This slideshare is prepared for last hour revision to students related with plant pathology. This slideshare may not contain enough informations about diseases.

1. Diseases of Field Crops and Etiology
By
Binod Pokhrel
Faculty of Agriculture
Agriculture and Forestry University (AFU)

2. Early blight of potato
Alternaria solani
The pathogen survives in infected plant debris (Pri.
Infection) left on the soil surface or buried in the
soil and on or in the seed. Under favorable
conditions in the spring, the fungi produce conidia
that are carried by wind (sec. infection) and
splashing rain. Secondary disease cycles can occur
as long as the weather remains favorable. Spore
production and secondary infections are favored
by heavy dews, rainy conditions and temperature
between 20 – 25°C.

3. Concentric ring of Alternaria
Conidia of Alternaria

4. Late Blight of Potato
Phytophthora infestans
• Infected potato tubers are the primary source
of inoculum.
• Zoospores act as source of secondary
infection.

6. Black wart of potato
Synchytrium endobioticum
The primary infection may be caused by seed
tubers, infected soil or by any plant for
planting
Zoospores act as source of secondary
infection.

7. Potato wart

8. Black Scurf
Rhizoctonia solani
• Primary inoculum is infected seed tubers
• Secondary inoculum is sclerotia and mycelium

9. Black Scurf

10. Bacterial wilt
Pseudomonas solanacearum
or
Ralstonia solanacearum

11. Brown spot of rice / Sesame leaf blight
Helminthosporium oryzae
Rice blast
Pyricularia oryzae (Pear shaped conidia)
False Smut of rice
Ustilagonoidea virens
Sheath Blight of rice
Rhizoctonia solani
Foot rot/Bakane/Foolish seedling disease
Gibberllia fujikuroi

12. Bacterial leaf Blight of rice
Xanthomonas oryzae pv. oryzae

13. Diseases of wheat/Barley
Black rust / stem rust
Puccinia graminis f.sp. tritici
Brown rust / Leaf rust
Puccinia triticina
Yellow rust / Stripe rust
Puccinia striformis
Teliospore of Puccinia is single septate and bicelled

14. Damping off of seedling
 Pythium aphanidermatum
 P. debaryanum
 P. ultimum
Other fungus also causes damping off
 Phytophthora
 Rhizoctonia
 Sclerotium
 Sclerotinia
 Fusarium
Lemon shaped, papillate conidia
Sickle shaped conidia

15. Powdery mildew of pea
Erysiphe polygoni
Powdery mildew of cucurbits
Erysiphe cichoracearum
Powdery mildew of wheat
Erysiphe graminis
Can germinate even at 0% humidity
Requires > 90% humidity for germination
Requires 100% humidity for germination

16. Diseases of maize
Southern leaf blight: Bipolaris maydis
Northern leaf blight: Exserohilum turcicum
Grey leaf spot: Cercospora zeae-maydis
Comparatively small leisons and vein limited
Comparatively long leisons crossing the veins
Prominent yelllow halo, spots become comparatively rectangular

17. Club root of cabbage/cauliflower
Plasmodiophora brassicae
 Primary infection form resting sporangium
 Secondary infection from sporangium
producing zoospores

18. Fungal wilt
Different pathogens are involved
 Rhizoctonia solani: Whole root system is
affected, only tap root remains unaffected
 Sclerotium rolfsii: Always attacks on collar
region at early stage, root system is not
damaged
 Fusarium oxysporum: doesn’t damage root
system, partial wilting of leaves
Yellowish color: just started wilting
Green color: not affected
Dried leaves and branches: completely wilted

19.  If soil is moist Rhizoctonia is active.
(Produces sclerotia of irregular shape and size)
 If soil is dry:
Sclerotium ( Affects at early stage) – produces
smooth, small sclerotia initially white which
changes into yellow
 Fusarium ( Affects at any stage of crop) –
produces 3 types of spores; Macroconidia,
microconidia and chlamydospores.

20. Stemphylium blight of lentil
Stemphylium botryosum ( Conidia have both
longitudinal and transeverse septa)

21. Sclerotinia blight of Mustard
Sclerotinia sclerotiorum ( produces ascospores
in apothecia)
Grey mold of chick pea
Botrytis cineria ( Knee shaped spporangia)

22. Panama wilt of banana
Fusarium oxysporum f.sp. Cubense
Moko Disease / Bacterial wilt
Ralstonia solanacearum
Vascular discoloration occurs from outer to inner side
Vascular discoloration occurs
from mid rib to margin.

23. Sigatoka disease
Pseudocercospora musicola (formerly Mycosphaerella
musicola)
Initial symptoms are more prominent in lower
surface of older leaves.
Oval to circular spots within vein.

24. Bunnchy top of banana
By Virus (ssDNA)
Anthracnose of banana
Colletotrichum musae

25. Bean anthracnose
Colletotrichum lindemuthianum
 Primary infection: Infected seeds
 Secondary infection: Conidia
Angular leaf spot of bean: Isariopsis griseola
Bean rust : Uromyces phaseolus
Stem rot: Sclerotinia sclerotiorum
Root rot: Rhizoctonia solani
Southern blight of bean: Sclerotium rolfsii

26. Citrus Decline
Due to different pathogens like:
 Virus (Citrus Tristeza Virus)
 Fastidious bacteria ( Candidatus liberibacter)
 Bacteria ( Xanthomonas axonopodis pv. Citri)
 Nematode ( Tylenchullus semipenetrona)
 Fungi ( Fusarium, Phytophthora)
 Nutritional deficiency ( Zn, Mg and Fe)

27. Citrus gummosis
 Exudation of amber colored juice ( gummy
substance), prominent cracking of bark.
 Caused by Phytophthora sps.
 Primary infection through chlamydospores /
oospores in infetced plant parts or trunk.
 secondary infection through air borne
conidia.

28. Citrus gummosis

29. Citrus canker
 By Xanthomonas axonopodis pv. Citri
 Lime and lemon are more susceptible than
mandarin
 Raised postules of cankers are formed on both
sides of leaves and mostly associated with yellow
halo.
 Volcanic crates with several cracking in postules.
 concentric ringsleaf miner ( Tuta absoluta)
 Transmission by
• Primary infection by infected seed
• Secondary infection by wind or water transmission

30. Concentric rings with yellow halo
Citrus canker

31. Citrus scab / Verucosis
 By Elsinoe fawselti (Teliomorph)
Sphaceloma fawseti (Anamorph)
 Circular depressions, reddish to brownish centre
 Leaves are excessively distorted and wrinkled.
 primary infection by infected tissue as acervuli.
 Secondary infection by wind borne conidia.

32. Distorted leaf

33. How to distinguish citrus canker and scab?
Citrus canker Citrus scab
Raised postules are formed on both sides of
leaves
circular depressions are formed
Mostly associated with yellow halo Yellow halo are rarely associated
Volcanic craters with several cracking on
postules
No volcanic craters
No such conditions Leaves show excessive wrinkling and
distorted

34. Citrus Greening/Hwanglongbing disease/Yellow dragon disease
 By fastidious phloem limited bacteria, candidatus
liberibacter asiaticus/africans/americanus
 Foliar interveinal chlorosis ( leaf cupping and
vein corking)
 often confused with Zn deficiency
 Simple pen test or Iodine scratch method can be
used to distinguish Zn deficiency and HLB.
 Looks like mosaic
 Transmitted by Citrus Psylla.

35. Citrus greening
Fruit is half yellow and half green
• Curved collumela
• One lobe is larger and another is
smaller

36. Quick Decline / Tristeza
 Caused by Tristeza virus
Transmitted by aphids in semi persistent manner.
 3 phases are seen:
1. Quick decline: more common in sour orange
2. Shrievling: more common in lemon and grape fruit
3. stem pitting: more common in grape fruit and
aleomolo
 Generally virus survives in latent infection which is
triggered by heat/water stress.
 Symptoms are seen in graft union; longitudinal linear
pits and stem looks like ropey.
 Honeycomb pattern of stem pitting in trunk below the
bark.

37. Bulging or swelling in graft union
Ropey appearance

38. Powdery mildew of citrus
 By Oidium tingitaninum
 Powdery mildew requires cool and wet
condition. ( Downy mildew requires dry and
humid condition)
 Powdery mildew can be controlled by Sulphur
based fungicides and downy mildew can be
controlled by Copper based fungicides.
 Symptoms are same in all crops.

39. Powdery mildew of citrus

40. Mango anthracnose/Blossom blight/Leaf spot/Fruit rot
 By Colletotrichum gloeosporiodes
 Alligator skin symptom is seen
 Fruit cracking occurs from drought stress
 generally other species of Colletotricum are
curved but it is straight, cylindrical and slightly
constricted
 Other species have single vacuole but it has
two vacuoles.

41. In older leaves, symptoms initiate from leaf tip or margin
Alligator skin symptom

42. Mango malformation / Bunchy Top
 caused by several agents and factors:
1. Physiological causes: High Nitrogen, water stress
2. Higher C:N ratio
3. Hormonal imbalance
4. Fusarium moniliforme var. subglutinans
5. Mite ( Vector of Fusarium )
Three phases:
1. Bunchy top
2. Vegetative malformation
3. Floral malformation

43. 
More common in Bombay green and Amrapali
Dassheri and Nangra are resistant varities
1. Bunchy top: shoots are swollen and appears as
small rudimentary leaves, stunted in young
seedlings
2. Vegetative malformation: apical growing parts
of stem suffer malformation or hypertrophized,
excessive veg. growth, peduncle and secondary
branches are swollen and stunted, gives bunchy
top effect
3. Floral malformation: flowers are clustured and
compact, dull green, floral parts are converted
into veg. buds and rudimentary leaves or small
secondary branches.

44. Red rust of litchi
Cephaleurus mycoides

45. Red rust of mango
Cephaleurus viricense
If rust is washed by rain water
then it appears as white mass
Causes twisting and
curling of leaves

46. Powdery mildew of mango
 By Oidium mangiferae
 Primary infection occurs through infected
parts
 Secondary infection occurs through wind
borne conidia

47. Stem rot / Foot rot / collar rot of papaya
 By Pythium aphanidermatum
 Primary inoculum is resistant spores i.e.
oospores.
 Secondary inoculum is sporangia.
 Water soaked spot in the stem at the ground
level which enlarge and griddle the stem.

48. Powdery mildew
 By Oidium caricae or Oidium indicum
 Requires cool and moist condition for diseases
development ( opposite in case of downy mildew)
 Primary inoculum: dormant mycelia
 Secondary inoculum: wind borne oidia

49. Papaya ring spot
 By papaya ring spot virus ( ssRNA)
 Vein clearing, puckering and chlorophyll leaf tissues
lobbing in.
 On fruits, circular concentric rings are produced. If
affected earlier no fruit formation.
 Vectored by aphids Aphis gossypii, A. craccivora (in
non persistent manner) and also spreads to
cucurbits not through seeds.

50. Margin and distal parts of leaves roll
downward and inwards
Shoe string symptom
circular concentric rings

51. Papaya Leaf curl
 Papaya leaf curl virus (ssDNA) is an obligate
parasite.
 Spread by whitefly (Bemisia tabaci) in non
persistent manner.
 Either upward or downward rolling of leaves.
 Petiole orientation is different
 Leathery, brittle, puckered leaves
 Thick clear vein is seen.

52. Downward orientation of petiole
Papaya Leaf Curl

53. Anthracnose of papaya / guava
 By Colletotricum gloeosporiodes
 On fruit initially brown superficial discoloration of
the skin develops which are circular and slightly
sunken.
 Under humid condition salmon pink spores are
released.
 Both seed born and soil born disease
 Primary infection is caused by fruit from field.
 Secondary infection by conidia by rain splashes

54. circular and slightly sunken skin

55. Rust of guava
 By Puccinia psidii
 Primary inoculum: Teliospore
 Secondary inoculum: wind born Uredospore

56. Fusarium wilt of guava
 By Fusarium oxysporum pv. Psidii
 Primary inoculum: surviving structure (
chlamydospore)
 Secondary inoculum: Air borne conidia

57. Stem gall of coriander
 By Protomyces macrosporus
 Numerous swelling in stem