Stem rust of wheat
Stem rust of wheat, also known as black rust, is a problematic disease worldwide. It is caused by Puccinia graminis subsp. graminis and P. graminis f. sp. tritici. A federal program and breeding efforts have kept the disease in check in the U.S. However, new variants of the stem rust pathogen virulent on many resistance genes currently used in the U.S. has been observed in Africa and the Middle East. Stem rust may infect any part of the aboveground portions of the wheat plant and is typically not found until after heading. Pustules containing red to orange spores erupt from stem and leaf tissue. The damage inflicted by the emerging spores tears leaf tissue, giving pustules a characteristic tattered appearance. Pustules can also be found on glumes and awns. Typically, stem rust is not found until after heading. Spores of the stem rust pathogen tend to be a deeper red color than those observed with leaf rust and stripe rust.
The stem rust pathogen requires living plant tissue to survive. The pathogen does not overwinter in northern areas of the U.S. and each year urediniospores are blown north along wind currents from southern regions of the U.S. Stem rust is favored by relatively warm daytime temperatures (77-85°F), wet weather, and prolonged dew periods. Each stem rust pustule produces thousands of spores that infect additional plants when favorable weather occurs. If this cycle continues, disease can reach epidemic levels in a short time. Infection cycles continue until host plant death or if unfavorable weather occurs. Yield loss is often dependent on timing of spore migration and disease onset.
Stem rust-resistant varieties of wheat are available. For spring wheat, early planting can help avoid infection during grain fill, reducing disease impact. Destroy volunteer wheat and barley plants after harvest as these serve as a green bridge for spores in the southern U.S. Fungicides are labeled for stem rust. The decision to use a fungicide is influenced by factors such as varietal susceptibility, forecasted weather, and yield potential. When necessary, fungicide application should be applied preventatively and target the flag leaf (FGS 8-9).
Gallery images: C. Grau, M. Burrows, and A. Friskop.