Soilborne wheat mosaic (SBWM) was first discovered in the central plains of the USA in 1919. By 1923, Soilborne wheat mosaic virus (SBWMV) was identified as the cause, making it one of the earliest known and first soilborne wheat viruses here.
SBWM was detected in Washington State for the first time in spring 2008, although it was detected in an adjacent county in Oregon about 1995. It has now been confirmed in three fields, all in the same vicinity near Walla Walla. How widely the disease occurs in Washington is not known.
Usually only autumn-sown wheat develops symptoms, although spring wheat also is susceptible. Symptoms occasionally develop on rye, barley, and other grasses.
Losses in winter wheat to SBWM vary with the variety, virus strain, and environment, but can be substantial when a susceptible variety is grown in a field that is completely infested. Entire fields or areas of fields can be damaged.
Symptoms of SBWM range from mild green to prominent yellow mosaics on the leaves. Symptoms are most prominent on early-spring growth and rarely appear in autumn. As new leaves unfold, they appear mottled and develop dashes and streaks parallel to the veins. Leaf sheaths are also distinctly mottled. Rising temperatures in spring slow and eventually stop disease development.
Stunting can be moderate or severe. Some strains of the virus cause rosetting in highly susceptible cultivars. Fields may be uniformly diseased but more often show a pattern of symptoms that corresponds to distribution of the vector, Polymyxa graminis, which preferentially inhabits low-lying wet areas.
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.