Wheat streak mosaic, High plains disease, and Triticum mosaic of wheat
Wheat streak mosaic, High plains disease, and Triticum mosaic are part of a virus disease complex found in most wheat growing regions of the U.S. and into Canada. These diseases are caused by Wheat streak mosaic virus (WSMV), High plains wheat mosaic virus (HPWMoV), and Triticum mosaic virus (TriMV), respectively. Co-infection of two or more viruses can increase symptom severity and yield loss over single infections. This virus complex causes major yield losses in both spring and winter wheat if infection is severe.
Symptoms vary depending on environment, plant stage at infection, infection by multiple viruses, and variety susceptibility. Symptoms can be observed from tillering stages through ripening. Plants with wheat streak mosaic typically exhibit stunting, and streaky discoloration of leaves. Discontinuous, yellow-green streaks run parallel to leaf veins with leaves eventually becoming completely yellow. Severe stunting and spread out stems result from infection at early growth stages. Mild mosaic of newest leaves gives way to older leaves that are more yellowed. Symptoms progress quickly during very warm conditions. High plains disease symptoms appear as small spots on leaves. Spots eventually turn into a yellowish mosaic and necrotic spots may form. Triticum mosaic is extremely difficult to discriminate from wheat streak mosaic. When severe, heads will be sterile or may not form. Plants may be severely stunted and can die. Grain may be shriveled. Laboratory diagnosis is required to distinguish these virus diseases from other diseases and disorders.
Feeding by wheat curl mites, the vector of WSMV, HPWMoV, and TriMV, results in leaf edges curling toward the midvein. This creates a protective space for the mites. The field distribution of this virus complex is often a very visible border effect or gradient extending from the wheat curl mite and virus source. When the source of the mites is within the field, there may not be a clear symptom gradient. Weather conditions conducive to volunteer crop establishment (pre-harvest hail storms) and mite reproduction (extended warm fall) and movement favor virus spread. Mites survive as adults, larvae, and eggs. Temperatures above 65°F with low relative humidity and winds above 5 mph are optimal for mite reproduction and spread.
Some virus-resistant wheat varieties exist. Manage volunteer plants that serve as hosts to the wheat curl mite. Create a two-week gap of host tissue availability (i.e., between wheat harvest and wheat emergence in the fall) to reduce the number of wheat curl mites. Avoid early fall planting of winter wheatand late planting of spring wheat for areas and seasons with high virus incidence. No acaricides are registered for management of wheat curl mites and insecticides are ineffective.
Gallery images: M. Burrows and E. Byamukama.