Figure 1. Common rust (Puccinia sorghi) develops on some corn every year in Nebraska. It is characterized by the development of brown to cinnamon red pustules that are sparsely scattered across both the upper and lower leaf surfaces. (IANR photos by Tamra Jackson)

Figure 2. Southern corn rust (Puccinia polysora) is less common in Nebraska than common rust. Southern rust pustules tend to be limited to the upper leaf surface and are clusters of pustules ranging in color from orange to tan.

Figure 3. Southern rust pustules can develop on corn leaves in dense clumps. Often they are surrounded by yellow halos that may be more visible when held up to the sunlight.

Southern rust identified in some Nebraska corn fields

Like in wheat, there is more than one rust disease of corn. In contrast to southern rust, common rust (Figure 1) develops every year on corn in Nebraska to some extent. Common rust , caused by the fungus Puccinia sorghi, is characterized by the development of brown to cinnamon red pustules that are sparsely scattered across both the upper and lower leaf surfaces. Southern rust (Figure 2) can be distinguished from common rust by the development of pustules that are predominantly on only the upper leaf surface and tend to be more orange to tan in color. Southern rust pustules can develop in dense clumps and are often surrounded by yellow halos that are more visible when held up to the sunlight (Figure 3).

Sometimes southern rust can cause severe and early senescence, resulting in yield loss if it develops early and weather conditions remain favorable. Southern rust can develop rapidly and large quantities of spores are produced in the pustules that cause repeated infections. Yields in susceptible hybrids may be compromised with as few as two lesions per leaf on the ear leaf or above on at least half of the plants in a field.

In seed production fields, a timely fungicide application is often cost effective, but in typical commercial production areas, it may not be. Strobilurin fungicides, such as azoxystrobin and pyraclostrobin, act as protectants and/or local systemics and provide the best control if applied preventatively or soon after infection. Triazole fungicides, such as propiconazole, act curatively and may provide control if they are applied after infection.

A major constraint for managing southern rust is its tendency to develop late in the season after the treatment window for some fungicides. When choosing a fungicide, read the label closely. Some fungicides may not be applied after silks turn brown and may suggest higher rates for some diseases.

Tamra A. Jackson
Extension Plant Pathologist

Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.

Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.

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