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July 6, 2001
Corn is tasseling or starting to tassel. This is the time (2 weeks prior to tasseling and up to 2 weeks after tasseling) to scout for foliar disease in corn. Fortunately for the grower, but unfortunately for the pathologists, the weather has been too dry in most areas for much disease development this year. Leaf blights and rust on corn were prevalent last year. If foliar diseases develop in the next few weeks, remember that a threshold exists. For leaf blights, including the rust fungi, it is not the individual identity of the blight that is important but rather the percentage of leaf area blighted as a whole on the entire plant. Control considerations may be justified when whole-plant infection reaches 15%. This will depend on the price of corn and the price of the fungicide. Fungicides are more useful and cost-effective in hybrid seed production. It is also important to remember that the type of symptoms or size and coloration of lesions for leaf blights on corn can vary with the genetic resistance of the hybrid that you plant. Refer to issue no. 13 of the Bulletin, "Plant Disease Musings," for more information and considerations about fungal leaf blights.

Figure 1 illustrates the percentage of leaf area blighted by southern corn leaf blight. Regardless of the leaf spot or blight, this should help with assessing the percentage of leaf tissue infected.

It is good once in a while to realize just how well a crop can yield in the absence of disease. Omar Koester, Monroe and Randolph counties, reported 80 bushels per acre yield from 'Clark' wheat, in the absence of wheat leaf rust. This helps keep in perspective all those management practices for disease control that we strongly encourage. Disease will develop given the appropriate amount of time, if the pathogen is present on a susceptible host, under favorable environmental conditions. When management practices or Mother Nature removes one of the four criteria necessary for disease development, disease is absent and yield is good.--Loretta Ortiz-Ribbing

Author: Loretta Ortiz-Ribbing

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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