Sorghum Diseases in India

the selection criterion that you have chosen is a
yes/no type test: the pathogen either does or
does not sporulate on day 6 after infection. The
single specific time you choose to evaluate the
latent period is but one (arbitrary) point observation
of a process (the latent period that varies
in time). Thus, if you do want to "tease out"
variation, as you put it earlier, it can readily be
achieved by making more observations on other
days.
It is axiomatic that the latent period varies its
quantitative expression in time. Thus, without
changing the target of your selection process —
evaluation of the latent period — you could obtain
the variability that you seek.
Please note that I am not necessarily recommending
a selection process based on the latent
period; rather, I am commenting about arbitrary,
categorical yes/no tests using your current commitment
to evaluating the latent period as the
starting point.
Balasubramanian: Do you find any varieties in
populations of M. phaseolina in sorghum with
reference to the history of previous crops, such
as maize, sorghum or castor?
Pande: We have not studied the variability in M.
phaseolina isolates.
Laing: In your list of research objectives, you
mentioned an examination for pathogenic specialization.
Macwphomina attacks more than 400
species, so it is safe to assume that there is specialization
and it is therefore only of academic
interest to pursue this issue. It also suggests that
it will probably be unproductive to pursue vertical
resistance against this disease.
Pande: Recent reports suggest that M. phaseolina
has pathogenic variation. Soybean isolates have
preference to soybeans, but not for other hosts—
such as sorghum and maize. In the light of this
information, it is important to study the variability
of the organism (M. phaseolina).
Odvody: Recently, Pearson in USA was able to
demonstrate that a Macwphomina phaseolina population
in soil may contain strains differing in
their sensitivity to chlorate. Growth of M.
phaseolina isolates from soybeans was inhibited
in a basal medium containing chlorate, but isolates
from maize were not. In some soils with a
362
predominant population of chlorate-insensitive
isolates the soybeans were preferentially infected
with chlorate-sensitive isolates. I provided
Pearson with sorghum and maize isolates of M.
phaseolina from Texas which he determined to be
chlorate-insensitive. These results do not indicate
that cross infectivity is impossible but the
host preferences indicate some virulence difference
on these hosts.
Pande: Thanks for the comment and information.
We would like to follow the methodology
particularly with respect to the sensitivity of M.
phaseolina isolates to chlorate in cross-inoculation
tests with isolates of M. phaseolina from
other hosts.
Teyssandier: Do you believe that drought allows
Macwphomina sp to colonize host tissue because
it has a competitive advantage over other
root/crown inhabiting fungi?
Pande: Fusarium spp are always associated
with sorghum root, crown, and nodal tissue
throughout the life-span of sorghum plant;
their frequency increases after drought stress,
but the end product is the symptom produced
by M. phaseolina. I believe that Fusarium spp
may predispose sorghums to attack by M.
phaseolina.
Butler: The relationship between root exudates
and the organisms which produce stalk rot was
not clear. The reason I ask is that under laboratory
conditions we find that the production of
root exudate acting as a germination stimulant
for Striga is greatly increased by drought stress.
There could be a relationship between the effect
of drought stress on exudate production and the
incidence of this disease.
Pande: I was referring to the nearly nonexistent
information on the mechanism of infection by
M. phaseolina in sorghum. However, there are
reports that root exudate triggers the germination
of sclerotia of M. phaseolina and, thereby,
these germinated sclerotia colonize roots of
sorghum.
Balasubramanian: Is the production of sclerotia
in sorghum ergot controlled by geontype or by
environment or by genotype-environmental
interaction?