Sorghum Diseases in India

leaf-disease resistance (polygenic) perhaps coupled,
where needed, with pathogen-specific
(monogenic or oligogenic) resistance has been
successful in other crops and might be a valuable
approach with sorghum (Hooker and Perkins
1980). Sorghums with the "stay-green" or
nonsenescent characters (Duncan 1984; Rosenow
1984) may be valuable sources for general
leaf-disease resistance.
Sorghum breeders often treat sorghum as a
self-pollinating crop which is most amenable to
identification of monogenic resistance, while
population breeding—being dependent on both
self- and cross-pollination—is useful in identifying
monogenic and polygenic sources of resistance.
Frederiksen and Rosenow (1986) suggest
that resistance traits should be present at levels
that can be readily evaluated and they must be
heritable and stable across environments- They
state that progress in disease-resistance breeding
can be made using several methods, including
pedigree line breeding, backcross breeding,
and population breeding.
Resistance (monogenic or polygenic) that reduces
the rate of disease development against all
races of a pathogen effectively is especially
needed for pathogens with known races (Frederiksen
and Rosenow 1986). This type of resistance
may also reduce selection pressure for
variants of other foliar pathogens. General or
durable resistance conferred by many genes can
have several additive and synergistic effects in
host response to pathogens but their collective
response under disease-conducive environments
is the important factor and more easily
measured.
The world collections of landrace and other
sorghums represent a vast and diverse resource
containing all types of resistances to many foliar
and other pathogens. Scientists have already
identified a large number of elite disease-resistant
materials and they are being used and evaluated
in sorghum programs around the world.
Yet, this represents only a small proportion of
what is available.
There are a multitude of germplasm-screening
techniques that range from fully natural disease
development in the field to strictly
controlled pathogen and environment conditions
in the greenhouse and laboratory (Sharma
1980; Williams et al. 1980; Frederiksen and
Franklin 1980). Mughogho (1982) stated that no
screening techniques were available for sooty
stripe and gray leaf spot. This was due largely to
the lack of laboratory techniques to mass produce
conidia for greenhouse or field inoculation.
Odvody (1981) developed a sporulation technique
for Cercospora sorghi that was successfully
utilized in germplasm screening by Wall (1983).
Ramulispora sorghi sporulates in almost a yeastlike
fashion on potato dextrose agar if incubated
under a continuous source of longwave UV light
(Odvody, unpublished). The conidia can be serially
transferred and streaked, as with bacteria,
onto media for mass production.
Other Disease Controls
Destruction of infested host debris
The ecology of foliar pathogens is of considerable
influence on how cultural practices and
farming systems affect the incidence of specific
diseases. Most foliar pathogens of sorghum are
favored by tillage that permits pathogen survival
in surface host-residue lesions. Control by
minimalization of host residues is more effective
with some pathogens than others. Pathogens
disseminated primarily by wind, like E. turcicum,
can more easily overcome distance limitations
imposed by such practices (Frederiksen
1986). Those pathogens surviving as sclerotia in
soil, e.g., R. sorghi, may be long-lived in the soil
but the more rapid destruction of lesion debris
increases the exposure of sclerotia to the deleterious
soil microenvironment. Colletotrichum
graminicola survives only in host debris, but its
ability to attack more than one plant organ compensates
somewhat for that dependency.
Destruction of collateral hosts
Puccinia purpurea, a wind-disseminated pathogen,
must survive on a living host. Destruction
of collateral hosts may at times be an effective
control for P. purpurea and several other pathogens,
but this practice is often impractical except
in the proximity of field areas.
Crop rotation and disease escape
Crop rotations are especially effective where
pathogen survival is dependent on debris, but
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