7. Brown spot of rice
CO: Bipolaris oryzae
Damagetoplants
seedlingmortality
qualityandnumberofgrainsaffected
The diseasecausesblight on seedlings,whicharegrownfrom
heavily infectedseeds,and cancause 10-58%seedlingmortality.
It alsoaffectsthe qualityand the number of grains perpanicle
and reducesthe kernelweight.
The reductionin yield canbe as high as 45%in severe infection
and 12%in moderateinfection.
The diseasewasconsideredtobe themajor factorcontributing
tothe “GreatBengalFamine”in 1942 resultingto yield lossesof
50%to90%and caused the deathof 2 millionpeople.
Economic importance
8. Symptoms
Infected seedlings have small, circular or oval, brownlesions,whichmay
girdle the coleoptileand cause distortionof the primary and secondaryleaves
(symptom is called seedlingblight).
Infected seedlings become stuntedor die.
Youngor under developed lesions on older leaves are small and circular, dark
brownor purplish brown.
Afullydeveloped lesionon older leaves is oval, brownwithgray or whitish
center withreddish brown margin.
When infectionis severe, the lesions may coalesce, killinglarge areas of
affected leaves.
Infected grains withblack discolorationor withbrownlesions
9. Youngor under
developed lesions
on older leaves are
small and circular,
dark brownor
purplish brown
Afully developed
lesion on older
leaves is oval,
brownwithgray
or whitishcenter
withreddish
brownmargin
Infected grains with
black discolorationor
withbrownlesions
Conidia of B. oryzae
16. Factors favoring disease development
presence of rice crops grown on problem soil
deficient in potassium
temperature from 25-28° C
susceptibility of the variety to the fungus
late growth stages of the rice crop