Sorghum Diseases in India

ghum, the other cereal important in the semiarid
tropics, is essentially nontoxic (Mantle
1968b, and unpublished).
Disease Escape
Research during the 1970s, by ICRISAT scientists,
to develop ergot resistant cultivars of pearl
millet has been successful and the topic has been
reviewed recently by Thakur and Chahal (1987).
Concurrently it had been recognized that improved
pollen management, facilitating early
and efficient capture by stigmas as they are exerted,
minimized disease expression. Complementary
studies, supported by UK's Overseas Development
Administration, sought to explain
the mechanism operating in the ergot-resistant
cultivars and revealed for the first time a fundamental
botanical event which had until then,
surprisingly escaped notice. Within about 5
hours of compatible pollen reaching the surface
of stigmatic lobes on exerted stigmas, a group of
cells in a region of the fused stylodia begin to
lose turgor and progressively collapsed (Willingale
and Mantle 1985). The collapsed region is
unsuitable for the passage of further pollen
tubes and soon leads to desiccation and abscission
of the distal stigmatic tissue. Although the
postpoUination constriction of fused stylodia is
possibly the most dramatic instance of a localized
response to potential fertilization, some
analogous events were being observed independently
and concurrently by Heslop-Harrison et
al. (1985) in maize and some grasses. Clearly,
such a quick response, either to the passage of
pollen tubes through the potential abscission
zone or by some hormonal response to invasion
of the upper ovary wall, will exclude ovary
pathogens for which the normal route of entry is
via the stigma, mimicking the behavior of pollen
tubes. Stigmatic constriction is an important factor
in escape from ovary disease in pearl millet
and should be exploited. This could best be
achieved by reducing protogyny within the inflorescence
to a minimum and if possible arranging
synchrony of anthesis with exertion of
stigmatic hairs from within the confines of the
floral cavity, as is evident in cultivars ICMPE
13-6-30 and 134-6-9 (Willingale et al. 1986). The
resulting economy of pollen could also allow selection
for minimum size of exerted stigmas to
reduce the effective receptive area for pathogen
inoculum. Stigmatic constriction is initiated also
following the passage of C. fusiformis hyphae
down the stylodial transmission tracks in the absence
of pollination (Willingale and Mantle
1987). Since the pathogen is much slower than a
pollen tube, taking about 16 h to induce the constriction,
it seems likely that it is not just a response
to invasion of the potential constriction
region. Otherwise, there might be a response before
the hyphal tips had passed through the
group of cells. Constriction also occurs simply as
an aging response (notable in the commercial
susceptible hybrid BJ 104) in inflorescences enclosed
in the conventional bag. If the bag is carefully
removed shortly before anthesis, long
white turgid stigmas will be seen as expected
but, since the fused stylodia have already constricted,
the stigmas can be wiped away with the
slightest touch. They are in effect already useless
as an effective route of pollination.
This mechanism for ergot disease escape
does not exclude the possibility that other forms
of resistance may exist, but it would be in itself
sufficient to ensure that ergot disease in pearl
millet becomes insignificant if, by arranging efficient
pollen management, the natural rapid
postpoUination stigmatic constriction phenomenon
in pearl millet is exploited to the full.
References
Heslop-Harrison, Y., Heslop-Harrison, J., and
Reger, B J. 1985. The pollen-stigma interaction in
the grasses. 7. Pollen-tube guidance and the regulation
of tube number in Zea mays L. Acta Botanica
Neerlandica 34:193-211.
Loveless, A.R. 1967. Claviceps fusiformis sp. nov.,
the causal agent of an agalactia of sows. Transactions
of the British Mycological Society 50(1): 15-
18.
Mantle, P.G. 1968a. Inhibition of lactation in
mice following feeding with ergot sclerotia
[Claviceps fusiformis (Loveless)] from the bulrush
millet [Pennisetum typhoides (Staph and Hubbard)]
and an alkaloid component. Proceedings
of the Royal Society, Series B, 170:423-434.
Mantle, RG. 1968b. Studies on Sphacelia sorghi
McRae, an ergot of Sorghum vulgare Pers. Annals
of Applied Biology 62: 443-449.
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