Fig. 1. Mosaic of "green islands" on yellowish background. (photo by Bill Willis)

Wheat soilborne mosaic virus (WSBMV) occurs throughout the eastern and central United States. It has been reported in most counties in the eastern two thirds of Kansas and in many irrigated circles in southwest Kansas. The symptoms, life cycle, and field pattern of WSBMV are similar to those of wheat spindle streak mosaic virus. This similarity probably prevented the recognition of spindle streak mosaic as a distinct disease until varieties with resistance to WSBMV became available. WSBMV differs from spindle streak mosaic virus in two important aspects: 1) it causes greater yield loss, and 2) symptoms persist longer into the spring.

SYMPTOMS

The optimum temperature for symptom development is 60°F. Above 68°F disease progress stops. Symptoms typically appear in early spring right after green-up. By the time of jointing, mosaic symptoms have usually faded, but stunting may persist until maturity.

Soilborne mosaic is usually first noticed as large, irregular areas of yellow, stunted wheat. The field pattern often follows the drainage pattern of the field because the fungal carrier requires moisture in order to infect. Symptoms of WSBMV can easily be mistaken for nitrogen deficiency when viewed from a distance. Unlike some other diseases, the patches of soilborne mosaic do not grow in size as the season progresses.

WSBMV leaf symptoms are typically described as a mosaic of "green islands" against a yellowish background (Fig. 1). Unfortunately, the typical symptoms are not always present. Symptoms of spindle streak mosaic virus are yellow spots or dashes on a green background. The situation is confused further by the fact that many plants are found to be infected with both viruses. In such cases, positive identification requires a serological test. Varietal resistance ratings are also helpful in distinguishing soilborne mosaic and spindle streak mosaic.

DISEASE CYCLE

Soilborne mosaic and spindle streak mosaic are both carried by a fungal vector called Polymyxa graminis.The virus particles are carried on or in the fungal zoospores (swimming spores).

The fungus invades root hairs of the young wheat in the fall during periods of high soil moisture. Apparently spring infections are possible, but inconsequential to the wheat. The fungus forms dark clusters of resting spores in the wheat roots which are released to the soil when the roots decay. Since the fungus survives in the soil as resting spores, the disease is always associated with infested soil. Soils may remain infective for many years. Neither soilborne mosaic nor spindle streak mosaic are insect borne or seed borne. In addition to wheat, WSBMV also occasionally attacks rye, barley and some species of annual bromegrass.

LOSSES

Losses are limited to the areas of the field showing symptoms. Therefore, the first task is to estimate the percentage of the field acreage which is affected. Within affected areas, losses caused by WSBMV are variable, depending on variety and weather. Losses are roughly proportional to the length of time during which plants show obvious symptoms. A long cool spring prolongs the symptomatic period and losses may be 30 to 50%. Losses may be only 10 to 20% in a warm spring.

CONTROL

Any field with a history of WSBMV should be planted to varieties with resistance. Many varieties are resistant to WSBMV. See publication MF-991 "Wheat Variety Disease and Insect Ratings" for current information on variety reactions.

Late planting is sometimes effective in avoiding infection periods in the fall. Wheat planted after the "fly- free" date is less likely to be attacked by soilborne mosaic as well as other viral diseases. Due to the longevity of the vector, crop rotation is not an effective control. There are no pesticides which provide economic control of soilborne mosaic.

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Web updated 9/01/06
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