Sorghum Diseases in India

pollination (Patil and Goud 1980), Clearly histological
studies upon simultaneous inoculation
and pollination, to trace the progress of pollen
tube and infection hyphae to determine if fertilized
ovaries can be colonized/would be valuable.
The susceptible period of spikelets, or what
has been termed as the 'window of infection' in
pearl millet ergot (Willingdale et al. 1986), begins
when stigma become receptive and ends (most
likely) when the ovary is fertilized. Any factor
that prolongs the susceptible period predisposes
sorghum to ergot infection. Conversely, factors
that ensure rapid fertilization would serve to reduce
ergot attack. For example, protogynous
spikelets of male-fertile plants would be more
susceptible than spikelets in which anthers and
stigma emerge together, because stigma would
remain unpollinated but receptive to conidia
longer.
Male sterility promotes ergot infection because
viable pollen is not produced (Futrell and
Webster 1965). The utility of readily available
fertile pollen in reducing ergot severity has been
shown in pearl millet ergot (Thakur and Williams
1980; Thakur et al. 1983). Conidia germination
is reduced on sorghum stigma pollinated by
viable pollen (Chinnadurai et al. 1970a).
Environmental factors
Favorable environmental conditions for infection
and disease development should coincide
with anthesis. Futrell and Webster (1966) reported
that near 100% relative humidity for 24 h
during anthesis of a male-sterile line was optimal
for infection. Molefe (1975) reported that inoculated
panicles held for 20 hours at high
humidity and at 20°C and 25°C, but not at 40°C,
showed honeydew within 7 days. From field experiments,
Anahosur and Patil (1982) concluded
that minimum temperatures ranging from .19° to
21 °C and relative humidity from 67 to 84% during
the 10-day period following panicle emergence
from boot leaf is sufficient to favor serious
ergot infection in a male-sterile. Temperature
and relative humidity are also reported to affect
the incubation period (Sangitrao and Bade
1979a).
In Botswana, Molefe (1975) observed 25-90%
ergot incidence, presumably on male-fertile
lines under natural conditions, only in a year
238
characterized by high rainfall (750 mm for the
epidemic growing season). Infected panicles
flowered when relative humidity was 90%, minimum
temperature 14°C, and maximum temperature
27°C Cloudiness during anthesis aids
disease development (Anahosur and Patil 1982).
Sangitrao and Bade (1979a) felt that in conditions
of high humidity, rainfall is not essential
for ergot development. Their conclusions are
from experiments in which abundant inoculum
was available in other plots sown nearby.
Much of the present knowledge on how environmental
factors affect ergot development is
mostly hypothetical and lacking experimental
evidence. Environmental conditions such as
cloudy weather and wetness (rainfall) affect anther
dehiscence, pollen viability, pollen deposition,
and pollen activity (Quinby 1958). Ovule
abortion and pollen sterility can occur due to
abnormal mega-sporogenesis and microsporogenesis
at 5-21°C (Brooking 1979; Dhopte 1984).
These conditions are likely to enhance ergot severity
because pollination and fertilization are
delayed or reduced.
Environmental factors, mainly through their
effects on fungal saprophytes, affect sclerotial
development. Sclerotia develop poorly in the
rainy season due to the growth of Cerebella (Sangitrao
and Bade 1979b), but wet conditions congenial
for infection followed by sclerotial development
in dry weather allow formation of less
contaminated mature sclerotia (Futrell and
Webster 1966).
Research to explain the relationship of sorghum
flowering biology to ergot development is
needed. Precise information on epidemiology is
required, not only for male-sterile lines on which
ergot is usually more severe, but on male-fertile
hybrids and varieties as well. Future epidemiological
studies should consider altered floral
behavior of the host and its effect on pathogen
development resulting from changes in environment.
Disease cycle
Primary infection in the field is probably established
by conidia from collateral hosts and infected
panicle debris in soil (Futrell and Webster
1966), Ascospores may also serve as primary inoculum
(Singh, 1964), but proof is lacking. Conidia
in dried honeydew in infected panicles that