This pdf contains various plant diseases that are caused by microorganisms, plant diseases,symptoms, cause and prevention

1. Bacterial, Fungal &
Viral Plant Diseases

2. Phytopathology
 Phytopathology or plant pathology is the
science of diagnosing and managing
plant diseases.
 It covers all infectious agents that attack
plants and abiotic disorders, but does
not include herbivory by insects,
mammals, etc.
 Approximately ten percent of food
production is lost to disease worldwide..

3. Phytopathogens
 Phytopathogens or plant pathogens are the pathogens such as Bacteria,
Fungi, viruses, protozoa and nematodes which attacks plant and cause
major economic loss.
 The majority of phytopathogenic fungi belong to the Ascomycetes and
the Basidiomycetes.
 Most bacteria that are associated with plants are actually saprotrophic,
and do no harm to the plant itself. However, a small number, around 100
species, are able to cause disease.
 There are many types of plant virus, and some are even asymptomatic.
Normally plant viruses only cause a loss of crop yield.
 Nematodes are small, multicellular wormlike creatures. Many live freely
in the soil, but there are some species which parasitize plant roots.
 There are a few examples of plant diseases caused by protozoa. They
are transmitted as zoospores which are very durable, and may be able
to survive in a resting state in the soil for many years.

4. Bacterial pathogens
 Bacterial diseases are much more prevalent in
sub-tropical and tropical regions of the world.
 Most plant pathogenic bacteria are rod shaped
(bacilli).
 In order to be able to colonize the plant they
have specific pathogenicity factors.

5. Pathogenicity Factors
• Five main types of bacterial pathogenicity factors are
known:
– Cell wall degrading enzymes – used to break down the plant cell
wall in order to release the nutrients inside. Used by pathogens
such as Erwinia to cause soft rot.
– Toxins These can be non-host specific, and damage all plants, or
host specific and only cause damage on a host plant.
– Effector proteins These can be secreted into the extracellular
environment or directly into the host cell, often via the Type three
secretion system. Some effectors are known to suppress host
defense processes.
– Phytohormones – for example Agrobacterium changes the level of
auxins to cause tumours.
– Exopolysaccharides – these are produced by bacteria and block
xylem vessels, often leading to the death of the plant.

6. Significant bacterial plant pathogens
 Burkholderia
 Proteobacteria
 Xanthomonas spp.
 Pseudomonas spp.

7. Bacterial Leaf Blight
Causal organisms: Bacteria
Important species:
 Bacterial blight of bean (Xanthomonas axonopodis pv.
phaseoli)
 Bacterial blight of cotton; Angular leaf spot of cotton
(X. axonopodis pv. malvacearum)
 Bacterial leaf blight of rice (X. oryzae pv. oryzae)
 Bacterial leaf streak of rice (X. oryzae pv. oryzicola)
 Bacterial leaf blight of tomato and pepper (X. campestris
pv. vesicatoria)
 Cassava bacterial blight (X. campestris pv. manihotis)
 Cassava leaf spot (X. cassavae)

8. Bacterial leaf blight of rice
 An infected leaf has yellow water-soaked lesions at the
margin of its leaf blade.
 The lesions run parallel along the leaf and when they join
together may cover the whole leaf.
 Bacterial discharge appears on young lesion early in the
morning that looks like a milky dewdrop.

9. Bacterial leaf blight of rice

10. Conditions that favor development
 Warm temperature, frequent rain, and high
humidity
 Over-crowded plants with poor air flow and
low sunlight penetration among plants
 Improper soil nutrient and irrigation
management
 Poor soil drainage
 Diseased-seeds and planting materials

11. Pathogenesis
 Penetration and entry by plant pathogens.
 Contact (propagules)
 Recognition between host and pathogen
 Host penetration an infection
 When a pathogen comes in contact with a host plant, a
complex of interacting factors including
 temperature,
 moisture,
 susceptibility of plant tissues,
 effects of other microorganisms present,
 aggressiveness of the pathogen
determine whether infection will occur or not.

12. Preventive control
 Crop rotation with crops that are not susceptible to the bacteria
 Use of disease free seeds, disease-free planting materials and cuttings,
and use of resistant cultivars.
 Hot water seed treatment
 Proper fertilization and water management
 Proper land preparation for better drainage
 Proper plant spacing for proper air circulation and sunlight penetration
within plants
 Insect pest control as they may serve as the carrier of the bacteria
 Weed control
 No farm activities when plants are wet
 Clean farm tools
 Field sanitation
 Removal and proper disposal of infected plant parts
 Deep plowing to bury plant debris
 Lemongrass extract
 Mint extract

13. Citrus Canker
 Causal organisms: Bacteria
 Scientific name: Xanthomonas axonopodis
pv. Citri
 Host Plant - Citrus
 Affected plant stages - Seedling to
reproductive stages
 Affected plant parts - All above ground
parts

14. Symptoms of Citrus Canker
 Citrus canker is a bacterial disease of citrus that causes
premature leaf and fruit drop.
 The spots have oily margins or yellowish rings.
 The cankers can be seen on both sides of the leaf.
 On fruit, the cankers appear scab-like or corky but
without the yellow halo (ring).

15. Citrus Canker

16. Citrus Canker
Conditions that favor development
 Strong winds and wind borne rain
 Citrus leafhoppers, birds and animals, farm equipments
that carry the bacteria
 Movements of infected plants or plant parts
Preventive Control
 There is no effective control on citrus canker, except to
pull out the infected tree and then burn it.
 Also, control citrus leafhoppers.

17. Bacterial Wilt
 Scientific name: Ralstonia (Pseudomonas) solanacearum
 Causal organisms: Bacteria
Host plants
 Potato, tomato, tobacco, eggplant, banana and plantain are the major
hosts but peanut, bell pepper, cotton, sweet potato, cassava,
Affected plant stages
 Vegetative and reproductive stages
Affected plant parts
 Whole plant
Symptoms
 Bacterial wilt is very destructive especially during hot and wet seasons.
Plants wilt and die suddenly.

18. Bacterial Wilt of Potato
 The infested leaves wilt during the (sunny) day and sometimes
recover during cool hours.
 The wilting is similar to the result of lack of water.
 During the rapid development of the disease, the entire plant
wilts quickly without yellowing.
 On the cut surface, a creamy fluid usually appears on the
vascular ring.

19. Bacterial Wilt of Potato

20. Bacterial Wilt
Conditions that favor development
 Crop residues left in the field that were infected
by Ralstonia solanacearum
 Injured roots caused by farm tools or by soil
pests
 Warm temperature and high moisture
 High soil pH
 Poor and unfertile soil
 Nematodes present in the soil

21. Bacterial Wilt
Preventive control
 Remove and destroy all infected plants immediately
 Control nematodes
 Rotate crops. Rice, corn, beans, cabbage, and sugarcane
are found to be resistant to bacterial wilt
 Since the bacteria can be transmitted through farm tools,
wash or expose them to heat before using in another field
 For banana, remove and chop the plants surrounding the
infected mat or within the radius of 6 meters from the
infected plant to prevent further spread of the disease.

22. Bacterial leaf spot
Scientific name: Pseudomonas spp.
Causal organisms: Bacteria
Important species:
 Bacterial leaf spot of cole (P. syringae pv. maculicola)
 Bacterial leaf spot of cucurbits, Angular leaf spots (P. syringae
pv. lachrymans)
Host plants
 Cucurbits, beans, cereals and many other plants
Affected plant stages
 All growth stages
Affected plant parts
 Leaves, stems, fruits, pods, seeds

23. Bacterial brown spot of Beans
Symptoms
 Infected leaf has ovale-shaped spots of dead
tissues surrounded with yellow-green to lemon-
yellow margins.
 As the spots enlarge, they may join together often
having torn out dead tissues, developing a shot-
hole appearance.

24. Bacterial Leaf Spot

25. Bacterial leaf spot
Conditions that favor development
 Warm temperature, frequent rain, and high relative
humidity
 Diseased-seeds and planting materials
 Over-crowded plants with poor air flow and low sunlight
penetration among plants
 Improper soil nutrient and irrigation management
 Poor soil drainage
Preventive control
 Rotate crops
 Heat water seed treatment
 Use diseased-free seeds or use resistant cultivars
 Removal and proper disposal of infected plants. Burn
infected plant tissues when possible.
 Clean contaminated equipment
 Avoid working when plants are wet

26. Bacterial soft rot
Scientific name: Erwinia carotovora
Causal organisms: Bacteria
Host plants
► Potato, sweet potato, cassava, onion, cabbage and other
crucifers, carrot, tomato, beans, corn, cotton, coffee,
banana, and many other succulent agricultural crops
Affected plant stages
► All growth stages
Affected plant parts
► Whole plant

27. Symptoms of Bacterial soft rot
► On carrot, an infected taproot has a soft and watery decay.
The bacteria sometimes rapidly consume the entire taproot
often leaving the epidermis (the peel) intact.
► On corn, an infected plant has a tan or dark-brown, water-
soaked, soft stalk that suddenly collapses and is usually
twisted.
► On onion and garlic, an infected bulb is pale-brown and,
soft and watery.
► On pepper, the infected fruits collapse and hang like
water-filled bags.

28. Bacterial Soft rot

29. Bacterial soft rot
Conditions that favor development
►Infected plant debris left rotten in the field.
►Presence of onion maggot as the vector for
soft rot in onion and garlic and root knot
nematodes that cause lesions on the roots.
►Plant wounds and injuries.
►Hot and damp weather with plenty of
rainfall trigger the disease to occur. Water is
required for the bacteria to invade.

30. Bacterial soft rot
Preventive control
There is no known effective control measure of bacterial soft rot.
The following practices can lessen its damage to the plant
population;
► For potato, plant the whole seed tubers
► Proper land preparation to have a well-drained soil
► Control nematodes and other insect pests that serve as vectors
(carriers) of the bacteria to invade the plant tissues
► If possible, avoid plant injury during weeding especially when
the disease symptoms are observed
► Remove infected plants immediately
► Remove plant residues after harvest
► Practice crop rotation by using crops that are not susceptible to
the bacterial soft rot disease like soybean, forage legumes, and
small grains

31. Fungal Pathogens
 The majority of phytopathogenic fungi belong to the
Ascomycetes and the Basidiomycetes.
 The fungi reproduce both sexually and asexually via the
production of spores.
 These spores may be spread long distances by air or
water, or they may be soil borne.
 Many soil borne spores, normally zoospores and capable
of living saprotrophically, carrying out the first part of their
lifecycle in the soil.
 Fungal diseases can be controlled through the use of
fungicides in agriculture, however new races of fungi often
evolve that are resistant to various fungicides.

32. Significant fungal plant pathogens
Ascomycetes
 Fusarium spp. (causal agents of Fusarium
wilt disease)
 Verticillium spp.
 Magnaporthe grisea causes blast of rice and
gray leaf spot in turfgrasses

33. Significant fungal plant pathogens
Basidiomycetes
 Rhizoctonia spp.
 Phakospora pachyrhizi Sydow; causes
soybean rust
 Puccinia spp.; causal agents of severe rusts
of virtually all cereal grains and cultivated
grasses

34. Significant fungal plant pathogens
Oomycetes
 The oomycetes are not true fungi but are fungal-like
organisms.
 They include some of the most destructive plant
pathogens including the genus Phytophthora which
includes the causal agents of potato late blight and
sudden oak death.

35. Rust Disease
Causal organism: Fungus
Important species
• Asian soybean rust; Soybean rust (Phakopsora pachyrhizi,
P. meibomiae)
• Bean rust (Uromyces appendeculatus)
• Coffee rust (Hemileia vastatrix , H. coffeicola)
• Common rust; Corn rust (Puccinia sorghi)
Host plants
• Beans, Allium, corn, sorghum, rice, banana, coffee, and a wide
range of agricultural crops
Affected plant stages
• Vegetative stage
Affected plant parts
• Stems, leaves, petioles, and pods

36. Rust Disease
Common rust, Corn rust
• The initial symptoms are yellow spots on leaf
surfaces.
• The spots develop into oval to elongate reddish-
brown powdery and elevated lesions that contain a
powdery mass of orange to reddish-brown spores
(pustules) on the upper and lower leaf surfaces.
•

37. Rust Disease

38. Rust Disease
Conditions that favor development
• Frequent rains, drizzle, or dew with cool temperature and high humidity
• Poor quality seeds used for sowing
• Poor field sanitation
Prevention and control
• Select only disease-free seeds for planting.
• If there was a heavy infestation of rust on your farm during the previous
crop, practice crop rotation by planting a different crop in the next cropping
season.
• Control weeds to facilitate air circulation and rapid drying of the plants'
canopy.
• Prune coffee plants properly.
• Aloe leaf extract
• Papaya leaf extract

39. Rice Blast
 Scientific name: Pyricularia grisea,
Magnaporthe grisea
 Causal organism: Fungus
Host plants
 Specific to rice but also infects some rice
field weeds
Affected plant stages
 All growth stages but the severe damage
occurs during the seedling stage
Affected plant parts
 All above ground parts

40. Rice Blast
Symptoms
 An infected leaf has diamond-shaped or elliptical or spindle-
shaped spots with gray or white centers and brown margins.
 The spots may merge leading to a complete drying of the
infected leaf.
 The infected panicle turns white, causing panicle blast, and
dies before being filled with grain.
 The infected node rots causing all above parts to die.

41. Rice Blast

42. Rice Blast
Conditions that favor development
 Infested or diseased seeds
 Excessive use of nitrogen
 Poor air flow and poor sunlight penetration
 Rainy days with high humidity
Prevention and control
 Use disease-free seeds
 Use resistant cultivars
 Proper plant spacing, transplanting is advisable rather than broadcasting
 Split applications of nitrogenous fertilizer
 Apply compost
 Avoid farm activities when plants are wet
 Burn diseased-straw and stubble
 Malabar nut extract

43. Powdery Mildew
Causal organism: Fungus
Important species
 Powdery mildew of tomato, pepper, eggplant, potato and
other solanacoeus crops, squash, cucumber and other
cucurbits (Erysiphe orontii)
 Powdery mildew of carrot (E. polygoni)
 Powdery mildew of cucurbits (E. cichoracearum;
Sphaerotheca fuliginea, Podosphaera fusca; P. fuliginea)
Host plants
 Cereals, grasses, vegetables, ornamentals, weeds, shrubs,
fruit trees, and forest trees
Affected plant stages
 Seeding, vegetative, and reproductive stages
Affected plant parts
 Leaves, petioles, stems, and sometimes, fruits

44. Powdery Mildew
Symptoms
 Powdery mildew is characterized by a dusty-white to gray
coating and talcum powder-like growth commonly infecting
plant's leaves.
 It begins as circular, powdery-white spots that turn yellow-
brown and finally black.
 In most cases, the fungal growth can be partially removed by
rubbing the leaves.
 Powdery mildew is commonly found on the upper side of the
leaf. It also infects the underneath surface of the leaf, young
stem, bud, flower, and young fruit.

45. Powdery Mildew

46. Powdery Mildew
Conditions that favor development
 Warm and dry climate with high humidity
 Infected seeds and planting materials
 Overcrowded plants
Prevention and control
 Plant resistant cultivars when available.
 Practice a wider spacing between the hills and the rows to
increase air flow among plants, and light penetration.
 Practice crop rotation.
 Remove infected plant materials and alternative hosts.
 Prune overcrowded plants to increase air circulation, reduce the
relative humidity, reduce infection, and increase the light
penetration.
 Plow under all plant residues or remove plant residues after
harvest. Do not place infected plant materials on the compost
pile.

47. Downy Mildew
Causal organism: Fungus
Important species
 Downy mildew on tobacco, crucifers, spinach, soybeans,
alfalfa, onion, and many herbaceous and woody plants
(Peronospora spp.)
 Downy mildew on cucurbits (Psuedoperonospora spp.)
 Downy mildew on sorghum and corn (Peronosclerospora spp.)
 Downy mildew on grasses and millet (Sclerospora spp.)
 Downy mildew on corn, rice, wheat (Sclerophthora spp.)
Host plants
 Beans, crucifers, cucurbits, maize, potato, tomato, rice, and a
wide range of herbaceous and woody plants
Affected plant stages
 Seedling, vegetative, reproductive, and post harvest
Affected plant parts
 Leaves, inflorescence, fruits, and seeds

48. Downy Mildew
Symptoms
 Downy mildew infection begins as angular yellow spots on the
upper leaf surface.
 Then they become brilliant-yellow.
 Eventually, the internal parts of these spots become brown
with yellow margins.
 The underside of this infected leaf has fine, grayish fungal
growth.
 Infected young shoots, fruits, and seeds have white coating of
fungal spores.

49. Downy Mildew

50. Downy Mildew
Conditions that favor development
 Cool moist and warm moist weather conditions
Prevention and control
 Select and use only diseased-free seeds for sowing. If
possible, procure seeds that are resistant to downy
mildew.
 Remove infested plants and prune infested shoots.
 Avoid overhead watering.
 After harvest, plow-under all the plant debris.

51. Viral Pathogens
 Normally plant viruses only cause a loss of crop yield.
 Therefore it is not economically viable to try to control
them, the exception being when they infect perennial
species, such as fruit trees.
 Most plant viruses have small, single stranded RNA
genomes.
 These genomes may only encode three or four proteins:
– a replicase, a coat protein, a movement protein to allow cell to cell
movement though plasmodesmata and sometimes a protein that
allows transmission by a vector.
 Plant viruses must be transmitted from plant to plant by a
vector.
 This is often by an insect (for example, aphids), but some
fungi, nematodes and protozoa have been shown to be viral
vectors.

52. Banana bunchy top virus
Causal organism: Virus
Host plants
 Banana, plantain, abaca, wild banana, bird of
paradise, ginger
Affected plant stages
 All growth stages
Affected plant parts
 Whole plant

53. Banana bunchy top virus
Symptoms
 The initial symptom is the presence of dark-green spots on
the petioles, midribs, and leaf veins of the newly emerging
leaves.
 The infected leaves are erect, narrow, stunted, and often
have yellow leaf margins.
 The leaves become smaller as the disease progresses.
 Eventually, the crown of the plant is composed of only
stunted leaves, producing a "bunchy top", hence the name.
 The infected plant does not produce any fruit or the bunch
does not emerge from the pseudostem.

54. Bunchy top Virus Disease

55. Banana bunchy top virus
Conditions that favor development
 Banana aphids (Pentalonia nigronevosa)
 Infected planting materials
 Volunteer plants that serve as alternate hosts; like bird of
paradise, ginger, and wild bananas
 Weeds
Prevention and control
There is no cure for bunchy top viral disease. However, it can
be avoided by;
– Using disease-free planting materials
– Controlling of aphids, using soapy water or insecticidal soap.
– Spray the plants thoroughly, importantly on petioles, furled leaves,
whorls, or on young suckers.
– Removing and rouging of infected banana plants. Chop, dry, and
bury the infected plants.

56. Cassava mosaic disease
Casual organism: Virus
Host plants
 Cassava
Affected plant stages
 Seedling to reproductive stages
Affected plant parts
 All above ground parts

57. Cassava mosaic disease
Symptoms
 An infected leaf has white or pale-yellow or pale-
green spots.
 It is often twisted, distorted, and stunted.
 The leaflets have uniformed or localized mosaic
pattern.
 Heavily infested plant is distorted, retarded, and
dwarfted.
 Whiteflies are the carrier of the disease and
transmission is through their feeding habits.

58. Cassava Mosaic Disease

59. Cassava mosaic disease
Conditions that favor development
 Whiteflies
 Infected planting materials
 Weedy fields
 Stressed plants
Preventive control
 Use of cassava resistant varieties or diseased-free
planting materials
 Control of whiteflies
 Removal of infected plants
 Field sanitation

60. Leaf Curl Virus
Geminiviridae: Begomovirus
Synonyms: Leaf curl, yellow leaf curl
Hosts
 Tomato, pepper, eggplant, okra, tobacco, beans, cotton,
cucurbits, citrus, and other crops as secondary hosts
Affected plant stages
 Seedling and vegetative stages
Affected plant parts
 Whole plant

61. Leaf Curl Virus
Symptoms
 Infected plant has stunted young leaves and
shoots.
 It grows very slowly, becomes bushy, and
dwarfed.
 The leaf margin rolls either inward or upward and
is rather stiff with yellowish margin.
 Its leaves are thicker than normal, with leathery
texture.
 The young leaves have yellowish color, cupped,
thick, and rubbery.

62. Leaf Curl Viral Disease

63. Leaf Curl Virus
Factors that favor development
 Presence of whiteflies
 Infected transplants
 Weedy fields
Prevention and control
 Use resistant or tolerant cultivars
 Protect seedlings from whiteflies
 Use only good seeds and healthy transplants
 Control weeds

64. Mosaic virus
 Synonyms: Tomato mosaic virus, cassava mosaic virus, soybean
mosaic virus, cucumber mosaic virus, tomato fern leaf virus
 Causal organism: Virus
Hosts
 Tomato, pepper, potato and other solanaceous, cucumber,
melons, squash and other cucurbits, beans, cassava are the ones
most affected but it will also infect several crops
Affected plant stages
 All growth stages
Affected plant parts
 Leaves and fruits

65. Mosaic virus
Symptoms
 The common symptom of an infected plant is
alternating spots of yellowish and light or dark green
(mottle) leaves.
 The mottled areas often appear thicker and somewhat
elevated giving the leaves a blister-like appearance.
 Other symptoms include curling and yellowing of the
leaves, stunted growth, and malformed fruits.

66. Mosaic Viral Disease

67. Mosaic virus
Factors that favor development
 Infected seeds and planting materials
 Presence of insect pests such as aphids, beetles, and
grasshoppers
 Infected crop debris left on the field
Prevention and control
There is no known method to control viral disease. The following
are the recommended preventive control measures;
 Use resistant cultivars.
 Control aphids and other plant sucking insect pests that spread
the disease.
 Remove and properly dispose of infected plants.

68. TOBACCO MOSAIC VIRUS
DISEASES
 Viral infections can reduce plant vigor by
disrupting cellular functioning, but do not
commonly kill plants. Tobacco mosaic is one of
the most highly persistent tomato diseases
because it can remain viable without a host for
many years and it is able to withstand high heat.
The virus is spread primarily by mechanical
methods. Gardeners can contaminate their
hands when they touch tobacco products or
infected plants and weeds and spread the virus
to healthy plants.

69. SYMPTOMS
 Tobacco mosaic virus is not as distinct as other
viruses, but yellow-green mottling on leaves is
the most characteristic symptom of the disease.
 Infected plants have stunted growth, and
flowers and leaflets may be curled, distorted,
and smaller than normal in size.
 Open blooms may have brown streaks through
them.
 Yellow, chlorotic spotting is characteristic of
pepper plants that are infected

70. TMV in pepper

71. LIFE CYCLE
 Tobacco mosaic virus survives on infected
seeds, plant debris, and even clothing for
months or years.
 The virus enters plants through wounds.
 It spreads rapidly once it is in the host. The
virus can remain viable for years on dried plant
debris and is extremely tolerant of very high
temperatures. Tobacco products are the most
common source of inoculum; smokers can infect
plants by handling them. The virus is not spread
by aphids.

72. Factors that favor development
 Infected seeds and planting materials
 Presence of insect pests such as aphids,
beetles, and grasshoppers
 Infected crop debris left on the field

73. Prevention and control
There is no known method to control viral
disease. The following are the
recommended preventive control
measures;
 Use resistant cultivars.
 Control aphids and other plant sucking
insect pests that spread the disease.
 Remove and properly dispose of infected
plants