Sorghum Diseases in India

sibly through doubled haploids. An in vitro
technique for maintenance of sporangia over
time is desirable. Tissue culture may be a possibility
for maintenance of sporangial cultures.
For studies on genetics of pathogen virulence, a
technique to induce oospore germination is also
needed.
At present, there seems to be little need for
pathologists to identify more sources of downy
mildew resistance in accessions of germplasm.
Sufficient resistance is probably available, in
most breeding programs, in agronomically superior
lines. Research efforts could better be directed
to reliable identification of resistances in
such material. In India, where downy mildew is
an especially critical problem in hybrids, a joint
effort by breeders and pathologists becomes essential
for rapid and effective progress. In view
of the recent experiences of resistance breakdown
in hybrids in India (Singh et al. 1987a), it
seems advisable that a strategy be developed
whereby the same hybrid is not grown in an
area for more than 4 successive years. This
might be achieved through production, distribution,
and sale of seed of specific hybrids. There is
an urgent need in India to increase the genetic
diversity of parent lines, especially female parents,
so that hybrids grown by farmers may be
genetically more diverse. A concerted effort to
increase the number of good hybrid parents is
under way at ICRISAT, and the possibility of
using topcross hybrids to reduce vulnerability to
downy mildew is being investigated.
Breeding programs in Africa can certainly
learn a lesson from the experience of resistance
breakdown to downy mildew in India, even
though the problem has not yet presented itself,
possibly because of the current minimal use of
improved cultivars, and of almost no hybrids,
by farmers in Africa.
Rust
Rust is generally considered to be a moderately
important disease in most pearl millet production
areas; although in Sahelian and sub-Sahelian
Africa it is usually of little consequence. It
reportedly attacks pearl millet at all stages of
plant growth (Ramakrishnan 1963), but severe
infection normally does not occur in the field
until late in plant development. This has little
98
effect on grain yield, though forage and fodder
value may be reduced considerably.
Four rust pathogens have been reported on
pearl millet: Puccinia penniseti, P. substriata var
indica, P. substriata var penicillariae, and P. stenotaphri
(Ramachar and Cummins 1965; Sathe
1969). The complete life cycle of pearl millet rust
is well known (Kulkarni 1958; Ramakrishnan
1963; Ramakrishnan and Soumin 1948). The
uredinial and telial stages occur on pearl millet
and the spermagonial and aecial stages occur on
the alternate host, Solatium melongena. Reports
on period of viability of urediniospores vary, but
generally they can be maintained for several
weeks at moderately low temperature (10°C)
and low humidity. Incubation period for symptom
expression is 6 to 8 days.
Control of rust by use of fungicides (Kapooria
1972; Bhadur et al. 1975; Parambaramani
et al. 1971) and cultural methods such as sowing
date and fertilizer application (Kandasamy et al.
1971; Sivaprakasham and Pillayarsamy 1975)
have been attained with varying degrees of success,
and disease inhibition by biological control
agents occuring in nature. However, none of
these methods currently hold much promise as a
practical control measure for farmers.
A number of sources of rust resistance have
been identified, although the extent of their use
in breeding seems limited. Resistance in some
cases is likely based on slow rusting traits (Singh
and Sokhi 1983). However, two cases of resistance
based on a hypersensitive reaction controlled
by single, and likely different, dominant
genes have been reported (Hanna et al. 1985;
Andrews et al. 1985b). The resistance identified
in Georgia, USA, is effective in India, but the
reverse does not seem to be true (Hanna and
King, unpublished). It seems unlikely, however,
that extensive use of rust resistance based on a
single dominant gene would be long-lasting, especially
since the alternate host, egg plant, is
commonly grown in most millet-production
areas.
Although rust may be a relatively unimportant
disease in most locations and years, there
are many areas of research that merit attention.
An understanding of the taxonomic identities
and relationships among the pathogens reported
to cause rust on pearl millet is needed.
Research is also needed to identify and quantify
environmental factors that promote epidemics,
and to better establish the relationship between