More questions and concerns about winter wheat have been coming in over the last week, especially from southwestern Missouri. The virus diseases of winter wheat likely to occur in Missouri were covered in the March 17, 2006, issue of the Integrated Pest and Crop Management Newsletter. So far this season, wheat samples positive for wheat soilborne mosaic, wheat spindle streak mosaic and barley yellow dwarf have been confirmed. The majority of the positive samples have been from southwest and central Missouri.

Wheat soilborne mosaic and wheat spindle streak are favored by cool temperatures and symptoms tend to fade as temperatures increase. With the increase is temperatures over the state during the past week, I would not expect these two virus diseases to be particularly evident in wheat fields for the rest of the season.

The barley yellow dwarf virus is vectored by a number of different aphid species. Infection can occur in the fall; over the winter if winter conditions are mild and aphids are present and active; in the spring of the following year; and through the growing season if aphids remain active. Fall and early spring infections tend to have the greatest impact on yield. In addition to the purple to reddish-purple to reddish pink or even yellow discoloration from the leaf tips and margins in towards the center of the leaves, the plants may be stunted, may not tiller well, may have a rigid, upright growth form and may have reduced yield. Moisture stress and unseasonably high temperatures may be compounding the effects of barley yellow dwarf.

We have not yet received any wheat samples that have tested positive for wheat streak mosaic. This is the wheat virus disease vectored by the wheat curl mite. Hot, dry conditions favor both the wheat curl mite and wheat streak mosaic. It might be wise to watch for this virus disease as scouting wheat fields.

There have also been some reports of powdery mildew showing up in wheat fields in southwest Missouri. Last week’s issue of the Integrated Pest and Crop Management Newsletter contained descriptions of foliage diseases of wheat including powdery mildew. Powdery mildew is fairly easy to identify in the field. White powdery lesions or streaks develop on the leaf surface. Initially powdery mildew appears as distinct lesions but as the disease progresses the mildew growth may completely cover the leaves and stems of infected plants. Young lesions are white to a very light gray in color. Older mildew growth may have a dull gray to even grayish-brown color. Environmental conditions which favor powdery mildew infection are temperatures in the range of 59 to 72 degrees F and relative humidity between 85 to 100 percent. Free moisture on the leaf surface is not required. Powdery mildew tends to be more severe on wheat that is growing rapidly, wheat that has been heavily fertilized with nitrogen, wheat that is lodged and when growing conditions are cool and humid. Digital images from wheat fields in southwest Missouri show unmistakable symptoms of powdery mildew, yet the fields do not meet any conditions which would favor development of the disease listed in the previous sentence. Susceptible of the variety to powdery mildew may be a factor but I really don’t have a good explanation for why powdery mildew is showing up in those fields.

Much of the state had unusually warm temperatures last week (April 10-14). Although temperatures for the week of April 17-21 are forecast to be more normal for this time of year, it is important to monitor wheat fields for stage of growth. Because of the high temperatures last week, wheat may be moving along more rapidly than usual. Most of the foliar fungicides labeled for use on wheat have application restrictions of either Feekes growth stage 8.0 or Feekes growth stage 10.5. If a fungicide application is being considered, scouting for stage of growth to insure proper application timing will be important this season.

Laura Sweets
Ag Ext.-Plant Sciences
573-884-7307