SBWMV is a rigid rod-shaped virus with hollow particles of two sizes: 140 and 280 nm long. Particles of both sizes are necessary for infection. SBWMV is an RNA-containing virus. SBWMV is transmitted by the soilborne, fungus-like protozoan Polymyxa graminis. Infection of plants by SBWMV and spread of the diseases does not occur in the absence of P. graminis.
Polymyxa graminis is an obligate parasite in the roots of many higher plants. It produces a spore, called a zoospore, that swims in the soil solution and carries SBWMV internally or externally. After penetrating host root hairs and epidermal cells, P. graminis expands and produces a plasmodium that replaces the cell contents. The plasmodium eventually divides into additional zoospores or develop into smooth, thick-walled resting spores 2 to 4 wk after infection. Clusters of resting spores are visible in cortical and epidermal cells under a compound microscope.
SBWMV survives in association with P. graminis, and fields in which SBWM is observed remain infective for many years. During cool, wet periods in the autumn, zoospores of P. graminis infect roots of wheat seedlings. Soil wetness following planting is more critical in determining disease incidence and severity than is the seeding date.
In the field, symptoms of SBWM on susceptible cultivars diminish as temperatures rise in the spring, but yield from susceptible cultivars can be greatly affected. Temperatures from 50 to 70F°F (optimum near 61°F) promote SBWM in the field.
SBWMV is spread by cultivation, wind, water, and other factors that disperse infested soil. However, for reasons not understood, the virus sometimes spreads more rapidly and over longer distances than can be explained by soil or water movement.
Planting resistant cultivars is effective and the most economically feasible method to control WSBM, and many wheat varieties adapted to the Great Plains, Midwestern and eastern wheat-producing states are resistant to WSBM. Additionally, resistance to WSBM has been identified in Thinopyrum intermedium (Intermediate wheat grass) and transferred to wheat. Recent variety tests by WSU and OSU have demonstrated that some PNW-adapted varieties have effective resistance to SBWMV, and most of these are hard red winter varieties.
Crop rotation has limited value in control because of the longevity of the vector in soil. Late-autumn seeding of wheat also has limited value because infection depends more on soil moisture and temperature shortly following planting than simply on planting date.
Symptoms of SBWM are suggestive but not diagnostic, and confirmation of infection requires a laboratory test. If you are concerned that you may have SBWM in one of your fields, samples can be submitted to the WSU Plant Pest Diagnostic Clinic in Pullman or other commercial diagnostic laboratories for confirmation.