Wheat fields with symptoms of wheat streak mosaic virus (WSMV) infection have been reported in the southwestern part of the state. The effect of the disease depends on environment, vector survival, and distribution and cultivar sensitivity.|
Winter wheat with symptoms of WSMV. (Photo by R. Bellm.)
Symptoms: Initial foliar symptoms of wheat streak mosaic virus (also known as yellow mosaic virus) typically show up in the spring. The pattern of the disease in the field is tied to the distribution of its vector, the wheat curl mite (Aceria tulipae). Affected wheat plants are typically stunted with mottled, streaked leaves. The streaks consist of yellow, discontinuous dashes running parallel to the veins.
Foliar streaking symptomatic of WSMV. (Photo by R. Bellm.)
We don't typically see the "leaf rolling" symptom in Illinois, but it has been reported in Randolph County so far. Heavily mite-infested leaves tend to remain upright, and the margins of the leaf may roll inward. Symptoms tend to worsen as the weather warms up, and severely infected plants may produce sterile heads or die. Yield loss is related to when infection took place. Fall-infected plants can experience severe yield loss; early-spring infection, light to moderate losses; and infection after jointing, minimal losses. Keep in mind that two other virus diseases in Illinois have very similar leaf mosaic symptoms: soilborne wheat mosaic virus (SBWMV) and wheat spindle streak virus (WSSV). For a discussion of these other virus diseases, refer to issue no. 2, 2000, of the Bulletin.
Vector and spread: Wheat curl mites are small, approximately 0.03 millimeters long. They are usually white and cylindrical with four diminutive legs near their head. The wheat curl mite thrives on lush young wheat and many grass hosts. The mite can develop from egg to adult within 8 to 10 days. In a favorable environment, 75° to 80°F, mites will reproduce the most rapidly. Dry conditions also favor mite development. The wheat curl mite is dispersed by wind. The mite can overwinter as an egg, nymph, or adult either in the winter wheat crown or other grass host. They are able to survive near-freezing temperatures for several months. There are no chemicals that are labeled for or effective against the wheat curl mite. Management is aimed at their weakness, which is the need for live plant material.
Hosts: Although wheat is the preferred host for the wheat curl mite and virus reproduction, many other grasses are good hosts, including corn, oats, cheat grass, green foxtail, barnyard grass, prairie cupgrass, crabgrass, and witch grass, as well as others. Volunteer wheat is the main culprit in wheat-growing areas that are significantly affected by this disease. But corn and the other grasses are excellent hosts as well. The mite needs live tissue to survive after the wheat crop is harvested, and that green tissue is typically found in Illinois as corn or other grass hosts. In the absence of living leaves, the mite will survive only a few days. The need for living tissue tells quite a bit about what the field pattern of the disease will look like. The disease spreads out usually from the edges of the field that were adjacent to "green corn" or grass hosts in the fall or just grass hosts in the spring. The pattern is generally very similar to what is seen with spider mites on soybeans. Corn, by the way, is not affected by infection with WSMV or by the wheat curl mite.
Management: Management of WSMV consists of trying to interfere with the life cycle of the wheat curl mite. This is attempted through planting after the Hessian fly-free date, control of weedy grass hosts, and not planting wheat while green corn (which may serve as a reserve for the virus and the wheat curl mite) is still in the area. Varieties are available with WSMV resistance.
Diagnosis: Although the presence of the wheat curl mite is a pretty good indication that WSMV is present, a definitive diagnosis of a virus disease can be made only with a serological lab test. The Plant Clinic at the University of Illinois can only make a visual estimation of the presence of a virus in a wheat plant. We cannot tell you which virus is actually present based on the visible symptoms. To have a virus identified, it is necessary to send it to a lab for serological testing. The cost of this procedure is $25 per specimen, if you desire to know exactly which virus is present; please indicate this on any specimen forms sent to the clinic. Fresh plant material is needed for serological analysis because the tests use fresh plant sap.--Suzanne Bissonnette