Sorghum Diseases in India

sclerotia or on leaf lesions are borne in a similarly
functioning water-soluble matrix (Olive et
al. 1946; Bain and Edgerton 1943). Under dry
conditions, single conidial masses of G. sorghi
and R. sorghi are held so tightly together that
their removal from lesions is possible only en
masse, but when the mass is placed into free
water, the conidia immediately disperse as
individual spores (Odvody, unpublished observation).
The spore-dispersal mechanisms are consistent
with the epidemiology of these pathogens,
especially the latter group that form spores in
protective structures and matrices. They have
high free water requirements for both inoculum
dispersal and initial infection.
The initial inoculum of S. rolfsii and Rhizoctonia
spp causing blights of the leaf sheaths is
primarily mycelial, and derives probably from
sclerotia and other colonized substrate, because
infection usually begins on basal sheaths near
the soil line (Odvody and Madden 1984; O'Neill
and Rush 1982). Rhizoctonia spp may also produce
basidiospores as initial and secondary inoculum
(O'Neill and Rush 1982).
Host Range and Pathology Variability
Exserohilum turcicum and C. graminicola can be
pathogenic on sorghum, maize (Zea mays), and
other grasses, but naturally occurring isolates
from host crops are generally genus-specific
(Frederiksen 1980,1984). Gloeocercospora sorghi is
reported to attack sorghum, maize, and some
other grasses, but more information concerning
host specificity (Tarr 1962) is needed.
Sclerotium rolfsii and the Rhizoctonia spp
(probably R. solani) causing sheath blights on
sorghum are pathogenic to a wide variety of
hosts, and might be described as facultative
parasites (O'Neill and Rush 1982; Odvody and
Madden 1984).
The other pathogens listed (Table 1) are generally
regarded as occurring only on Sorghum
spp. Isolated reports of some of these pathogens
on other hosts could be the result of error in
identification of the pathogen, saprophytic instead
of parasitic attack, or extremely unusual
conditions allowing infection. The large number
of minor, seldom-reported diseases of sorghum
could be due to similar factors, especially as
senescing sorghum leaves become vulnerable to
170
weak pathogens and saprophytes (Frederiksen
1986).
Of the pathogens listed in Table 1, only
C. graminicola (Frederiksen 1984) and P. purpurea
(Bergquist 1974) currently have pathotypes
(races) described on sorghum.
Geographic Distribution
Frederiksen (1982), S.B. King, and N.V. Sundaram
classified the major diseases of sorghum
on the basis of prevalence and severity in temperate
(outside 34° latitude), subtropical (between
23 °15' and 34° latitude), and tropical (within
23°15' latitude) regions. According to their
classification, the foliar pathogens E. turcicum,
C. graminicola, C. sorghi, P. purpurea, G. sorghi,
R. sorghi, and A. sorghina are all commonly or
generally found on sorghum grown in the subtropics
and on sorghum grown in the tropical
lowland during summer. Occurrence of these
pathogens was less consistent in the cooler temperate
and tropical highland or tropical winter
environments. The pathogens C. sorghi, E. turcicum,
and P. purpurea, easily wind-disseminated,
are apparently the most consistent in their
occurrence and incidence across all these diverse
sorghum-growing environments. Bipolaris
sorghicola also occurs, but only occasionally, in
all these environments. The potential misidentification
of B. sorghicola lesions as those of
C. sorghi could partially account for its lower reported
occurrence.
The reported observations of C. sorghi may
also include occurrences of the newly described
C fusimaculans. Ramulispora sorghicola seems to
be restricted to warmer conditions in the subtropics
and tropical lowland summers, and it
usually does not have a high disease incidence.
Phyllachora sacchari is limited to high-rainfall
tropical areas (Dalmacio 1986), possibly due in
part to requirements for a living weed-host reservoir
for significant pathogen survival and the
need for wet conditions for infection, disease development,
and inoculum dispersal.
Host: Parasite Interaction
Initial infection of sorghum by foliar pathogens
requires high moisture conditions for specific
periods to allow spore germination and penetra-