Wednesday, March 23, 2011

Wheat Soil-Borne Mosaic Virus Prevalent

The following is in the Kentucky Pest News from Don Hershman  

By Don Hershman

Over the past few weeks, we have been getting questions about fields that look similar to Fig. 1.  Initially, we thought the problem was related to nitrogen deficiency in areas of fields that were subject to water saturation. In some cases, inadequate availability and/or uptake of nitrogen may be the culprit. However, we have since determined that most of the fields exhibiting patches of stunted and yellowed plants are most likely affected by Wheat Soil-borne Mosaic Virus (WSBMV).  We have come to this conclusion based on samples from symptomatic fields that have tested positive for WSBMV using a very reliable antibody-based diagnostic test known as ELISA.  

Fig. 1.   Wheat field exhibiting areas of plant yellowing caused by Wheat Soil-borne Mosaic Virus.

WSBMV is transmitted to wheat by a very common soil-borne fungus, Polymyxa graminis. P. graminis itself does not harm wheat.  WSBMV symptoms first appear as crops begin to “green up” in late February to early March.  Leaves of affected plants exhibit a mild green to prominent yellow mosaic pattern (Fig. 2). Infected leaves may be somewhat elongated and have rolled edges; tillering of plants may be reduced.  Symptoms tend to fade as the weather warms up, but in severe cases plants will be permanently stunted. Symptoms caused WSBMV can occur throughout fields, but are usually most severe in poorly drained or low areas. This is because infection of seedlings by P. graminis is favored by high soil moisture.  
​The extent of yield loss caused by WSBMV depends on how much of the crop is affected and the severity of symptomatic plants.  In mild cases, yield effects will be barely noticeable.  However, if symptomatic areas persist and make up a significant portion of the field’s surface area, yield losses may be substantial.  Part of the dilemma at this time is the difficulty in knowing if affected fields should be destroyed and replanted to corn or soybean, or maintained in wheat.  With considerable financial resources yet to be spent on wheat this spring, as well as the cost of replanting, this is no small decision.  Unfortunately, there is no sure way to determine how much yield loss will be caused by WSBMV in affected fields.  However, fields like the one shown in Fig. 1 would be a good candidate for replanting.  

Fig. 2. Symptoms in wheat leaves caused by Wheat Soil-borne Mosaic Virus.

Decisions to manage WSBMV must be made in the fall, before planting.  The best and most consistent results are achieved by planting wheat varieties that are resistant to WSBMV (or “soil-borne viruses”). Delaying planting operations past the “Hessian fly-free date” in the fall can also help by limiting the extent of fall infections.  Fields with persistent WSBMV problems may be helped by improving internal and surface drainage of fields.          

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