RA 00110.pdf - OAR@ICRISAT

Seed and Soilborne Inoculum
The seed or soilborne inoculum acts as a primary
inoculum source to cause the first expression of the
disease in a host population by infecting the seedlings.
Severely infected young seedlings mostly die within
30 d without producing oospores. Michelmore et al.
(1982) reported heterothallism in S. graminicola
which indicated that if compatible mating types colonize
the same host's tissue, oospores are produced
even in 12-day-old seedlings. Perhaps compatible
mating types are not colonizing the same leaf during
the early growth period of the host seedlings. But
plants which express disease symptoms about 20 d
after sowing due to soilborne oospores, seedborne
inoculum, or secondary infection by airborne sporangia
(Reddy 1973, Subramanya et al. 1982), produce
millions of sporangia during earlier growth
periods (Figs. 1 and 2). However, towards the end of
the growing season, they give rise to numerous oospores
in the infected leaves, which are perennating
structures. The oospores produced in the leaves get
mixed with soil or seeds to initiate the disease in the
next season. The airborne sporangia can infect the
unfertilized florets during the heading stage of the
crop, and this might lead to the mycelium establish-
Symptoms appear
when s y s t e m i c a l l y
c o l o n i z e d t i s s u e s
grow o u t & u n f o l d
S p o r a n g i o p h o r e s and
s p o r a n g i a produced
o n d i s e a s e d p l a n t s
Zoospores emerge
f r o m s p o r a n g i a t o
i n f e c t t i l l e r s o f p l a n t s
o r s e e d l i n g s i n a d j a c e n t
f i e l d s
S p o r a n g i a r e l e a s e d
f r o m s p o r a n g i o p h o r e s
S y s t e m i c a l l y
d i s e a s e d
p l a n t s
produce
malformed
s t e r i l e
i n f l o r e -
scence
S e e d l i n g s
i n f e c t e d
by zoospores
o r o o s p o r e s i n s o i l
or oospores & m y c e l i u m
i n t h e seed
Oospores r e l e a s e d t o e n v i r o n m e n t d u r i n g
t h r e s h i n g and/or d e c o m p o s i t i o n o f d i s e a s e d
t i s s u e s
Figure 1. Disease cycle of pearl millet downy mildew.
Oospores produced
i n d i s e a s e d t i s s u e s ,
a b u n d a n t l y i f comp
a t i b l e m a t i n g t y p e s
a r e t o g e t h e r
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