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| Figure 1. Southern rust pustules tend to develop on the upper leaf surface of corn. (IANR photos by Tamara Jackson) | Figure 2. Sometimes larger pustules and sporulation can occur on the lower leaf surface on or near the midrib. |
September 8, 2006
Plan order of field harvest accordingly
Southern rust may increase risk of stalk rots this year
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| Figure 3. Southern rust pustules on the leaf sheath. |
Over the past several weeks, southern rust (Figure 1) has become severe in some areas of Nebraska, particularly in the southcentral part of the state between York and Holdrege. Severe disease has been reported near Kearney and Clay Center in south central Nebraska where all leaves on some plants are more than 50% covered with rust pustules. Unfortunately, because of its late development, there is a low likelihood for an economic return for foliar fungicide applications in most producers’ situations, except in fields with higher value corn crops.
Southern rust is not an annual problem in Nebraska. The pathogen is favored by high temperatures and high humidity. Southern rust was first identified in July in southcentral Nebraska (see CropWatch story) following a week of daytime high temperatures that exceeded 100°F that was followed by rain. Temperatures and humidity remained high enough to support spore production and disease development for several more weeks thereafter and contributed to the current epidemic in some areas. Southern rust pustules tend to develop only on the upper leaf surface (in contrast to those of common rust which sporulate on both the upper and lower leaf surface); however, larger pustules and sporulation can occur on the lower leaf surface when it is on or near the midrib (Figure 2), as well as on the leaf sheath (Figure 3).
Many people have complained of "turning orange" during scouting trips to corn fields. The orange discoloration is from the large volume of fungal spores produced on the leaves’ surfaces by the fungus that causes southern rust. There may be enough spores in heavily infested corn fields to aggravate respiratory conditions in people predisposed to such ailments. It is recommended that these individuals wear a dust mask when spending time in these fields and during harvest.
Severe infections of southern rust can cause early senescence of leaves. Grain fill is a priority for the plant, so when leaves are no longer available to provide resources, it will cannibalize resources from the stalk. This will compromise the integrity of the stalk and weaken it, which may lead to stalk rots and lodging. It is a good idea to monitor fields with heavy infestations of southern rust (and any other foliar disease) for stalk rot damage by scouting and testing stalk strength with the "push or pinch" test on at least 100 plants. Fields with more than 10% stalk rot should be harvested first, before plants lodge, to minimize further yield loss.
Tamra A. Jackson
Extension Plant Pathologist
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| 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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