Sorghum Diseases in India

Craig: On the basis of our experience in Texas, it
seems futile to expect satisfactory control with
the monogenic resistance factors we have used.
We should be examining the possibility of combinations
of "minor" genes, or better still, escape
mechanisms (for example, faster maturation
of seedling tissues or possible morphological
characters that prevent or reduce the probability
of invasion by the pathogen).
Putter: You point out that both horizontal resis-
tance (HR) and partial vertical resistance (VR) exert
an effect upon the rate of disease progress. How
can one select for HR and avoid particular quantitative
VR when "dilatory resistance" could be
either?
Frederiksen: We are attempting to evaluate the
effects of various sources of host resistance on
changes on pathogen populations. Those host
genotypes that do not show directional selection
pressure are preferred.
de Milliano: In our selection for resistance to
downy mildew, conidia resulted in more disease
than oospores. Do you think new pathotypes are
more likely to develop from conidia or from oospores?
Should we infect mainly with conidia, or
should we also keep on adding oospores?
Craig: In our experience in Texas, resistance to
conidia provides resistance to oospores; I do not
believe shift in pathogen virulence is necessarily
related to sexual recombinations in the oospore
stage, but rather represents shifts in frequencies
of virulent genotypes in a heterogenous population
in response to the host genotype present. If
so, conidial inoculum should give the same
range of virulence as oospores.
King: I just want to include the concept of recovery
resistance to downy mildew in pearl millet
in this discussion. It is a phenomenon that may
be useful for desirable resistance, but we don't
know much about it We are investigating durable
resistance and should know more about its
potential value in another year.
Laing: Downy mildew and anthracnose of sorghum
show great genetic plasticity. How do you
suggest we should tackle these diseases?
Frederiksen: With antiiracnose we are looking at
368
levels of dilatory resistance and studying the effect
of these resistance traits on changes in the
pathogen population. Sources of dilatory resistance
with minimal effects on the pathogen population
will be preferred.
Kuwite: Breeding for horizonal resistance or tolerance
is an excellent approach when breeding
at the national level, but with international exchange
of genetic material it could introduce
some pathogens that are seedborne. Exclusioncontrol
techniques, like meristem-tip culture, are
inadequate for handling thousands of lines sent
to national programs from international institutions.
There is therefore a chance of introducing
new pathogens in areas where they do not
exist.
Frederiksen: Fortunately, there are few seedborne
pathogens of sorghum and these, naturally,
are widely distributed. Seed exchanges
should not present unusual hazards to plant
protection under normal circumstances.
Claflin: Seemingly, information concerning anatomical
differences and/or gene products when
breeding for resistance is limited. Perhaps other
programs, such as Dr Rosenow's at Texas where
he has developed genotypes with anatomical
traits that greatly reduce logging, would be beneficial.
Consulting biochemists and plant anatomists
for input into the breeding effort might
pay considerable dividends.
Balasubramanian: Physical barriers with sclerified
cells in Striga-resistant sorghums have
been demonstrated since 1977. Why can't we exploit
this character, because most of the physical
resistance traits are easily introduced?
Frederiksen: If the resistance works and can be
recognized among segregating individuals in a
population, it will be used.
Toler: We believe that the resistance to viruses
includes five mechanisms: tolerance, field resistance,
immunity, resistance to infections, and
multiple resistance.
Guiragossian: In our long smut research, we inoculate
plants with abundant spores without
controlling concentration of the inoculum. Do
you think we should conduct an experiment