RA 00110.pdf - OAR@ICRISAT

oospores, or in the form of internal dormant mycelium.
The standard procedures for detecting seedborne
inoculum of S. graminicola were evolved by
Shetty et al. (1978). Oospore inoculum on the seed
surface was detected by using the washing method
and mycelial infection by the modified embryo
count procedures. Out of 93 seed samples obtained
from different sources, 59 were surface contaminated
with oospores whose viability was checked by
the tetrazolium chloride (TTC) test.
Although mycelial infections were detected in all
parts of the seed tissue, tests conducted by Shetty et
al. (1980a) indicated that only the mycelium present
in the embryo takes part in causing seedling infection.
A direct correlation was observed between the
percentage of infected embryos and percentage of
plants that expressed symptoms (Shetty et al. 1980a).
However, Williams et al. (1980) questioned whether
the surface sterilization technique used actually
killed oospores on the seed surface.
Soilborne Oospores as Inoculum Source
Large numbers of oospores are produced in downy
mildew-infected plants which can easily get incorporated
in the soil during natural shredding of the crop
or during harvesting.
The survival of oospores in soil has been investigated
by several workers, with details provided by
Nene and Singh (1976). Oospore survival in soil is
reported from 8 months to 10 years. In the absence
of a reliable and repeatable technique for germinating
oospores, all tests for oospore longevity are
based on the infectivity test. The TTC test (Shetty et
al. 1978) can be very effectively employed to determine
viability of oospores during storage. However,
Williams et al. (1980) were of the view that the TTC
test is unreliable to determine the viability of graminicola
oospores.
Several workers claim to have germinated oospores
of S. graminicola under laboratory conditions.
Hiura (1930), Evans and Harrar (1930),
Chaudhri (1932), Tasugi (1933), and Suryanarayana
(1956), reported the production of germ tubes from
the germinating oospores, while Pande (1972), Safeeulla
(1976b), and Sundaram and Gurha (1977)
reported the indirect germination of oospores in the
sense that no germ tubes were produced from such
germinating oospores. However, the author has
repeated all existing techniques to germinate oospores,
in addition to several other methods, but with
little success.
Commonly, all oospores from a particular source
do not germinate at the same time. Safeeulla (1976b)
noticed that five successive crops grown on soil containing
oospore inoculum (added at the beginning)
were infected. The importance of host factors in
stimulating oospore germination has been realized
in several downy mildew pathogens (Kaveriappa
1973, Pratt 1978, Shetty and Safeeulla 1980). Specific
host factors for oospores to germinate rule out
the possibility of controlling the disease by crop
rotation. Even avoiding cultivation of this staple
crop for 1 -2 years may be of no use because soilborne
oospores can remain infective for several years.
It is possible to isolate soilborne oospores by flotation
methods, but no correlation can be made
between disease severity and oospore density in the
soil. Pratt (1978), who worked on Peronosclerospora
sorghi (Weston & Uppal) C.G. Shaw came to a
similar conclusion. This low correlation between
oospore densities and disease incidence may be due
to the percentage of oospores in the soil that are
viable and able to germinate, and the secondary
spread of the disease due to airborne sporangia.
Infectivity of Soilborne Oospores
It is generally agreed that oospores have a higher
infection rate after weathering, and that 1-year-old
oospores infect at a higher rate than fresh oospores
or those more than 1 year old. Early workers
believed that oospores have a dormant period.
However, Safeeulla (1976b) reported that newly
formed oospores infected 55% of the plants. Borchhardt
(1927), Chaudhury (1932), Suryanarayana
(1952, 1963), I A R I (1955, pp. 87-99), and Bhandar
and Rao (1967), all of whom tested the viability of
the oospores, incorporated them into the soil and
observed the disease. Siddiqui and Gaur (1978)
observed that oospores coated on seed and incorporated
in the soil infected plants. However, after the
initiation of systemic infection from the oospores
often insufficient care has been taken to prevent
secondary infection. So high incidences of infection
have been found. The author's study demonstrated
that the plants which received both oospore and
sporangial inocula were 68% diseased, but those
which received only oospore inoculum showed 31%
disease incidence and those which received only sporangial
inoculum showed 36% disease incidence
(Subramanya et al. 1982).
The airborne behavior of oospores of 5. graminicola
has not been reported thus far. It is possible,
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