Sorghum Diseases in India

insecticides, fungicides, or other chemicals must
likewise be cataloged and separated (Merkle
1986). Another type of discoloration is chlorotic
streaking or striping, appearing as wide or narrow
bands of chlorosis running parallel to the
midvein. This type of chlorosis may appear in
younger leaves and in the sheaths. Streaking
and striping caused by a virus or viruses must
be differentiated from those caused by genetic
anomalies or chemicals (Rosenow 1986).
Red coloration produced by anthocyanin developing
in spots or stripes is often associated
with virus infection. This is sometimes referred
to as red-leaf reaction (Toler 1986). In the case of
soighums with tan pigments, the spots appear
as tan or brown instead of red. The red areas
usually appear early as spots that enlarge, coalesce,
and finally become necrotic. As a precaution,
this symptom must be distinguished from
chemical damage and from nonviral pathogens
causing similar symptoms.
Sorghum viruses may also induce ring spots.
These are usually chlorotic, but may be necrotic.
Rings may appear in conjunction with other
symptoms, such as mosaic. Ring spots are not
commonly diagnostic on sorghum. Discoloration
also includes necrosis. Some hypersensitive
sorghums infected with virus react by producing
local necrotic spots that may eventually encompass
the entire leaf and cause death of the
plant (Toler 1986). Again, agents such as insects
and other pathogens must be eliminated as
incitants.
Mosaics or mottling, either chlorotic or necrotic,
are often found on leaves and sheaths of virus-infected
sorghums. Mosaics may occur on
the entire leaf area or in defined patterns on the
leaf. This symptom is more often associated with
younger leaves or tillers. Mosaic, and particularly
necrosis, may be confused with chemical
damage or with symptoms caused by other
pathogens, and the latter must be recognized
and eliminated as possible inciters. Similarly,
symptom patterns induced by mixed infections
of viruses or virus strains must be distinguished
from symptoms produced by either strain separately
(Giorda and Toler 1985).
Stunting and dwarfing often accompany virus
diseases of sorghum. With these symptoms,
the affected plant must be compared to healthy
plants of the same cultivar growing in the same
location* Stunting occurs when the intemodes in
a particular portion of the stalk are noticeably
154
shortened. This occurs most often in the upper
portion of the plant, and may be associated with
a particular growth phase. Stunting at a particular
growth stage may be indicative of the time of
virus infection. Dwarfing connotes reduced size
of the entire plant, usually uniformly. The degree
of dwarfing or stunting may be measured
by comparing the height of the diseased plant
with associated healthy plants (Alexander et al.
1983; Giorda 1983). Drought stress, excessive
moisture, malnutrition, genetic root rots, and
nematodes also cause stunting and dwarfing
and must be eliminated as causes (Jordan and
Peacock 1986; Rosenow 1986; Starr 1986).
Delayed flowering is a symptom associated
with virus disease in sorghum. Flowering in diseased
plants may be delayed from 1 to 12 days.
Delay in heading may contribute to an increased
incidence of other diseases and damage by
midge (Toler 1985). Grain yield losses are reflected
in lower total grain weight and test
weight (Alexander et al. 1984; Alexander et al.
1985; Giorda 1983; Henzell et al. 1979; Toler
1985). Other factors associated with yield loss
include smaller heads and seeds on the diseased
plants. Confusion of virus symptoms with those
caused by other pathogens or parasites can usually
be systematically eliminated by visual inspection
and light microscopic examination of
the plants for nematodes, midge, and dodder.
Facultative fungal causal agents can be identified
by inspection or by light microscope, and by
culturing on media. Bacterial infections can be
identified by culturing and examination by light
microscope. Pathogens such as mycoplasma and
rickettsia can be separated from viral pathogens
by axenic culture, electron microscopy, and immunofluorescent
microscopy (Rocha et al. 1986).
Viroids consist of naked single-stranded RNA
(Diener 1983) and do not produce nucleocapsids
or virions (have no protein or lipoprotein). Thus
they can be separated from viruses by nucleic
acid hybridization (Owens and Diener 1981).
Transmission and Host Range
Plant viruses are obligate pathogens, so the
identification procedure usually begins with
modified Koch's postulates: transmission of the
causal agent from a diseased plant to a healthy
plant of the same cultivar (genome) with the resulting
production of the symptoms previously