Sorghum Diseases in India

GM-resistance in the savannas of southern
Guinea, western Africa, possibly by ICRISAT.
Mukuru: I agree that screening for a mold-resistant
photoperiod-sensitive germplasm collection
should be taken up at locations where this
can be done adequately and reliably
Mughogho: The seedborne diseases of sorghum
listed by Noble and Richardson contain references
to publications which have not been reviewed
or meet the criteria for seedborne
diseases. There is need to ensure that only those
reports of scientifically proven associations are
listed as seedborne.
McGee: I agree it will be an enormous task to
validate the literature on seedborne diseases of
sorghum.
Claflin: Bacterial streak and bacterial stripe are
reported to be seedborne by Noble and Richardson,
yet their source of information is based
on conjecture. Therefore, how and why are they
included on the list?
McGee: The publication indicates that an association
between a microorganism and seed has
been reported.
Duncan: What is the potential of viruses being
introduced by sorghum seed? This problem recently
emerged when peanut mottle virus was
introduced on groundnuts from China, even
though proper health testing and quarantine
regulations were observed.
McGee: There is always a hazard, for any crop,
of an exotic virus on other microorganism being
introduced by seed. Even with vigorous genetics
directed against well-defined pathogens, there
may be loopholes.
Claflin: Can scientists be intimidated when conducting
research on seedborne diseases if the
potential results would be harmful to the
industry?
McGee: This is possible.
lating: In your talk on screening techniques,
you suggest that selection for resistance should
use Robinson's single-pathotype-susceptible ge-
notypes for starting. The conventional procedure
is to use resistant material to start with
and expose it to as many pathotypes as are available.
Please explain the limitations of the conventional
approach and the Robinson approach
a little.
Putter: Robinson is the original author of the
one-pathotype technique. An explanation of the
reasons why it serves its purpose is provided by
Vanderplank. The technique was also later endorsed
by Parlevliet in a letter to the editor of
Phytopathology, 1983, 73:379.
Craig: How does one handle this avoidance of
numerical values in your scoring when dealing
with diseases in which severity and incidence
are the same, i.e., systemic phase of sorghum
downy mildew?
Putter: Incidence/severity transformations are
accommodated by the multiple-infection transformation
developed by Gregory. Incidence percentages
can be used directly, but they assume
that all plants have the same severity of disease
and that they were equally (i.e., stochastically
equal) exposed to infection. Systemic diseases
that involve vectors create problems due to the
need to differentiate between pathogen and vector
resistance, respectively.
Downy mildew does not involve a vector, you
are dealing almost exclusively with systemic infections
developing from soilborne oospores. If,
as you suggest, from your experience, severity
and incidence are the same, then this is equivalent
to saying that all plants with systemic symptoms
were infected by the same number of
oospores or that severity is equal and representative
of the number of separate oospore infection
events. Obviously there is room for clarifying
these points.
Would it not be logical to assume that one
could prepare soil with known—or at least standard—concentrations
of oospores. Batches of
seedlings to be screened can then be exposed to
this ''sick soil" and the incidence of successful
infections can be measured directly Adequate
replication and the inclusion of a reference variety
should (in the normal statistical way) provide
a means of interpreting the results in spite
of variability and other factors that contribute to
experimental error. The incidence counts will be
the data for analysis of variance.
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