Sorghum Diseases in India

fusimaculans Atk. (Wall et al. 1987). Despite similarities
in lesion size and shape, the scleriform
lesions caused by C. fusimaculans (pale-brown
with darkly-pigmented borders and ladder-like
markings) are distinct from those of Cercospora
sorghi Ell. & Ev. (discolored throughout with the
host's pigmentation) (Tarr 1962). These two Cercospora
spp also differ in morphology and pathogenicity
(Wall et al 1987).
Pathogen Survival
Many foliar pathogens of sorghum survive, in
the absence of the host, as mycelia or spores
within sorghum host residues on or in the soil.
In some environments these and other pathogens
are perpetuated on living weed sorghum
hosts that provide a readily available source of
inoculum for newly established susceptible
sorghums.
Exserohilum turcicum (Pass.) Leonard and
Suggs is known to form chlamydospores within
cells of the conidium; these chlamydospores can
survive in soil without host tissue, but their
function as inoculum on sorghum has not been
verified. A related pathogen, Bipolaris sorghicola
(Lefebvre and Sherwin) Shoem, also produces
chlamydospores and, although pathogenic, they
were thought to represent a minor contribution
to survival (Odvody and Dunkle 1975). Overwintered
lesion residue of B. sorghicola often produced
mycelium from the open ends of leaf
veins incubated under high humidity in the laboratory
(Odvody and Dunkle 1975).
The pathogens Ramulispora sorghi (Ell. and
Ev.) L.S. Olive and Lefebvre, Ramulispora sorghicola
Harris, and G. sorghi survive primarily as
sclerotia on or in soil; the sclerotia can be within
or free from host residue (Girard 1980). Coley-
Smith and Cooke (1971) classified these sclerotia
as sporogenic because they germinate by producing
a mass of conidia similar to that later
produced on foliar lesions. The other two sclerotial
classifications of Coley-Smith and Cooke,
based on type of germination, are myceliogenic
(mycelia production) and carpogenic (production
of sexual fruiting structure). The common
leaf sheath blight organisms, S. rolfsii and Rhizoctonia
spp can survive in soil as mycelia in tissue
residue or as free myceliogenic sclerotia.
Phyllachora sacchari P. Henn. and P. purpurea
survive within lesions on living weed sorghum
hosts. Ascochyta sorghina Sacc., C. sorghi, and
probably C. fusimaculans survive primarily as
mycelia in lesions on living or dead tissue of
crop and weed host sorghums (Dalmacio 1986;
Tarr 1962).
Initial Inoculum and Dispersal
All known leaf blade pathogens of sorghum are
dependent upon wind-dissemination of their
initial and secondary inoculum, but some appear
to be better-adapted to long-distance
dispersal than those whose dispersal is more
closely linked with the presence of free water.
The conidia of E. turcicum are easily wind-disseminated,
with most spores released in the
morning hours (Meredith 1965). Leach (1980a,
1980b) demonstrated that release of conidia of
E. turcicum (and Bipolaris maydis) from conidiophores
was a "spore-discharge" influenced
by infrared irradiation and changes (usually a
reduction) in relative humidity and electrostatic
forces. This phenomenon probably has implications
for other foliar pathogens, e.g., B. sorghicola,
C. sorghi, and C. fusimaculans, whose
conidia are freely borne on conidiophores in an
aerial environment. Though not aerially-borne
on conidiophores, urediospores of P. purpurea
are highly dependent on wind dispersal from
erumpent uredia on living host plants that provide
the source of initial inoculum.
Many of the other foliar pathogens produce
inoculum in some kind of protective structure,
probably in association with an external, viscous
water soluble matrix. The initial inoculum of
P. sacchari is ascospores produced within locules
of stromatic fungal tissue containing paraphyses
described as "slimy," which could indicate such
a matrix (Tarr 1962). Pycnidiospores like those
produced by A. sorghina are often associated
with viscous liquids that may protect spores
from dessication. This is evident when pycnidiospores
are extruded from the pycnidium in
a slimy cirrhus. The acervuli of Colletotrichum
graminicola (Ces.) G.W. Wilson also produce a
protective mucilaginous matrix in which conidial
masses are borne (Ramadoss et al. 1985).
The matrix functions to protect conidia from
dessication, and with free water, wind, and rainsplash
allows rapid dispersal of conidia to other
infection sites. Conidial spore masses of G. sorghi,
R sorghi, and R. sorghicola produced from
169