Sorghum Diseases in India

pathogen(s) being evaluated. Isogenic lines and
fungicides have been used to evaluate the
economic impact of several sorghum diseases
(Craig 1982). For foliar pathogens, fungicides
have generally been used to demonstrate variable
loss effects (Hepperly, in press; Hepperly et
al. 1987). These types of measurements are useful
as they indicate potential disease loss but the
results are often locational and environment
specific, or valid only with cultivars having similar
reactions to the pathogen(s).
Commonly, evaluation of the severity of foliar
diseases involves an estimation of the leaf
area visibly affected by the disease. There is often
a lack of correlation between leaf area affected
by disease and expected yield loss; foliar
diseases of sorghum are no exception (Hepperly,
in press). A pathogen at a low disease incidence
may cause economic loss, but another occurring
at a much higher incidence may cause no measurable
loss. It is always more convenient to do
single-disease evaluations at crop maturity, but
periodic evaluations would be more meaningful.
The occurrence of several foliar pathogens in
the later growth stages of the plant make it more
difficult to distinguish between cultivars differing
in susceptibility or resistance at their earlier
growth stages. This also makes it imperative to
consider differences in cultivar maturities even
when they have been sown on the same date
and spatially exposed to the same pathogen and
environmental stresses.
In Puerto Rico, multiple levels of control with
specific chemicals have been utilized to create
multiple disease levels in plants of the same genetic
background (Hepperly et al. 1987; Hepperly,
in press). Levels of chemical control,
especially using the systemic fungicide oxycarboxin,
can be related by regression analysis to
yield levels. These responses can be used to
make comparisons between genotypes with differing
resistance levels. However, some fungicides,
including systemic ones, give variable
results and can affect other diseases or factors
that influence yields.
Isogenic lines resistant and susceptible to a
disease or diseases can be used to measure disease
losses without the pleiotropic effect of fungicides;
but they take several years to develop
and are most practical with resistance governed
by single major genes.
A modification of the isoline strategy uses F3
families developed from susceptible by resistant
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crosses and then separated by their disease reactions
(Burton and Wells 1981). This strategy was
successfully utilized in assessing loss associated
with systemic sorghum downy mildew in Texas
(Craig and Odvody 1985) and the Honduras
(Wall, personal communication). The technique
seems particularly adapted to evaluation of foliar
diseases and should allow greater utilization
of polygenic resistance factors. The populations
can be developed rapidly, but choices for the
parental crosses should consider other factors
that may affect the assessment of disease loss—
such as yield potential, vulnerability to other
pest factors, and local adaptability.
Evaluation of Host-Plant Resistance
Several control approaches will minimize damage
by foliar pathogens, but each must be
viewed in the context of complementing hostplant
resistance. The methods by which we evaluate
host-plant resistance depend upon several
interrelated factors. Some of these factors are (1)
type of resistance desired or available in germplasm,
(2) growth stage at which host expresses
susceptibility or resistance to the pathogen, (3)
regularity of occurrence of the disease(s) or a
disease-conducive environment, (4) interference
of environmental and pest factors with evaluation,
(5) ease of inoculum production in culture
and its pathogenicity and efficacy as inoculum
in the field and in controlled environments, (6)
representative nature of cultural inoculum to at
least the local naturally occurring local pathogen
populations, (7) efficacy of introduced or resident
natural inoculum sources, and (8) opportunity
for multilocational evaluation to determine
effects of variable environments and variable
pathogen populations.
The type of resistance sought against a pathogen
is ultimately dictated by that existing in
available germplasm, influenced in part by other
desirable characters in such resistance sources,
and by the degree of need for the resistance.
With foliar diseases of sorghum there is often a
greater need to develop moderate resistance and
avoid high susceptibility, rather than develop
high levels of resistance (Frederiksen and Franklin
1980). Sources of resistance to most foliar
pathogens of sorghum have been identified and
at least partially characterized (Williams et al
1980; Frederiksen and Rosenow 1986). General