Sorghum Diseases in India

Starr: Although not normally classified under
the category of seedling diseases, plant-parasitic
nematodes can affect seedling vigor and stand
establishment.
Odvody: Yes, they can be very important, as the
additional stress factor may make seedlings
more vulnerable to seedling diseases.
Frederiksen: Have you found literature on the
biotic effects on stand establishment in the
warmer tropical soils?
Odvody: No, not on stand establishment, but
there are reports on fungal associations with
roots.
Vidyabhushanam: Is there data to indicate that
one leaf disease has a dominant effect over
other(s)?
Odvody: 1 am not aware of an active mechanism,
other than possibly competition for infection
sites, since multiple diseases can occur on a
single leaf. Currently, I have a graduate student
evaluating the potential of inducing resistance in
susceptible sorghum by initial inoculation with
avirulent or low-virulence isolates of C graminicola.
After allowing time for a host biochemical
response, the same and distal leaves are
challenge-inoculated with a virulent isolate of C.
graminicola to determine if there is increased resistance
to the pathogen.
Axtell: Sorghum is used in the midsouthern
United States in rotation with soybeans, as a
control of soybean cyst nematode. Is it known if
sorghum has a negative effect on soybean cyst
nematode populations, or is it merely the absence
of soybeans in the sorghum year that
helps nematode control?
Starr: I am unaware of specifics with regard to
direct affects of sorghum on Heterodera glycines.
Sorghum is a nonhost for H. glycines, and therefore
nematode populations would be expected
to decline under sorghum cultivation.
Sithole: Has the effect of nematodes on sorghum
yield been quantified? If not, are there
plans to do so?
Starr: There has been little effort to quantify ef­
fects of nematodes on yield. This is a high-priority
item; we only lack people who want to work
on nematode problems in sorghum.
Sithole: Are nematodes really a problem in sorghum
production?
Starr: We lack sufficient data to support general
statement, but the data available suggest that
nematodes do affect yield. We need data on frequency
of fields infested with populations of
pathogenic nematode species.
Sithole: How feasible is the idea of breeding for
resistance where nematodes are the problem?
Or, is breeding for tolerance a possibility here?
Starr: Tolerance (where there is high nematode
reproduction without host damage) is possible
and should be part of management systems if
identified.
Resistance can be difficult to identify, especially
for the less-specific ectoparasitic species.
But based on progress made in identifying resistance
to nematodes in other crop species, and in
sorghum to other pathogens, I believe that development
of cvs resistant to important nematodes
should be attempted.
Page: In Zimbabwe, our survey showed that it is
the farmers with the least land who have the
most serious pest/disease problems, including
nematodes. These farmers live in the most marginal
areas of the country, where the soil is
sandy, exhausted, and lacking in organic matter,
and rainfall is low. For these farmers even the
possibility of controlling nematode pests by rotation
is not open, as they have insufficient land
and are forced to continuously crop with cereals.
The only solution for these farmers is land
reform.
Thomas: Was population density of nematodes
in Zimbabwe estimated, and if so, what kind of
numbers are we talking about?
Starr: In Botswana, sorghum in fields with populations
of Tylenchorhyncus spp of about 600
nematodes in 500 cm 3 soil and Longidorus pisi of
about 100 nematodes in 500 cm 3 soil exhibited
poor growth.
Page: In Zimbabwe, populations of Pratylenchus
357