Sorghum Diseases in India

semination, zoospore release, and infection, especially
during early growth of test materials
(Williams et al. 1981; Singh et al. 1987a). Sporadic
rains accompanied by high soil temperatures are
often characteristic of early-season conditions in
pearl millet cultivation areas of the semi-arid
tropics; supplemental irrigation at this time may
help alleviate the problems of high temperature
and low moisture, thus increasing the reliability
of screening. In extreme cases of drought and
high temperature, irrigation may fail to assure
conditions necessary for effective field screening.
Many control measures for downy mildew
have been suggested. These include removing
diseased plant debris at the end of the season
and roughing diseased plants as they appear
during the season (Kenneth 1977; Thakur 1980).
Reports are available on the influence of soil nutrients
on downy mildew (Deshmukh et al. 1978;
Singh and Agarwal 1979), but the soil nutrient/
DM relationship is not well defined. The systemic
fungicide metalaxyl has been shown to be
effective as a seed treatment against early infection;
as a foliar spray it causes symptom remission
(Williams and Singh 1981; Dang et al. 1983;
Singh et al. 1984). However, the most practical
control measure is host resistance.
Numerous sources of resistance have been
identified among accessions of germplasm and
in breeding lines in India and in some countries
of western Africa (Singh et al. 1987a). However,
there seems to have been relatively little direct
use of accessions selected for their downy mildew
resistance. ICRISAT screening indicates
that resistance is most frequently found in accessions
from central to east-central western Africa
(Andrews et al. 1985a; Williams 1984b). Multilocational
testing in India and in countries in
western Africa have indicated stable resistance
in a number of lines (ICRISAT 1986). It is interesting
to note, however, that in multilocational
testing in western Africa, local landraces growing
at their center of origin often appear to be
more susceptible than landraces introduced
from elsewhere in the region (J. Werder, personal
communication). The variability for downy mildew
reaction among plants within lines has allowed
selection for resistance in susceptible
lines and the conversion of susceptible lines into
resistant lines (Singh 1983; Singh et al. 1988). The
phenomenon of recovery resistance, whereby an
infected plant outgrows downy mildew, has recently
been reported (Singh and King 1988); the
phenomenon appears to be common and widespread
in accessions of germplasm and breeding
lines. A number of studies have been made on
inheritance of resistance to downy mildew (Appadurai
et al. 1975; Gill et al. 1978; Singh et al.
1978; Basavaraj et al. 1980; Shinde et al. 1984).
The picture is not entirely clear, but reports generally
suggest that resistance is polygenic and
involves one or more dominant genes.
Because downy mildew is of major importance
in western Africa and the Indian subcontinent—the
world's two main areas of pearl millet
cultivation—it is essential that research be conducted
to fill gaps in our knowledge. One research
area involves the oospore. It plays an
important role as a primary inoculum, but its
biology is little understood. There is a need to
know the manner in which oospores germinate
and infect plants, and the factors (environment,
host) that influence germination and infection.
Development of reliable tests for oospore viability
and ways to influence viability, longevity,
and germination could lead to more effective
screening and control of this disease.
There should be a constant effort to improve
the efficiency and reliability of downy mildew
screening techniques both in the field and in the
greenhouse or laboratory, with a view to also
increase flexibility to better meet the needs of
breeding programs. In this regard, we have initiated
mass screening of breeding materials at the
seedling stage at ICRISAT Center.
It is likely that S. graminicola must invade
meristematic tissue for expression of downy
mildew symptoms (Williams 1984a), but relatively
little is known relating infection with
symptom expression or the reasons for the great
variability in symptoms. The phenomenon of recovery
resistance is, in the broad sense, related
to this matter of symptom expression. Studies
that will elucidate what happens to S. graminicola
at the tissue level during recovery of resistance,
as well as the possible influence of
environment, especially temperature, on symptom
expression, are needed.
Knowledge of the genetics of resistance in
pearl millet and the genetics of virulence in
S. graminicola is incomplete. A more complete
understanding could lead to the development of
a successful strategy of resistance breeding and
gene deployment to control the disease. For
such studies, it is necessary to first increase the
level of inbreeding in so-called inbred lines, pos-
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