Sorghum Diseases in India

ment. Do you exclude the possibility that reduced
moisture may have terminated the development
of this disease? What would've
happened if the same plants were grown in a
greenhouse under controlled conditions?
King: The recovery phenomenon is not dependent
on low moisture for expression. It occurs in
humid greenhouses and humid field conditions.
Vidyabhushanam: What is the role of alternative
hosts in initiating an ergot infection?
King: We do not know the importance of alternative
hosts in the epidemiology of pearl millet
ergot Ergot can develop to a high degree in the
absence of infected alternative hosts. Presumably
the inoculum is ascospores from germinating
sclerotia in the soil.
Laing: In your data for several differential interactions
between cultivars and sites of testing; do
you see this as a vertical resistance (VR) result,
arising from a cultivar x pathotype interaction;
or a cultivar x environment interaction?
de Milliano: I only suggest that there may be
VR; however, a cultivar x environment interaction
may also be involved.
Laing: Are you actively seeking to expose vertical
resistance?
de Milliano: Yes, we would like to screen at one
location for the entire region. With VR, this is not
possible when different pathotypes are present
in the region.
Mbaye: Are there morphological differences in
stigmas of ergot-resistant and ergot-susceptible
cultivars? Are there differences in speed of germination
of conidia and pollen grain?
Mantle: There may well be variations in the size
of the exerted stigmas, and small stigmas are
likely to favor escape from ergot The constriction
response to pollination seems to be a constriction
feature in pearl millet Ergot conidia
germinate much more slowly than do pollen
grains. If both are applied at the same time, pollen
grains achieve fertilization before the ergot
conidia germinate
Vidyabhushanam: Would you explain the effects
of the resistance mechanism through stylar
constriction after fertilization or the ability of the
fungal hyphae to cause disease.
Mantle: In pearl millet, invasion of the ovary by
a pollen tube or a pathogen hyphae via the stylodia
is presented by stigmata constriction in response
to initial pollination. At the first sign of
constriction, fertilization is probably complete. If
fungal hyphae were able to enter the ovary at
this stage or probably for a few days thereafter
there is no reason to believe, by analogy with
other ergot fungus/host examples, that disease
could not be fully and effectively established.
This constriction of stigmas in pearl millet is an
exceptionally effective barrier to infection when
initiated by pollination and provides a unique
mechanism. You have probably confused fertilization
as a genetic event, and one which does
not alter the structure of the ovule or ovary as
far as a potential pathogen is concerned. The
differentiation and structural changes which
proceed subsequently, over several days, provide
physical barriers through which a pathogen
invading via the stigmas would be unable to
penetrate.
McGee: Do you have electromicrographs to
show the infection tubes are blocked by the constriction
in the style?
Mantle: Yes, the tissue in the collapsed region
has its special integrity and the transmission
tracts cease to be prominent. The stigmas distal
to the constriction desiccate quickly on account
of a failure to maintain turgor through the collapsed
stylodal region.
Njuguna: From your talk, it is evident that ergot
millet does not constitute a serious health hazard
in humans; is this true also about ergot in rye?
Mantle: No, ergot of rye (Claviceps purpurea) often
contains alkaloids which have potent pharmacological
properties additional to those expressed
by the millet ergot fungal alkaloids.
Some of the G purpurea alkaloids have an established
place in modern medicine and are used
extensively in obstetrics and in the prophylaxis
and treatment of migraine. Excessive dose in the
human is certaily hazardous-
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