systemic insecticide, such as monocrotophos, is recommended in someareas.
Cocoa BeetleThe longhorn beetle, Steirastoma breve, is a pest of cocoa in manycountries.Eggs are laid in holes in the bark. The larvae bore a chamber in thecambium and bark. From the chamber a tunnel is bored in a spiral, whichoften rings the stem so that it dies. A pupal chamber is then bored in theheart wood, weakening the stem. A gummy, gelatinous exudate appearsaround holes through the bark made by larvae. Trees from 6 months to 5years are attacked.The intensity of attack increases as the amount of shade is reduced.
Ambrosia BeetlesAttack by Xyleborus beetles are of economic importance as they alsocarry spores of fungal species of Ceratocystis. These beetles causecharacteristic small round holes in the trunk and branches.Systemic insecticides like Endosulfon, Quinolphos, Dimethoate are usedto control the beetles.
VertebratesSeveral mammalian pests like rodents, squirrels and civets cause damageto pods in all cocoa growing countries (Thorold, 1975).In South India, squirrels and black rat (Rattus rattus) caused severedamage to cocoa (Keshava Bhat et al., 1981).The palm civet and bonnet monkeys caused minor damages. The squirrelsmake holes in center or terminus of pods, while rats feed near the stalkportion (Keshava Bhat, 1980).For controlling squirrels live box traps were most effective (Keshava Bhatand Mathew 1983). The anticoagulants like Warfarin and Fumarin areeffectively used to kill rats. These rodenticides (0.05% brodifacum) havebeen effectively used in controlling rats in cocoa gardens (Keshava Bhatand Sujatha, 1989; Smith and NOtt, 1988).
References:Diseases of Horticultural Crops - Dr. G. Arjunan, G. karthikeyan, Dr. D. Dinakaran, Dr. T. RaguchanderPlantation crops Volume 1 - V. A. Parthasarathy, p. K. Chattopadhyay, T. k. BoseCoffee, Cocoa and Tea - K. C. WillsonIntroduction to Spices, Plantation crops, Medicinal and Aromatic Plants - N. Kumar, JBM Md. Abdul Khader, P. Rangaswami, I. Irulappan