Sorghum Diseases in India

served (Sundaram 1969). The inoculum consists
of airborne ascospores produced by germinating
sclerotia lying in the soil and also by conidia
from the same season's crop (Thakur et al. 1984).
Although cross-inoculation studies have been
carried out by Small (1922) and Reddy et al.
(1969), the role of collateral hosts in initiation of
the disease, during the same season, is not clear.
In cross-inoculation studies, Reddy et al. (1969)
observed that the pearl millet ergot pathogen
was unable to colonize sorghum, but ergot sorghum
on the sugary-disease pathogen was able
to infect pearl millet. This can be a potential
threat to pearl millet where ergot of sorghum is
endemic.
It is generally observed that ascosopores of
C fusiformis do not infect fertilized flowers. Artificial
pollination before the application of inoculum
has been found to prevent infection
(Thakur and Williams 1980). This poses an obvious
hurdle in screening varieties for resistance.
Spraying heavy (10 6 conidia mL -1 ) suspensions
of conidia into the previously protected
protogynous inflorescence (Thakur et al.
1982) is a significant advancement in creating
ergot infection artificially. Gill et al. (1979) demonstrated
low levels of susceptibility at preprotogyny,
high levels of susceptibility during
protogny, and high levels of resistance during
anthesis.
An approach to dealing with ergot would be
to exploit host-plant resistance. Unfortunately,
there are few well-documented sources of resistance,
in spite of screening large numbers of
germplasm accessions in the national and international
programs. Development of resistant
lines by intermating moderately resistant lines
has been suggested by Gill et al. (1979). Reaction
of developed lines can be scored using a standard
rating scale (Thakur and Williams 1980).
Lines resistant to ergot have been bred through
pedigree selection from accessions available at
ICRISAT (Thakur and Chahal 1987). This could
be the starting point of the long journey to find
and incorporate resistance to ergot into pearl
millet. Chemistry of ergot-affected earheads can
contribute significantly to the quest for resistance.
Kannaiyan et al. (1973) reported tryptophan
content of spikelets was maximum before
fertilization. They also observed that in vitro,
asparagine and proline supported growth of the
fungus, while tryptophan impeded growth.
Variations in the morphology of the ergot
fungus and its capacity to attack pearl millet cultivars,
have been reported by Chahal et al.
(1985). These workers grouped their isolates,
based on pathogenicity, into highly virulent (Kovilpatti,
Jobner, Ludhiana, and Aurangabad)
and weakly virulent (Coimbatore, ICRISAT, and
Mysore) groups. These results can be confirmed
by the study of protein isozymes by electrophoresis.
Future studies should focus on the epidemiology
of this disease and the mechanisms and
genetics of resistance.
Smut
Smut incited by Tolyposporium penicillariae Bref.
has become a major disease, especially since the
introduction of high-yielding varieties. The infection
is confined to the individual spikelet;
smut sori are produced in place of grains. Infection
is by sporidia produced by soilborne teliospores,
acting through the stigmas (Bhatt
1946). Of the several inoculation methods tried,
injection of artificially cultured sporidia into the
leaf sheath at the boot stage appears to be dependable
(Gill et al. 1979; Thakur et al. 1983).
Earheads can be graded for their resistance or
susceptibility on a standard severity index
(Thakur and Williams 1980). Although application
of the fungicides oxycarboxin, carboxin,
and captafol as sprays helped to reduce smut
significantly (Gill et al. 1979), this type of management
has not made an impact and host-plant
resistance is the sought-after method of managing
smut in pearl millet.
The International Pearl Millet Workshop in
1986 emphasized the exploitation of host-plant
resistance to contain the disease. In the absence
of adequate information the aim of future research
should be the understanding of the biology
of the pathogen and the epidemics of the
disease. Mechanisms of resistance and in-depth
studies of variation in the pathogen population
are the other important items for concentrated
study.
Rust
Once considered a minor disease, rust (P. penniseti)
has in recent years gained considerable
importance in pearl millet production. A little
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