Sorghum Diseases in India

Detection and Identification of Viruses
and Virus Diseases of Sorghum
R.W. Toler 1 and L.M. Giorda 2
Abstract
Plant virology is the study of viruses that infect plants. Sorghum viruses are distributed worldwide
and cause economic damage to the host. A wide range of abnormalities is caused by these
viruses. Some of the most noticeable effects are changes in color, death of tissue, and changes in
growth habit. Most if not all sorghum viruses infect the plant systemically. Symptomology may
play a role in identification, but symptomology alone is not definitive. Diagnosis of a sorghum
virus disease on a routine basis is required for the development of control measures. Indicator host
and host-range studies are important in initial diagnosis. Early diagnosis may include microscopic
studies of cellular inclusions. These may be followed by quick-dip electron microscopy (EM).
Serological tests may follow. Combined EM and serology studies, e.g., immunosorbent electron
microscopy, may be in order. The EM studies should provide information on particle morphology,
serology, and on the relationship of known viruses and strains. For finite criteria of a virus,
purification is in order. Antiserum specific to the virus being studied can be prepared to the
purified virus. After purification, information including sedimentation coefficient, molecular
weight, isoelectric point, extinction coefficient, 260/280 ratio, and buoyant density can be obtained.
Molecular studies should identify protein species, terminal amino acids, and nonparticle proteins.
Lipids and polyamines present are characterized. The nucleic acid is analyzed for type and strandedness.
Number and size of nucleic acids are determined, as well as base composition and ratios.
Other distinguishing characteristics of the nucleic acid include presence of a methylated cap,
amino acid-accepting ability, or polyadenine at the 3' end. Subgenomic RNA may or may not be
present. Some viruses, such as the rhabdoviruses and reoviruses, may contain polymerases within
the particle. Virus identification encompasses evaluation of the sum total of all information available,
demonstrating relatedness or nonrelatedness to knoum sorghum viruses, or establishing the
identity of a new virus.
Symptomology
The earliest detection of sorghum viruses in the
field is made by noting their observable effects
on the host. However, observable changes induced
by plant viruses may be similar to those
associated with other diseases and disorders in
the field or area and cause misidentification.
Usually, the first symptom observed is some
type of leaf discoloration. Discolored plants are
compared to healthy plants of the same cultivar
in the same sowing. General chlorosis or yellowing
may indicate infection by viral or other
pathogens. However, one must eliminate possible
nutritional effects, particularly those resulting
from minor element deficiency or excess
(Clark 1986). Viral chlorosis must also be differentiated
from genetic chlorosis or anomalies
caused by biotic or abiotic agents (Jordan et al.
1986). Chlorotic effects caused by herbicides,
1. Professor of Plant Pathology and Microbiology, Texas A&M University, College Station, TX 77843, USA.
2. Sorghum Research Leader, INTA, Cordova, Argentina 5988.
Toler, R.W., and Giorda, L.M. 1992. Detection and identincation of viruses and virus diseases of sorghum. Pages 153-159 in
Sorghum and millets diseases: a second world review. (de Milliano, W.A.J., Frederlksen, R.A., and Bengston, G.D., eds). Patancheru,
A.P. 502 324, India: International Crops Research Institute for the Semi-Arid Tropics.
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