Sorghum Diseases in India

tion of host tissue. As wind and rain-splash are
the important modes of dispersal of G. sorghi,
R. sorghi, R. sorghicola, A sorghina, C. graminicola,
and P. sacchari, host proximity is more important
than with E. turcicum, B. sorghicola, and the Cercospora
spp.
These pathogens can occur on younger plants,
but in environments favoring disease most of
them cause greater damage to older plants because
of a greater susceptibility in postanthesis
foliage and the concomitant increase in inoculum
for infection. For most of the pathogens,
individual lesion development is less restricted
on older foliage of susceptible sorghum. The latent
periods between infection, lesion appearance,
and sporulation are shorter than on
younger tissue of the same cultivar. Lesion size
also increases on older leaves; delimitation by
leaf veins is reduced and lateral and longitudinal
coalescence of lesions is more common.
Often, the spread of a pathogen from one or a
few initial lesions is evident through the occurrence
of younger lesions on the same leaf or surrounding
leaves of the same or other plants.
Those pathogens disseminated by wind and
rain-splash, like G. sorghi and R. sorghi, often
produce a series of coalescing leaf-margin lesions
beneath an older lesion on the upper part
of the same leaf.
After initial infection, hyphae of E. turcicum
invade leaf vessels and cause a localized wilt as
the lesion extends first longitudinally within
vessels then laterally to give a fusiform appearance
(Frederiksen 1980). Susceptibility to E. turcicum
is reported to decrease with sorghum
maturity (Frederiksen 1980), but the occurrence
of leaf blight on leaves of all ages in the field
suggest that this host : pathogen interaction is
more complex.
Zonate leaf spot can occur as a seedling disease,
but the typically small rectangular lesions,
nonsporulating and highly pigmented, are indicative
of greater resistance in young foliar tissue
in comparison to older leaves. If economic
damage occurs on young seedlings, it may be
due in part to an overwhelming number of lesions
incited by initial inoculum from soilborne
sclerotia (Odvody, unpublished observation).
The type of damage that pathogens cause is
somewhat related to their preferred colonization
site on the host plant. Since it is the foliage of the
plant that produces the energy for all other parts
of the plant, foliar dysfunction at critical stages
of plant growth can inhibit roots, stalks, and
seed heads. The most direct effect of foliar
pathogens might be from reduced photosynthesis,
but foliar infections could increase leaf transpiration,
reduce carbohydrate translocation to
other organs, and/or increase uptake of carbohydrate
and other nutrients from the other
plant organs to dysfunctional leaves. Premature
foliar and plant senescence is a logical
consequence.
One of the sites of earliest and progressive
dysfunctions of leaves by foliar pathogens involves
the stomata, because many of the foliar
pathogens produce large numbers of specialized
fungal structures (conidiophores, fungal strornata,
and sclerotia) directly beneath and extruded
through the stomata. Rust causes a
physical disruption of the epidermis during lesion
development and may be more damaging
per unit of lesion area than are some other
pathogens.
Foliar diseases may dramatically affect other
disease incidence and severity (e.g., increased
stalk rots) on sorghum, possibly through direct
effects on host physiology.
Sorghums with a high susceptibility to one or
more foliar pathogens usually develop a higher
disease incidence and severity much earlier in
their maturity than the more resistant sorghums
growing in the same environments. Differences
in susceptibility may be unnoticed, however, if
evaluations are conducted too late in the maturity
of the sorghum. In some cases, the factors
of environment and inoculum can be so overwhelming
that differences in disease reaction
are obscured.
Economic Impact Assessment
The economic impact of foliar pathogens is a
topic of constant debate. Yield losses attributed
to a particular pathogen will usually vary with
sorghum-growing region and with environment
(Frederiksen 1982). The direct effect of foliar
pathogens can easily be related to loss of forage
value in terms of killed foliage, but it is harder to
demonstrate that foliar diseases cause a loss in
other forage components (stalk, sugars, etc.) or
reductions in grain yield. Meaningful loss assessment
values require a standard assessment
procedure, e.g., same or an equivalent cultivar
either protected from or more resistant to the
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