Sorghum Diseases in India

the length of rotation necessary to substantially
reduce soilborne pathogens surviving as sclerotia
and other long-lived propagules may make
this practice impractical. Selection of appropriate
sowing dates is successfully used in many
areas of the world to avoid or minimize foliar
disease losses. The plants are still fully susceptible
but the environment or the lack of inoculum
may limit disease incidence. Examples include
sowing in the dry season or when air temperatures
are unfavorable for disease development
and spread. Using taller genotypes or reducing
plant populations can reduce disease spread, because
the lower leaves (where most initial infections
occur) are less exposed to environments
favorable for infection.
Exclusion or elimination of the pathogen
Foliar pathogens, due to diverse survival mechanisms,
are very hard to eradicate from sorghum-growing
regions, especially if weed sorghums
provide survival sites away from field
locations. Only C. graminicola is known to be
seed-transmitted, although others are sometimes
reported as seed-surface contaminants.
Certainly, most of the foliar pathogens are easily
transported on infected soighum forage. Such
dissemination is usually prevented through routine
quarantine procedures, but within geographic
regions where diseases are already
established there may be significant pathogen
spread and introduction in this manner.
Chemical control
Foliar application of chemicals to control foliar
diseases of sorghum is usually impractical, because
control is either unwarranted or the cost
exceeds benefits, especially with soighum grown
in stressful environments with low-cost inputs.
Chemicals applied to seed to avoid the transmission
of seedbome pathogens, such as C. graminicola,
may be quite beneficial.
Biological control
The efficacy of many disease-control practices
such as tillage, crop rotation, and host-residue
destruction is dependent on biological processes
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that may accurately be described as biological
controls. Without understanding their specific
mechanisms, we manipulate, exploit, and enhance
these natural processes. There is always a
potential for the use of one organism to control
another. There are many naturally occurring
soilborne and aerial parasites of pathogenic
fungi, but their isolation and use apart from specific
natural habitats may not be feasible. Eudarluca
australis Speg. is a fungal hyperparasite
of rust fungi, including P. purpurea, but its occasional
abundant appearance in rust pustules
probably comes too late to have a practical effect
on rust epidemiology (Frederiksen 1980). The
potential of using low-virulence or avirulent
strains of a pathogen to induce resistance in susceptible
genotypes should be investigated for
pathogens with known variability like C. graminicola
(Dean and Kuc 1986).
Future Research Needs
1. Determine economic impact of foliar diseases
on sorghum in all geographic regions and
environments.
2. Develop foliar-disease resistance that is (a) effective
across environments, (b) durable
through reduced selection pressure for physiological
variants within a pathogen population,
and (c) active against several foliar
pathogens, especially during vulnerable
growth stages of the host.
3. Increase international cooperation in multilocational
evaluation and characterization of
germplasm response to foliar pathogens and
exchange of elite germplasm.
4. Increase efforts to locate, identify, and collect
landrace and other sorghum germplasm
sources endangered by adoption of introduced
elite germplasm or other factors.
5. Develop locally-effective control alternatives
to complement and conserve existing hostplant
resistance.
6. Integrate field and controlled-environment
screening and research techniques to simultaneously
obtain correlative information concerning
(a) host-pathogen interactions, (b)
characterization of germplasm response to foliar
pathogens, and (c) identity of sources of
resistance.
7. Employ multidisciplinary team research to
evaluate the physical and physiological fac-