Leaf rust of wheat
Leaf rust, also called brown rust, occurs across the U.S. and Canada and can cause severe yield loss in epidemic years. It is caused by the fungal pathogen Puccinia triticina. Leaf rust pustules rupture the leaf epidermis, but lack conspicuous tear marks. These pustules contain orange-brown, rusty colored spores. Later in the season, dark brown to black spores are produced which are not easily rubbed off the leaf surface. Pustules first develop on lower leaves and progress up the plant. Symptoms can be present from seedling stages through ripening. Leaf rust does not leave conspicuous tears on leaves as does stem rust. Leaf rust pustules are scattered, not in a linear pattern like stripe rust. Foliar symptoms of leaf rust. Image: C. Grau
Pustules of leaf rust in foliar tissue. Image: A. Friskop
The fungus causing leaf rust requires living tissue to survive. Thus, it does not overwinter in northern areas. Each year, urediniospores are blown north on wind currents from tropical areas and begin new infections when they land on leaves. High humidity and temperatures around 65-77°F favor disease development. When conditions are favorable, the infection cycle will continually repeat and result in secondary infections. Since inoculum has to build up in southern North America and blow north, yield loss will depend primarily on timing of spore migration and weather conditions when spores arrive in an area.
Host resistance is an important management tool for leaf rust, therefore the use of disease resistant varieties is recommended. Fungicides are labeled for the management of leaf rust. The decision to apply a fungicide is influenced by several factors such as varietal susceptibility, forecasted weather conditions and yield potential. If a fungicide is used, the application should be applied preventively and target the flag leaf (FGS 8-9).