Thus far this year the primary disease problems on winter wheat throughout the state have been virus diseases. Barely yellow dwarf is quite wide spread this year. Symptoms range from scattered plants showing discoloration of the flag leaf and leaf below the flag leaf to small, somewhat circular patches of plants that are stunted, more upright in growth, poorly tillered and discolored to large areas of fields that are showing symptoms. The barley yellow dwarf virus is spread or vectored by several species of aphids. At this point in the season it is difficult to determine if infection of wheat plants with the barley yellow dwarf virus occurred last fall, over the winter months, early this spring or later in the growing season. Although, the earlier that plants were infected with the virus, the more severe symptoms are likely to be and the greater the potential for yield loss. In parts of the state in which moisture stress has been severe some of the plants infected with barley yellow dwarf may now be showing a firing of the leaf tips and a more brown than purple discoloration of the leaf tips from a combination of moisture stress and barley yellow dwarf.

Scab or Fusarium head blight. Photo from IPM1022, Management of Soft Red Winter Wheat.

In addition to barley yellow dwarf, winter wheat samples submitted to the Plant Diagnostic Lab have also tested positive for wheat spindle streak mosaic virus, wheat soilborne mosaic virus and most recently wheat streak virus. Symptoms of wheat spindle streak mosaic and wheat soilborne mosaic usually occur early in the season when temperatures are cooler and then decline or fade away as temperatures increase. Wheat streak mosaic is favored by hot, dry conditions. The unusually hot, dry conditions throughout much of the state the first three weeks in April could explain the earlier than normal appearance of wheat streak mosaic.

Management options for winter wheat virus diseases are primarily preventative treatments. At this point in the season there are no effective management options.

Foliage diseases have not been very evident this season. It is possible to find a few lesions of Septoria leaf blotch in some fields. Powdery mildew has been found in some fields that had high levels of greenbug or other aphids earlier in the season. Leaf rust and stripe rust have not been very prevalent or severe. Most of these foliage diseases are favored by wet, overcast, humid and cloudy weather. The unusually hot and dry conditions for much of April have limited the development of foliage diseases on winter wheat. The recent change to cooler temperatures and rain over the past weekend might lead to the development of leaf rust, stripe rust or Septoria leaf blotch. The later in the season that these diseases develop the less likely they are to cause significant yield loss.

Over the last few days I have had several phone calls related to loose smut of wheat. Loose smut is quite easy to see in the field at this point in the season. The kernels on infected heads are replaced with masses of powdery black spores. So the heads have a very obvious, black, powdery appearance. These spores are eventually dislodged by wind and rain, so later in the season the smutted stems are less evident and only the bare rachis will be left. Spores produced on smutted heads now are being wind carried to adjacent plants in the field and infecting through the flowers. The fungus that causes loose smut survives within the embryo of wheat seeds. If infected seed is planted, the plants growing from those seeds will be infected and develop smutted heads the next season. If seed from a field that has a “small” amount of smut in one season is used for seed, the field planted with that seed may have a substantially higher level of smut. Loose smut is best controlled by planting either disease-free seed or using a systemic fungicide seed treatment. Now is also the time that we expect scab or Fusarium head blight to infect flowering wheat plants. See separate article in this issue for more information on Fusarium head blight.

Laura Sweets,
884-7307