University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

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Figure 1. Aspergillus ear rot, characterized by a light green mold, can develop into the mycotoxin aflatoxin. Figure 2. Charcoal stalk rot is indicated when internal stalk tissues turn gray-black. While not common in Nebraska, it is usually associated with drought-stressed fields.

August 11, 2006

Potential impact of recent heat and drought stress on corn diseases and mycotoxins

Despite the much needed rain that most of Nebraska received this week, the damage caused by recent drought and extreme heat might be irreversible in some fields. Many of the diseases that we are typically concerned about require wetness and humidity to develop, so we have avoided them. However, several important diseases can occur after periods of heat and drought stress. One of them is southern rust, caused by Puccinia polysora. It was discussed in an article in the July 28, 2006 issue of CropWatch,

The high temperatures and drought experienced by much of Nebraska in recent weeks increases the risk of aflatoxin contamination in this year’s corn crop. Severe growing conditions in 2002 contributed to the contamination of more than 30% of the corn in southeast Nebraska with aflatoxin at concentrations that exceeded the action levels recommended by the U. S. Food and Drug Administration for food grade corn intended for human consumption that year.

At high concentrations, aflatoxin can cause serious health effects in humans and livestock that consume contaminated grain, making it important to monitor. The fungus that produces aflatoxin is called Aspergillus flavus and causes a common ear rot and grain mold. The grain mold can be recognized by the growth of patches of yellow-green mold (Figure 1), often near the ear tip in association with wounds created by insect feeding. However, the presence of grain mold is not well correlated with aflatoxin contamination, so identifying the grain mold in your crop does NOT necessarily indicate that aflatoxin will be present, too. Laboratory testing is necessary to definitively diagnose aflatoxin contamination. Corn in some Nebraska fields that sustained damage may be at higher risk for aflatoxin this year.

Figure 3. Gray leaf spot of corn. (IANR Photo by Tamra Jackson)

Another disease, charcoal rot, caused by the fungus Macrophomina phaseolina, is also favored by hot dry conditions and can occur in Nebraska. This pathogen produces tiny black microsclerotia inside plant tissue that give the infected stalk the appearance of having charcoal dust (Figure 2). This fungus has a wide host range and also can infect sorghum and soybean with similar symptom development.

These are just two examples of diseases in corn that can be favored by heat and drought stress. We can’t predict how this week’s rain will impact the development of these diseases, but the increased humidity will likely increase some of our foliar diseases, such as gray leaf spot (Figure 3).

Tamra Jackson
Extension Plant Pathologist — Lincoln

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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