Sorghum Diseases in India

molds. In the early 1980s, sources with high and
stable levels of mold resistance were identified
and are being used in developing white-grained
breeding lines with good levels of mold resistance.
Results and progress made in breeding
for mold-resistant, white-grained sorghum genotypes
using these high and stable sources of
resistance are presented here.
Sources of Resistance
The sorghum germplasm collection was screened
for resistance to grain molds at ICRISAT Center
in mid-1970s and low levels of resistance were
identified. Not one of the germplasm collection
lines was immune to grain mold, and none
showed a consistently high degree of resistance
at all locations. However, in some lines mold
development was consistently low at many locations
and across seasons (Rao and Williams
1980). Search for higher levels of mold resistance
in the sorghum germplasm collection continued
and, by 1985, 7132 germplasm lines that flowered
and matured during the rainy season had
been screened; of these 156 colored-grain lines
exhibited high levels of mold resistance (Bandyopadhyay
et al. 1988). Threshed grain-mold
rating (TGMR) of the selected colored-grain
germplasm lines was 3 or lower on a 1 to 5 scale,
(1 = 0%; 5 = mold on more than 50% of the grain
surface). Most significant in these observations
was that these lines maintained resistance for
2 to 3 weeks after physiological maturity. Pericarps
of the highly resistant lines were colored; a
testa layer was present in all but 14 lines. The 14
lines without the testa layer were low in tannin
content, and diverse in geographical origin and
taxonomic race. No white-grained germplasm
line, however, has showed such a high level of
grain-mold resistance.
Mechanisms of Resistance
Certain plant and grain characteristics are reported
to be associated with grain-mold resistance
(Glueck and Rooney 1980). Grain hardness
and density and the structure of the outer layers
of the seeds and chemical composition are reported
to be partially responsible for improved
weathering resistance. Brown seeds with high
tannin contents and the presence of a pigmented
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testa layer have been associated with GM resistance
(Harris and Burns 1970, 1973). However,
some lines highly resistant to molds have been
found to be low in tannin. These have been
found to contain significant amounts of flavan-4-ol;
the susceptible cultivars did not
(Jambunathan et al. 1986). Hahn and Rooney
(1986) reported that GM-resistant sorghum cvs
contain a greater variety and quantities of phenolic
acids than do susceptible cultivars.
It is apparent that several factors, independently
or in combination, contribute to GM resistance.
The most important are tannin, flavan-
4-ol, and phenolic acids content, and grain hardness.
A sorghum cv combining grain hardness
with all or some of these polyphenols compounds
should possess a high level of mold resistance.
Unfortunately, high-tannin content is
nutritionally undesirable.
Genetics of Resistance
There is little information available on the genetics
of mold resistance, but it is known that
GM resistance is complex and may be the result
of additive effects of many genes affecting several
plant characteristics (Williams and Rao
1981). The high level of grain-mold resistance in
colored-grain sorghums is associated with high
tannin or flavan-4-ol content, or both.
Inheritance of the testa layer is controlled
by the complementary B1 and B2 testa genes
(Glueck and Rooney 1980). Presence of the testa
layer in the presence of a dominant spreader gene
"S" produces grain with high tannin content.
Information on the inheritance of grain-hardness
is likewise scanty. That available indicates
that grain hardness is governed by several additive
genes.
The inheritance of flavan-4-ol is not yet
understood.
Breeding for Resistance
The use of host-plant resistance to control grain
mold is recognized as the logical approach to
this problem. Breeding for grain-mold resistance
therefore receives considerable emphasis in
many national and international sorghumimprovement
programs. At ICRISAT Center,
breeding for grain-mold resistance began in the