Powdery mildew of wheat
Powdery mildew is a common disease of wheat throughout the U.S. and Canada wherever winter wheat is grown. It is caused by Blumeria graminis f. sp. tritici and is an economic problem primarily in the eastern soft winter wheat region. The characteristic sign of the powdery mildew pathogen is fluffy, white to gray fungal growth on the top surface of leaves. Yellowish spots below the fungal growth can often be observed on the underside of leaves. Signs are generally most prevalent on lower leaves. If tillers are severely infected early, heads may not form. In susceptible varieties, stems and heads may also have mildew. When the disease is advanced, small, dark fungal fruiting bodies can form within fluffy growth. Symptoms can be observed from tillering stages through ripening.
Powdery mildew severity is strongly influenced by weather. Mild winters favor early and severe mildew development in winter wheat areas. Cool, humid, dry conditions favor infection, while prolonged periods of cool, humid weather in spring can allow the disease to reach the flag leaf and cause yield losses. Excess nitrogen, high plant populations, or other conditions that encourage heavy tillering can increase disease severity. Lighter, sandier soils are more conducive to disease.
Many varieties have complete or partial resistance to powdery mildew. By itself, even partial resistance is often enough to manage powdery mildew. In areas prone to severe epidemics, selecting resistant varieties is of primary importance. Several fungicides are labeled for the management of powdery mildew. The decision to apply a fungicide is influenced by varietal susceptibility, forecasted weather, and yield potential. Resistant or moderately resistant varieties will not require a fungicide. Fungicide should be applied soon after the disease appears. Triazole-containing fungicides are most effective because they have some systemic activity. Practices that encourage thick stands and high yields encourage mildew development; follow fertility recommendations and avoid over-fertilization.
Gallery images: J. Marshall, A. Friskop, M. Burrows, E. Byamukama, and C. Grau.