Sorghum Diseases in India

and Australia. In India, the disease has been reported
in all areas that grow pearl millet.
The downy mildew pathogen, S. graminicola,
produces large numbers of sporangia at temperatures
between 15 and 25 °C At cardinal temperatures
of 5-7°C, 18°C, and 30-33°C the
sporangia produce oospores. The number of oospores
released from a sporangium varies from 3
to 13 (Suryanarayana 1965; Bhat 1973). Products
of sexual reproduction, the oospores, pose serious
problems in determining their viability
and their ability to germinate. Great variation,
anywhere from 8 months to 10 years, appears in
reports on the survival and dormancy of oospores.
The main reason for such a variation
seems to be the differences in conditions under
which the survival and dormancy tests were
conducted. Many workers have claimed successful
germination of oospores (Nene and
Singh 1976). But no one method appears to be
reproducible. The degradation of oospores in
soil is also highly conjectural. In the absence of a
reliable method to germinate oospores, the survival
and viability of oospores is indirectly inferred
from their infectivity on pearl millet
seedlings in artificially inoculated sick plots.
That the inoculum is associated with the seed
is generally accepted. The debatable point is
whether the inoculum is seedborne internally or
externally. It is generally accepted that the seeds
carry oospores on their surface. The role of sporangia
in inciting the disease has been proven,
and there is sufficient evidence that young seedlings
of pearl millet become systemically affected
with sporangial inoculum in the presence
of adequate humidity (Williams et al. 1981).
Information on physiological specialization
in the fungus is far from complete. The differential
interactions of pearl millet varieties at different
locations are, in the absence of data from
well-defined experiments, discussed in terms of
environmental differences or of physiological
races.
The host range of S. graminicola is considerable,
embracing the tribes Maydae, Andropogonae,
Paniceae, and Agrostidae of Graminaceae.
With this wide host range, it is not surprising
when downy mildew appears on pearl
millet in areas where it has not before been
grown.
Among the several types of inoculation used
in screening varieties for reaction to downy mildew,
the combination of sick plot method con­
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taining oospores in the soil with sporangial inoculation
appears to be more dependable. This
suggests the possibility of two types of resistance
in pearl millet, one type to soilborne oospores,
and the other type to airborne sporangia.
S. graminicola, being a biotroph (strict or physiologically
obligate parasite), refuses axenic culture.
A close scrutiny of the physiology of the
fungus is required to bring this fungus from the
tissue-culture stage, as it is today, to the axenicculture
stage. The achievement could perhaps
unravel the mystery surrounding the germination
of S. graminicola oospores.
Among agronomic management practices for
avoiding downy mildew, early sowing is generally
observed to be better than other methods.
Metalaxyl fungicide treatment holds good
promise to keep downy mildew under check,
providing sustained protection to the millet
crop. However, this fungicide is usually not
available in the free market in developing
countries.
In the absence of a good and easily available
fungicide for seed treatment, the alternative
strategy becomes the use of resistant varieties.
Sources of resistance have been found in several
national and international programs. Genes involving
resistance have to be systematically
identified, especially those providing durable
resistance.
Once identified, these genes must be incorporated
in adequate levels to perceptibly change
the pearl millet/downy mildew profile. It is
doubtful that this can be achieved until we have
strong and adequate supporting data on the
physiology and biochemistry of pearl millet
plants affected by downy mildew.
Ergot
Apart from a 1956 epidemic in India (Bhide and
Hegde 1957; Shinde and Bhide 1958), ergot of
pearl millet incited by C. fusiformis assumed importance
only with large-scale commercial cultivation
of F1 hybrids (Thakur and Williams 1980).
In addition to reducing yields, the adverse effect
of ergot is mainly contamination of grain with
toxic alkaloids. Sclerotia of the fungus contain
the toxic substances.
The infection takes place mainly through the
stigma of the flower, but entry through the
ovary wall prior to fertilization has been ob-