Sorghum Diseases in India

tion into which the resistance is to be incorporated.
When dealing with a disease resistance
source that is good agronomically and the resistance
is easily transferable, pedigree breeding
involving a single direct cross to an existing
adapted elite cultivar is an ideal method. In
many cases, however, the disease resistant
source will have poor agronomic traits. In such
cases, a modified procedure may be required.
One example would be crossing disease resistance
sources "A" with elite line "B". As soon as
progeny "C" is stabilized for the desired disease
resistance and hopefully some improved agronomic
traits, it is immediately crossed to elite line
"D" which possesses several desirable traits
missing in "C". Strong selection pressure should
be applied in early generations of resulting
progeny "E" for both agronomic performance
and disease resistance and for other desirable
plant and grain traits. This provides an opportunity
to select away from low yield traits associated
with the disease-resistant source. The
opportunity to develop a cultivar which combines
desirable traits in unique combinations is
now available.
Population breeding can be a very effective
tool for enhancing disease resistance at the population
level, especially if individual plants of S1
progeny can be effectively screened. However, it
is difficult to simultaneously maintain or concentrate
a large number of desirable traits in a
population. Extracting individual lines for direct
use becomes real problem due to the lack of control
over the many undesirable traits usually
found in a population, as well as the seemingly
dominance of lower yield. Many of the cultivated
sorghums possess weedy traits—good for
survival, but not for high yield. Population
breeding can be a very effective tool to develop
diverse germplasm with high levels of resistance
to certain diseases. Likewise, it is a useful approach
for maintaining a large number of diverse
sources of resistance in a segregating
situation, from which individual disease-resistant
selections may be extracted. In pearl millet,
population improvement is very useful since
most present varieties are actually random mating
populations.
A number of sorghum groups such as Guineaense,
Dochna-Subglabresent derivatives, and
Nervosum (Kaoliang) tend to be low yielding
with the low yield behaving much as a domi-
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nant trait. Breeding with such lines requires
using other breeding techniques such as additional
backcrossing and intergression.
One example of an indirect method of selecting
for disease resistance involves charcoal rot
and head blight. Resistance or tolerance to postflowering
drought stress correlates very well
with resistance to charcoal rot, since it is a disease
that requires predisposition by drought
stress during grain development. In the Texas
A&M drought program, we select genotypes
which do not prematurely senesce and thus tolerate
this late-season stress, possessing what we
call the "stay-green" trait. At the same time we
exert heavy selection pressure for stalk and peduncle
(weak neck) lodging resistance. Resulting
lines developed in this way show a very high
degree of resistance to charcoal rot and head
blight. Discussion of this may be found in Rosenow
(1977,1984) and Rosenow et al. (1983).
When breeding parental lines for use in hybrids,
special techniques are required, but the
same principles apply. If disease resistance or
other traits are dominant, it is somewhat easier
to combine them by using hybrids. However,
recessive traits are very difficult when dealing
with hybrids as both parental lines must possess
the trait.
Efficient and effective screening of a large
number of genotypes is critical in a successful
disease-resistance breeding program. Simultaneous
selection for disease and agronomic
traits is the ideal. I strongly encourage field
screening whenever possible, using natural infection
if possible, but inoculation if needed.
Large field screening nurseries, combining disease
screening and breeding at one or more location
where several important diseases naturally
occur would be an ideal facility.
In the humid south Texas, we sow large sorghum
nurseries where many internationally
important diseases are endemic. We observe excellent
development of grain mold, downy mildew,
head smut, and often head blight,
anthracnose, charcoal rot, zonate leaf spot, gray
leaf spot and MDMV in nearly every growing
season. In other areas we sow field screening
nurseries for charcoal rot and MDMV. The best
breeding lines are then identified from nurseries
and tested extensively in Texas and other areas
in the All Disease and Insect Nursery (ADIN)
and in various head smut, downy mildew, anthracnose,
charcoal rot, and lodging tests. The