Sorghum Diseases in India

direct effects on sorghum seedling growth and
vulnerability to soilborne organisms.
The major biotic and abiotic diseases affecting
sorghum seedlings, from germination through
the establishment of the permanent root system
are discussed here.
Seed Germination and the Primary
Root System
Freeman (1970) reviewed the root-system morphology
of sorghum. The radicle emerges from
the coleorhiza of the germinating seed and develops
into the primary root that then forms several
lateral roots. The coleoptile emerges from
the seed and extends toward the soil surface by
elongation of the mesocotyl (first internode).
The upper limit of the mesocotyl is delimited by
formation of the coleoptilar node near the soil
surface, where the plumule emerges through the
coleoptile.
The mesocotyl produces lateral (adventitious)
roots similar to the primary root and together
they form the primary, transitory or
temporary root system of sorghum. Most of
these are small in comparison to later roots and
are nearly uniform in diameter throughout their
length. This transitory root system is progressively
displaced by the permanent root system
which is entirely adventitious; whorls of roots
develop sequentially from the coleoptilar and
higher nodes of the crown. Later, buttress roots
are initiated from nodes above ground. Depending
upon plant-growth rate and soil environment
(including soil microflora activity), the
temporary (primary) root system may cease to
function within a few weeks.
Seedling diseases and the soil environment
can delay or impair establishment of the permanent
root system, causing pre- and postemergent
damping-off of sorghum seedlings, reduced
stands, and poor seedling growth.
Abiotic Factors
Soil temperature and moisture
Many soils have physical properties that are not
conducive to emergence of sorghum seedlings,
and those factors may be exacerbated by moisture
and temperature relationships related to
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rainfall patterns and subsequent solar radiation
and surface drying that forms hard crusts on the
soil surface. Such conditions are common to
many sorghum-growing regions. Other soil factors
that can affect or influence seedling growth
include pH, salinity, texture, and pore size;
chemical composition; and organic content.
These factors also indirectly affect seedling establishment
by influencing the composition and
numbers of the resident soil microflora. The latter,
including the facultative parasites and
pathogens of sorghum seedlings, in turn influence
one another.
Soil temperature directly affects the growth
of sorghum; temperatures cooler than 18°C are
generally deleterious to germination of sorghum
seed and emergence (Staffer and Riper 1963). In
the field. Germination response varies widely
(4.6-16.5 °C) in sorghum germplasm (Miller
1982). These temperatures are not favorable for
seedling establishment and growth. Cool soil
temperatures can inhibit seed germination and
emergence, but cold itself (if not below 0°C)
does not damage the sorghum seed.
Hot temperatures at the soil surface can damage
emerging seedlings and cause them to grow
parallel to, but beneath, the soil surface instead
of emerging. Optimal soil temperatures for germination
are from 21 to 35°C Temperatures of
40-48 °C at germination may be lethal to sorghum
(Peacock 1982).
Soil moisture must be spatially adequate to
allow germination and subsequent seedling
growth and establishment of the temporary root
system. In many areas the rapid depletion of soil
moisture after sowing either disallows germination,
or germination proceeds but is arrested by
lack of moisture and the seedling dies prior to
emergence. An excess of soil moisture depletes
soil oxygen, damaging roots of emerging and
established seedlings. The other negative effect
of high soil moisture is through its effect on soil
microflora activity against sorghum seedlings
(Leukel and Martin 1943; Forbes 1984).
Organic matter and allelopathy
Organic material affects soil structure and chemistry
through its direct physical attributes and
through the dynamic microfloral communities
active in its decomposition in soil. Some of the
decomposition products of certain weed and