Sorghum Diseases in India

Host resistance
Sowing of resistant cultivars is probably the
most practical, economical, and effective method
to control ergot. However, resistant lines
are not available.
Identification of sources of resistance requires
a long-term systematic approach, and a
major prerequisite is an effective screening technique
to identify resistant lines in a large population.
Screening technique
Artificial inoculation methods must take into
account the importance of flowering biology in
disease development. It is improper to pollinate
spikelets prior to inoculation, because pollination
interferes with infection. Evaluation of pollinated
and then inoculated spikelets would
measure escape, rather than resistance. Therefore,
it is important to remove all spikelets flowering
prior to inoculation. This was not taken
into account in the inoculation methods followed
earlier (Singh 1964; Sundaram 1971;
Khadke et al. 1978). At ICRISAT Center, we have
devised a screening methodology after carefully
considering these factors. It was effectively used
for resistance screening by the national programs
of Ethiopia and Rwanda, in cooperation
with ICRISAT. In brief, the technique is as
follows:
All spikelets flowering before inoculation
were cut and removed, retaining all spikelets in
priinary branches borne on two or three nodes
below the cut portion. Only spikelets in these
primary branches were inoculated and evaluated.
Trimmed panicles were inoculated with a
suspension of fresh honeydew (10 6 spores ml/ 1 )
twice at 2-day intervals and bagged for 15 to 20
days. Overhead sprinkler irrigation was used
for 1 h each morning for about 20 days after
flowering.
For evaluation, 100 spikelets were observed
in the inoculated primary branches. The number
of spikelets with fungal stromata were counted,
and ergot severity was recorded as the percentage
of ergot-infected florets. Qualitative visual
rating (Sundaram 1980), although simpler, is less
appropriate than the quantitative ergot-severity
measurement employed at ICRISAT Center; this
is because all of the infected spikelets may not
be visible (sorghum grains, unlike those of pearl
millet, are not on one plane in the panicle), and
honeydew from an infected spikelet may smear
a neighboring healthy grain, causing it to appear
to be infected.
Sources of resistance
Reliable information on sources of resistance is
meager. Several thousand gennplasm accessions
have been screened under artificial inoculation
to identify resistant sources (Singh 1964; Chinnadurai
et al. 1970b; Sundaram 1970,1980; and
Khadke et al. 1978). However, these reports were
based on results of 1 year's nonreplicated trials,
and lines thought resistant proved to be otherwise
in subsequent trials.
A major resistance screening activity, in collaboration
with ICRISAT, is presently underway
in the east African countries of Ethiopia and
Rwanda. It is being done without, overhead
sprinkler irrigation at 'hot spots', using the
screening technique described above. In 1987,
the sorghum collection in Rwanda was screened
at the Rubona and Karama research stations of
Rwanda. Not one accession showed a high level
of resistance. In Ethiopia, 248 genotypes with
intermediate and highland adaptation were
tested at Arsi Negele Research Station. We selected
27 accessions with less than 5% infected
florets, and among these ETS 2454, ETS 3135,
ETS 3147, ETS 2148, ETS 3286, ETS 3753, ETS
4762, and ETS 4485 were least infected (1-2%
infected florets). These selections will be
screened again in Ethiopia and Rwanda to see if
resistance can be confirmed.
The lack of confirmed sources of resistance to
sorghum ergot underscores the urgency to identify
ergot-resistant sources. The greatest need is
in male-sterile lines, although chances of identifying
or breeding ergot-resistant male steriles
appear slim in view of the role of pollen in the
disease. Control of ergot must integrate all
methods known to reduce the disease, but more
research is needed in this direction.
Research Needs
This review shows that research on sorghum ergot
has been largely superficial, and that wide
gaps exist in knowledge of the biology of the
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