Barley yellow dwarf continues to be the most widespread disease of winter wheat in Missouri this season. Over the last two weeks or so, red to purple flag leaves have been evident in wheat fields across the state. In some fields the disease appears to be quite widespread and in other fields it is sporadic within the field. Yield losses will vary with time of infection, susceptibility of the variety, other stresses which might have affected plants and weather conditions from now to harvest. At this point in the growing season it is too late for any control measures.
Much of the state has been dry, especially over the last few weeks, so foliage diseases have been slow to develop and to move onto the flag leaves. Susceptible varieties may be showing Septoria leaf blotch, leaf rust and in some cases stripe rust. When foliage diseases build up so late in the season, impact on yield is minimized. Again, at this point in the growing season it is too late for any control measures.
Fusarium head blight or scab of wheat develops on plants in the flowering to early grain fill stages of growth and the symptoms are most evident at that time. The wheat crop is still about three weeks ahead of normal and in many areas wheat heads are beginning to dry down. So the time for possible infection by the Fusarium head blight fungus is past. Symptoms of Fusarium head blight are more difficult to detect once heads begin to dry down. Infection is very dependent on environmental conditions while wheat is in susceptible stages of growth, i.e. flowering. Moderate temperatures in the range of 77-86°F, frequent rain, overcast days, high humidity and prolonged dews favor infection and development of scab. Weather conditions over the past several weeks were not particularly favorable for the development of Fusarium head blight in most of the state. There may have been isolated geographic areas which did receive rain at the critical time of wheat flowering so Fusarium head blight may be a problem in localized areas this year but doesn't appear to be a statewide issue.
The characteristic symptom of scab on wheat is a premature bleaching of a portion of the head or the entire head. Superficial mold growth, usually pink or orange in color, may be evident at the base of the diseased spikelets. Bleached spikelets are usually sterile or contain shriveled and or discolored seed.
Scab is caused by the fungus Fusarium graminearum. This fungus overwinters on host residues such as wheat stubble, corn stalks and grass residues. Spores are carried by wind currents from the residues on which they have survived to wheat heads. If environmental conditions are favorable, i.e. warm and moist, the spores germinate and invade flower parts, glumes and other portions of the spike. Scab infection occurs when favorable environmental conditions occur as the wheat crop is in the flowering to early grain fill stages.
Unfortunately, the detrimental effects of scab are not limited to its adverse effects on yield. The fungi which cause scab may also produce mycotoxins. Vomitoxin (deoxynivalenol or DON) and zearalenone may occur in wheat grain infected by scab fungi. This is a primary concern where grain is fed to non-ruminant animals. Ruminants are fairly tolerant of these two mycotoxins. Also, the fungi which cause scab may survive on the seed and can cause seedling blight and root rot problems when scabby grain is used for seed.
Crop rotation, variety selection, residue management and fungicide applications are preventative measures for managing scab in wheat. With the early wheat crop it is too late for fungicide applications this season. Wheat planted on corn, sorghum or wheat residue (even wheat double cropped with soybeans) has a greater risk for scab.
REVISED: May 21, 2012