Small Grain Disease Update

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Wheat virus diseases can be hard to diagnose especially when wheat is not growing quickly due to the cool weather. Both soilborne viruses, soilborne wheat mosaic and wheat spindle streak mosaic virus can cause stunting of plants at this time of the year. Varying degrees of mottling and yellow streaks or dashes may be present. It is hard to generalize what the patterns in the field might look like, but wheat spindle streak can be uniform across the field or in scattered spots that generally coincide with low spots in the field. Soilborne wheat mosaic virus is seen typically in large irregular spots in low or poorly drained areas of the field. A wheat sample sent to the lab last week was positively identified with wheat soil born mosaic virus by ELISA testing at Agdia, Inc. For a list of wheat varieties and their ratings for susceptibility to both virus diseases go to http://www.rec.udel.edu/Update09/wheatdiseaseresistance08.pdf. There is no control for these diseases other than planting resistant varieties. When warm weather returns the plants will generally grow out of the symptoms but the effect on yield can be very variable depending how early the plants were infected. If the symptoms persist until the flag leaf emerges, more yield loss can be expected. For additional information on soilborn wheat mosaic virus and wheat spindle streak mosaic virus see this factsheet from University of Maryland http://www.rec.udel.edu/Update09/wheatsoilbornvirusesMD.pdf.

Another disease that has been seen in the lab is Ascochyta leaf spot. This disease is caused by a very weak pathogenic fungus that infects winter damaged leaf tissue primarily. Most fieldmen and growers get excited when they see this because it looks like Septoria leafspot which does not occur until much later in the season in this region. Ascochyta occurs primarily on lower leaves and begins as small chlorotic flecks that then develop into elongated gray-brown spots that can resemble Septoria nodorum spots. This disease does not warrant fungicide applications and usually disappears once wheat growth resumes wholeheartedly.

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