Fungal Leaf Disease Showing Up in Corn

June 30, 2000
Despite some earlier problems with dry conditions, that's certainly not the case now, especially in the northwestern part of the state where several fungal diseases are making themselves very obvious.

Common rust on field corn is here. We rarely worry about common rust causing many problems on the field corn. Why? Because it doesn't usually show up with any density until we get into late July, and that is past the point when it will make much difference in terms of final yield. Usually the only people who see it build up with any intensity are those that plant their favorite susceptible sweet corn right next to their field corn, and the sweet corn serves as an inoculum (spore) source to infect the field corn. However, it seems that about every 5 or 6 years it shows up early in the season in field corn and causes concern. Kevin Black and Paul Klemme of Novartis and Matt Montgomery of the Sangamon/Menard Extension unit report active common rust in the northwestern and west-central parts of the state.

Common rust on field corn.

Our usual recommendation for fungal leaf diseases is to scout for them during the 2 weeks before tasseling to 2 weeks after tasseling. At that point, you would be looking for about 15% whole-plant infection to justify a fungicide treatment. Well, we aren't to that point yet, so how can you make a reasonable decision about treatment? First, consider the weather. Fungi in general and this one in particular need free water (on the leaves) and continued wet weather to continue to flourish. Next, consider the probability of other fungal leaf blights developing in the field. Knowing what type of resistance is in the hybrid is helpful to answer this. Is the field continuous corn with high residue? Fungal leaf blights are more likely to develop under those circumstances.

Why do you need to know all this other stuff to decide whether or not to spray field corn for common rust now? You need to know the probability of other fungal diseases developing because there's not a big arsenal of fungicides for treatment. We recommend chlorothalonil (Bravo, Zeneca) or propiconazole (Tilt, Novartis) for common rust, the drawback being that chlorothalonil is registered only for sweet corn and seed corn, and 16 ounces (four applications prior to brown silk) of propiconazole can be applied. So, spraying this early in the season may leave you with few options or none when the "other" leaf blights become active. The price of corn and cost per application also need to be considered in your decision.

So, where does that leave us? The probability of continued wet weather is the hardest thing to be sure of, but rain is forecast as I write. The probability of other leaf blights developing is good especially in high-residue situations. The fungicide treatments are effective if applied in a timely manner. Application and fungicide cost money. The price of corn is low. The risk is greater in seed corn than in field corn.

Brown spot and anthracnose are also showing up in the same areas of the state. Paul Klemme also reported eyespot in Ogle County. More on how these diseases are developing will be in next week's Bulletin.--Suzanne Bissonnette

Author: Suzanne Bissonnette

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

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