Sorghum Diseases in India

crop plants are inhibitory to sorghum growth
through a process called allelopathy (Mohamed-
Saleem and Fawusi 1983; Shahid Shaukat et al.
1985). Conversely, sorghum residues are reported
to have allelopathic effects on some
weeds, including one genus (Digitaria) reported
to be allelopathic to sorghum (Defrank and Putman
1979). The quinones produced as hydrophobic
root hair exudates may prove interesting
in this regard (Netzly and Butler 1986). The
other important effect of soil organic material is
its function as a site for survival and colonization
by microorganisms pathogenic to sorghum
seedlings and or by organisms inhibitory to
these pathogens.
Pesticides
Soil-applied pesticides, especially herbicides, are
often phytotoxic to sorghum and can inhibit
seedling growth. McLaren (1983) demonstrated
that four herbicides, including atrazine, inhibited
secondary root formation and promoted
pre- and postemergent damping-off of sorghum
seedlings.
Biotic Factors
Seed-related
Many organisms have been isolated from sorghum
seed, but their relationship to seedling
diseases or reduction in seedling growth differ
and are often unknown. Some of the fungi are
passively associated with the seed and are of no
significance, but others, like Rhizopus spp, can
cause seed rots in cold, wet soil (Leukel and
Martin 1943). Others, such as Colletotrichum
graminicola (Basu Chaudhary and Mathur 1979)
and Gloeocercospora sorghi (Watanabe and Hashimoto
1978), are seedborne pathogens that later
attack seedlings and may or may not also damage
the seed. Some fungi deleteriously affect
seed and subsequent seedlings through direct
damage to the seed before sowing.
Seedborne fungi damaging the seed are usually
divided into two groups, one based on biological
behavior and species and the other on
where the attack on the seed occurs. The storage
fungi (primarily Aspergillus spp and Penicillium
spp) usually will damage sorghum seed in stor­
age under moisture conditions lower than those
in which field fungi attack seed before maturity
(grain mold) or after maturity (grain weathering
fungi) (Castor 1981). The most common grain
mold fungi are Fusarium moniliforme, F. semitectum,
Curvularia lunata, and Phoma sorghina (Bandyopadhyay
1986); of these, F. moniliforme and
C. lunata may be the most important (Frederiksen
1982). The most common fungi associated
with grain weathering are species of Fusarium,
Alternaria, and Cladosporium (Frederiksen 1982).
Gaudet and Kokko (1986) reported that Pseudomonas
syringae pv syringae was seedborne in
sorghum seed produced in southern Alberta,
Canada, and was responsible for stunting and
discoloration of roots and coleoptiles of seedlings.
The cumulative effect of damage by these organisms
can be loss of seed viability, seed rotting
in soil (by these or other fungi), retarded germination,
low seedling vigor, and sometimes
blighting of the seedlings (Fusarium) (Williams
and Rao 1980).
Soilborne organisms
Some of the seedborne organisms causing damage
to seed are also soilborne and under conducive
soil environments can attack ungerminated
and germinating seed (Leukel and Martin 1943).
The primary seed rotting fungi in soil are soilborne
and seedborne species of Fusarium and
Penicillium and seedborne species of Rhizopus
and Aspergillus.
The primary organisms associated with preand
postemergent damping-off and seedling
diseases of sorghum are species of Pythium (Leukel
and Martin 1943; Pratt and Janke 1980;
Forbes et al. 1985). Odvody and Forbes (1984)
reviewed Pythium root and seedling rots of
sorghum. The Pythium species most commonly
associated with seedling root loss are P. arrhenomanes
and P. graminicola.
In the field, Pythium predominantly infects
and damages the roots and mesocotyls of sorghum
seedlings in cool, wet soils before or after
emergence. Symptoms on seedlings are either
brown or gray water-soaked roots or root tips,
or lesions on roots that become flaccid and necrotic
(Forbes 1984). The mesocotyl produces
greater pigmentation response to the pathogen
than do the roots. Most of the seedlings suc-
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