Sorghum Diseases in India

cumbing to preemergent damping-off die prior
to emergence and those that do emerge may wilt
rapidly and die (postemergent damping-off).
Often development of the permanent adventitious
root system will proceed at different rates
in plants stunted by Pythium attack, and this becomes
apparent in affected fields by wide variations
in plant height and spacing.
Soil moisture and temperature are the most
important factors associated with Pythium attack
of sorghum. Oospores of Pythium spp probably
supply the initial inoculum for infection, but it is
not known if they germinate directly or if they
produce zoospores. Free water would be required
for zoospore production and infection.
Low temperatures (15 °C) in association with
high soil moisture has historically been associated
with Pythium attack of sorghum (Leukel
and Martin 1943) but the optimal temperature
for growth of Pythium spp is normally much
higher (Hendrix and Campbell 1973). The transitory
root system of sorghum seems especially
vulnerable in cool wet soils, because seedling
growth slows and development of the permanent
root system is delayed. However, with field
soil in a controlled environment, Forbes (1984)
demonstrated that high soil moisture was the
key requirement for most damage caused to sorghum
seedlings by Pythium arrhenomanes although
there were some important temperature
x moisture interactions.
Any stress factor that delays establishment of
the permanent root system probably increases
the potential for Pythium attack in wet (and especially)
cool soils. Lines and varieties, because
of their reduced vigor, may be more vulnerable
than hybrids. Seedlings from old seed are more
susceptible than those plants of younger seed,
because the older-seed seedlings show reduced
vigor, especially in cool, wet soils. Other factors,
such as seed damage, may reduce seedling vigor
or allow other organisms to reduce vigor, and
increase vulnerability to Pythium spp.
Controls
Control of all negative effects of some soil environmental
factors against sorghum seedlings is
probably not possible, but cropping and tillage
practices should be utilized, when possible, to
rerduce or avoid deleterious soil environments.
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Seed rots caused by seedborne and soilborne
fungi are easily controlled by fungicide seed
treatments. Fungicide treatment also provides
needed extra protection for low-vigor seedlings
emerging from seed damaged by storage and
grain-weathering fungi and by grain mold. Traditional
contact fungicides (like captan and
thiram) do not provide protection against attack
by Pythium spp, partly because Pythium attacks
at root and mesocotyl sites distal to the fungicide,
but also because these fungicides do not
act against Pythium, New systemic fungicides,
such as the acylanilides (metalaxyl) and phosphonates
(fosetyl-Al), are active against Pythium
in other crops and may provide some control in
sorghum.
Differences in response to temperature and
moisture stress may be found in sor-ghum germplasm,
so cultivars may be chosen or avoided
to minimize the deleterious effects of these and
other soil abiotic factors. Seed of these sorghums
also differ in susceptibility to grain mold and
weathering fungi. Cultivars can be selected to
reduce seed damage associated with these fungi,
or cropping sequences that will allow seed maturation
in environments unfavorable for grain
mold and weathering may be possible. Sources
of resistance to Pythium seedling disease have
been identified (Forbes et al. 1987), but more
rapid and consistent screening techniques are
needed.
Research Needs
1. Major studies of abiotic and biotic factors affecting
sorghum growth, especially as they
function in areas where seedling diseases are
unrecognized and little understood.
2. Better understanding of Pythium and its occurrence
on sorghum in field environments,
including incidence, severity, parameters of
conducive environments, frequency of those
environments, and interactions with host
development.
3. Determine the value of fungicide treatments
to protect seed against fungal attack in areas
where sorghum is grown under conditions
believed to be nonconducive to their development.