 |
 |
|
|
||||
|
||||
Wheat Streak Mosaic
ExEx 8045Wheat streak mosaic is an important disease of wheat, particularly in the central Great Plains of the United States, where each year it affects a portion of the winter wheat crop. In addition to winter wheat, wheat streak mosaic also can occur on spring wheat, barley, oats, rye, corn, other crops, and a number of annual and perennial grasses, although wheat is the most important host. In South Dakota, wheat streak mosaic is a concern not only in traditional winter wheat production areas, but also in spring wheat areas where winter wheat is also grown.
Although infection often takes place in the fall, symptoms of wheat streak mosaic in winter wheat generally do not appear until spring, as temperatures rise and plants begin to grow. Affected areas of fields have an overall yellow, stunted appearance. Individual plants, in addition to stunting, have scattered green-yellow streaking, striping or mottling of the leaves parallel to the veins. Eventually the entire leaf may turn yellow, brown and die. Cool, moist weather in the spring reduces severity of the problem, while hot, dry weather intensifies symptoms and increases yield loss.
The causal agent of wheat streak mosaic is wheat streak mosaic virus (WSMV). WSMV survives only in green, living tissue, and cannot propagate itself in or be spread from dead plant material or stubble. Winter wheat, and in some cases other winter annual or perennial weeds, are important in the over-wintering of the virus, while these and other spring crops and annual weeds may be involved in persistence throughout the season. In the field, the natural means of transmission of WSMV is by the wheat curl mite, Aceria tulipae Keifer, although it can also be transmitted mechanically. Direct feeding damage of the mite appears as trapping and curling of leaves emerging from the whorl. The mite is extremely small and crawls very slowly, so movement from plant to plant depends on the wind. Mite populations build up most rapidly under warm, dry conditions. While very hot weather is not favorable for the mite, such conditions may contribute to spread of the virus by increasing movement of mites from stressed plants. Infected plants often appear near the edges of fields as the virus-laden mites blow in from an adjacent field or strip. Where volunteer wheat was a problem in a field prior to planting, infection may be spread throughout much of the field.
Wheat streak mosaic is controlled primarily by breaking the life cycle of the mite vector. Planting date and elimination of volunteer wheat are important. At least one to two works prior to planting winter wheat in the fall, volunteer wheat should be destroyed in any fields to be planted and in adjacent fields. Complete control is essential. Since the mite and virus survive only on green tissue, they will die out during this period. The same applies to spring wheat if any volunteer wheat remains in the spring. Along with volunteer control, nearby crops should also be considered. Corn is an efficient host of both the mite and virus. Winter wheat should not be planted next to corn that has not yet matured. If such planting is done, it should be as late as possible. In the case of spring wheat, growers wheat fields which may have the disease present.
Later planting of winter wheat helps avoid mosaic infection. Not only is the time period between harvest of current season crop and emergence of new winter wheat extended, but mite populations will be decreasing late in the season. For South Dakota, recommended planting dates are September 5-10 in the north, September 15 in the central part of the state, and September 20 in the south. Planting on or after these dates will normally reduce wheat streak mosaic pressure, yet allow sufficient growth of the wheat plants prior to dormancy. Spring wheat planting should be done as early as possible to avoid mite populations that may build up during the season. Finally, wheat varieties do differ in their reaction to wheat streak mosaic. While complete resistance does not exist, some winter wheat varieties, including Abilene, Agate, Brule, Sage and Scout 66, express some tolerance to the disease, and will perform better relative to other varieties under heavy disease pressure. Most spring wheat varieties are also very susceptible to wheat streak mosaic. However, data from North Dakota suggests that some varieties, including Butte, Oslo and 2369, have some tolerance.
by Dale J. Gallenberg, Extension Plant Pathologist
|
Plant Science Department, South Dakota State University |