Sorghum Diseases in India

Variation in pathogen populations
Sporisorium reilianum is composed of physiological
races. The importance and status of our understanding
of the populations of these fungi
were reviewed by Frederiksen and Reyes (1980).
For the most part, the populations of the pathogens
described then (Frowd 1980; Frederiksen
and Reyes 1980) have changed little. The same
differential cultivars are used in Texas and
the widely grown uniform head smut nursery
(UHSN) contains national program differential
cultivars, along with cultivars with resistance
from other areas.
Sources of resistance to head smut in USA
Many of the lines identified as resistant by Frederiksen
and Reyes (1980) remain useful in 1988.
Changes in the pathogen population have not
caused an unusual erosion of host resistance.
Currently, populations and cultivars of sorghum
are being bred for higher levels of smut resistance
in Texas.
Long Smut
The disease has wide distribution in middle
eastern countries, the Indian subcontinent, and
in the far eastern countries. (Briton-Jones 1922;
Tarr 1962; Mahdi 1962; Ragab and Mahdi 1966;
and Frowd 1980).
Long smut has gained prominence as a serious
problem in certain countries in eastern and
western Africa. In Sudan, on hybrids especially,
annual crop loss was estimated to be 2-5%, but a
much higher value (40-60%) had been reported
from individual fields in Pakistan (Hafiz 1958).
Symptoms of long smut
Long smut appears as conspicuous sori affecting
individual grains. The sorus is cylindrical and
elongate, usually slightly curved with a thick
membrane. The spore sac, about 4 cm long and 7
mm wide, splits when mature, exposing a black
mass of spore balls. Within the spore mass are
eight to ten filamentous structures arising from
the base. These are the remnants of the vascular
tissues of the ovary. The spore balls are variable
in size, globose to oblong, black to dark brown
(Fig. 2). Within the spore ball, spores are cemented
together and their number varies, depending
on the degree of breakage. Thus the
size of the spore ball has no significance since its
permanency is greatly affected by physical factors.
Spores in the middle of a spore ball are
somewhat angular, light colored, and thin walled,
whereas those on the periphery are dark,
echinulate, and globose. Teliospores are about 12
µm in diameter.
Seasonal persistence and spread
Since the first International Workshop in Hyderabad,
in 1978, our knowledge of long smut
has expanded. Mahdi (1962) reported that the
thermal death point of teliospores was 67°C.
However, recent investigations indicate that the
spores remain viable after 1 week at 85°C (Hago,
personal communication 1988). The highest recorded
soil-surface temperature in sorghumgrowing
areas of the Sudan is 65 °C. The longevity
of spore balls may extend to 2 years or
longer. Sporidia in the laboratory retained their
viability for at least 255 days (Mahdi 1962)
whereas dry sporidia, presumably short-lived,
remained viable at room temperature for longer
than 1 month (Hago, personal communication
1988).
Kamat (1933) claimed that most spores in a
spore ball readily germinate on different media
and over a wide range of temperature (19-39°C).
In sterile distilled water only a few teliospores in
a spore ball germinate, forming three- to sixcelled
stout promycelium bearing sporidia terminally
and near the septa. Sporidia are 8-12
mm, spindle shaped, single-celled, and rarely
with a transverse septum. In solid medium,
branched germ tubes with clusters or chains of
sporidia develop. The optimal temperature for
growth is 25-30 °C. The fungus grows well in a
variety of media and does not lose pathogenicity
after a series of transfers.
Evidence from various sources refutes the reports
(Mahdi 1962; Ragab and Mahdi 1966) that
the long smut pathogen is internally seedborne.
The work of Vasudeva et al. (1950), Manzo
(1976), and Omer et al. (1985) demonstrates that
the pathogen is airborne and infects single florets.
Attempts to simulate the disease using arti-
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