Southern rust confirmed in parts of Nebraska
click image to zoomPhotos courtesy of Casey Schleicher, UNLFigure 1. Southern rust spores are typically orange to tan in color and produced in pustules predominantly on the upper leaf surface (Figure 1a, left), although they can also be produced abundantly on/near the midrib on the underside of the leaves (Figure 1b, right). Southern rust (Figure 1) was confirmed on corn recently (July 5) in samples submitted to the University of Nebraska-Lincoln Plant and Pest Diagnostic Clinic and Pioneer Hi-Bred laboratory from Adams, Clay, Fillmore, Hall and Thayer counties in south central Nebraska and Burt County in northeast Nebraska. These samples were from pivot-irrigated fields that had very low incidence of disease at this time. Warm temperatures and high humidity may promote development and spread of disease.
Rust diseases produce large amounts of spores that can be easily moved by wind over long distances. Having a history of southern rust in corn does not affect current disease development because this pathogen does NOT overwinter in infected residue. The spores are carried into a field by wind from southern or western diseased areas. If the disease continues to spread and worsen, those fields planted later are at higher risk for disease and potentially severe yield impacts. We recommend scouting fields, especially those at higher risk, particularly later planted fields that are overhead irrigated and/or in south central Nebraska.
The characteristics used to differentiate common rust and southern rust are described and illustrated in the NebGuide, Rust Diseases of Corn in Nebraska. The simplest and most reliable way to differentiate the diseases without a microscope is to examine both leaf surfaces for spore production. Southern rust spore production is usually limited to the upper leaf surface and tends to be tan or orange in color. The most reliable way to identify corn rust diseases is through microscopic examination of spore characteristics. For this analysis, submit samples to the UNL Plant and Pest Diagnostic Clinic.
click image to zoomPhotos courtesy of Casey Schleicher, UNLFigure 2. Common rust also has begun to develop at low incidence and severity in Nebraska and can be confused with southern rust, a more aggressive and damaging disease problem. Common Rust
Common rust (Figure 2) has also begun to develop in some areas of the state, but at very low incidence and severity. Recent very high temperatures likely slowed the progress of this disease. (This is different than southern rust which can be exacerbated by warmer night temperatures and historically been the much more aggressive rust disease in Nebraska corn fields. Common rust spores are usually brick-red to brown in color, however, the color difference is not a reliable method for identification unless both diseases are available for comparison. Also, the spore type can change and, with both diseases, turns black later in the season.