RA 00110.pdf - OAR@ICRISAT

Figure 4. Figure assembly to demonstrate the movement
of zoospores on the leaf surface (S = sporulating
leaf, WB = wet blotter, B - belljar, SS = susceptible
seedling, W = whorl, £ = Excised plant, T =
trough).
zoospores (Hiura 1935, Suryanarayana 1965, Safeeulla
1976b), but occasionally, production of the
germ tube has been noted. Hiura (1935) reported
that the pathogen from Italian millet had a minimum
sporangial germination temperature of 5-7°C,
an optimum of 18°C, and a maximum of 30-35°C.
Melhus et al. (1927) noticed that an isolate of S.
graminicola from Setaria viridis (Linn.) P. Beauv
had an optimum temperature requirement of 17-
18°C for sporangial germination. Suryanarayana
(1965) noticed that sporangia germinate readily in
drops of distilled water, liberating 3-8 zoospores.
The formation and liberation of zoospores from
sporangia took 35-180 min. Only the fully mature
sporangia germinated, but none however, germinated
directly to produce germ tubes. Safeeulla et al.
S
WB
B
SS
W
E
T
(1963) noted that sporangia of S. graminicola from
pearl millet germinated at 18-29°C, with an optimum
of 24-25°C, while Suryanarayana (1965) reported
22-23°C as optimal. Germination tests with
sporangia in both light and dark germinated at 80-
90% (Suryanarayana 1965). Researchers seem to
agree that sporangia require free water for germination.
Production of 3-8 zoospores (Suryanarayana
1965) and 3-11 zoospores (Bhat 1973) from a sporangium
have been noted, however Shetty and
Ahmed (1981) reported 1-3 zoospores produced
from the sporangia of the HB 3 race of S. graminicola.
Zoospores produced by germinating sporangia,
which normally swim for 30-60 min, undergo encystment
leading to retraction of the flagella, and then
germinate by producing germ tubes. Appressorium
production by the germ tubes when they are still in
water suspension has been noted (Safeeulla 1976b).
Suryanarayana (1965) noticed that germinated zoospores
swim freely at 16-22° C. At higher (near 32° C)
and lower (4°C) temperatures, movement stops and
they are presumed dead since no activity resumes
when returned to favorable conditions.
The mode of infection of pearl millet plants by
zoospores has only recently been studied because the
importance of zoospores in causing secondary infection
was not understood. Studies have shown that
roots, root hairs, coleoptiles, and the base of young
and emerging leaves still present within the whorl are
the infection courts for zoospores to infect pearl
millet plants (Subramanya et al. 1983).
Often the germinated zoospores produced an
appressorium from which a tube-like infection peg
developed. The infection peg gave rise to a primary
vesicle which developed into intercellular mycelium
(Subramanya et al. 1983). Bhat (1973) noticed the
unidirectional movement of the zoospore germ
tubes toward the roots, indicating the prevalence of
a chemotactic stimulus. In studies on the biology of
systemic infection, the aggregation of infecting zoospores
at a single infection site has been noted (Subramanya
et al. 1983). Zoospores have been observed
entering leaves through stomata. Very often more
than two zoospores entered through one stomata,
and soon after entering the substomatal chamber
they produced both primary and secondary vesicles.
From the secondary vesicles thread-like infection
hyphae originated and developed as intercellular
hyphae.
Not much work has been done to understand the
factors influencing the infection process. It is quite
likely that temperature, humidity, dew period, tissue
susceptibility, and inoculum load influence the infec-
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