Plant diseases, their unique symptom and the causal organism, (bacterial or fungal) primary inoculum and secondary inoculum and management

1. Plant Diseases
By
Binod Pokhrel
Faculty of Agriculture
Agriculture and Forestry University

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/striformis
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