The unusually mild winter in much of Missouri could have given some winter wheat diseases a slight advantage so it would be worthwhile to check wheat fields for signs of foliage diseases and virus diseases. Septoria leaf blotch, powdery mildew and leaf rust can develop on young plants during the fall months. In most years, fall infections of these diseases are minimal and cold winter temperatures prevent further disease development. This year with the unusually mild winter temperatures, it is possible that these foliage diseases could have remained active through the winter and symptoms or signs of these foliage diseases might be found if fields were scouted now.
Septoria leaf blotch lesions begin as light yellow flecks or streaks. These flecks expand into yellow to reddish brown, irregularly shaped blotches. As the lesions mature, the centers may turn lighter gray in color. Dark brown specks (fruiting bodies or pycnidia of the causal fungus, Septoria tritici) may be scattered within the centers of mature lesions. Lesions may coalesce killing larger areas of leaf tissue.
Powdery mildew infections begin as light green to yellow flecks on the leaf surface. As powdery mildew develops, the leaf surfaces become covered with patches of cottony white mold growth of Erysiphe graminis f. sp. tritici, the causal fungus. These patches eventually turn a grayish white to grayish brown.
Leaf rust lesions appear primarily on the upper leaf surfaces and leaf sheaths. Initially, lesions are small, yellow to light-green flecks. Eventually, leaf rust appears as small, circular to oval-shaped, orange red pustules. These pustules break open to release masses of orange red spores of Puccinia recondita.
Again, although these diseases may occur in the fall on young plants, typical Missouri winter temperatures usually slow or halt disease development. This year's mild winter temperatures could have allowed any disease which started last fall to remain active through the winter months and symptoms might be evident now. Leaf rust is the least likely of the three to be a problem because our inoculum for this disease comes from southern states such as Texas and Oklahoma. The drought conditions crippled those states during 2011 meaning that little leaf rust developed during the 2011 season so little inoculum was available to be wind carried to Missouri.
If any of these are present in wheat this spring, how much of a problem they will be and how much yield loss they might cause is dependent, in part, on weather conditions now through heading and harvest. These foliage diseases are favored by warm, wet or humid weather. A wet spring with frequent rains, overcast days and heavy dews is more likely to result in higher levels of disease incidence and severity and greater risk of yield loss. A warm, dry spring would reduce the risk of disease development and thus for yield loss. Of two recent forecasts for 2012 spring weather conditions in Missouri, one calls for a our spring to be a bit warmer and dryer than normal while the other suggests a normal to slightly wetter-than-normal spring.
Perhaps the best recommendation is to scout fields now to determine if any foliage diseases are present, to access their severity, to follow weather conditions in your area and to be prepared to apply a foliar fungicide if disease pressure and weather conditions warrant such a treatment.
The other wheat diseases which might be showing up are virus diseases such as barley yellow dwarf, wheat spindle streak mosaic and wheat soilborne mosaic. Barley yellow dwarf is the virus disease which is vectored by numerous aphid species including the oat bird cherry aphid. Symptoms may develop in the fall or early spring, especially if temperatures have been mild and aphids active. Symptoms include leaf discoloration ranging from a light green or yellowing to a red or purple discoloration of leaf tissue. Discoloration tends to be from the leaf tip down and from the leaf margin in toward the center of the leaf. Plants may be stunted or have a rigid, upright growth form. Symptoms may be more severe and yield losses higher if plants are infected in the fall or early spring. Infections developing in the late spring or summer may cause discoloration of the upper leaves, especially the flag leaf, but little stunting of plants or yield loss. At this point in the season, monitoring aphid levels and taking appropriate actions to control aphids would be the principle management strategies. Maintaining good plant vigor with proper fertility may also help minimize the impact of barley yellow dwarf.
Wheat spindle streak mosaic and wheat soilborne mosaic symptoms typically show up during spring greenup and are most pronounced when air temperatures are around 50 F. With wheat spindle streak mosaic, yellow green streaks or mottling develop on the dark green background of the leaves. These lesions usually run parallel to the leaf veins and tend to be tapered at the ends giving the lesions a spindle-shaped appearance. Plants may be slightly stunted, off-color and have fewer tillers than normal.
Wheat soilborne mosaic causes light green to yellow green to bright yellow mosaic patterns in leaf tissues. Symptoms of wheat soilborne mosaic are not always distinctive and might occur as a more general yellowing, similar to that caused by nitrogen deficiency. Infected plants may be stunted and slow to green up in the spring.
Both wheat spindle streak mosaic and wheat soilborne mosaic tend to be more severe in lower, wetter areas of fields. Symptoms of both diseases are more obvious early in the spring with cooler air temperatures and tend to fade as air temperatures increase late spring to summer. In years with extended periods of cool spring temperatures, symptoms may be evident for longer periods of time and the possibility for impact on yield may increase. Proper fertility may help reduce the impact of these virus diseases on wheat.
REVISED: February 24, 2012