Disease Report from Around the State

July 9, 1999
Common rust and gray leaf spot. Lots of other diseases are starting to show up around the state. Ria Barrido of Growmark in Bloomington reports common rust and gray leaf spot on field corn in central Illinois. It is a little early for both of these diseases, particularly common rust. But we have had pretty timely rains for fungal disease development in central Illinois. Fungicide application is recommended if infection reaches about 15 percent whole plant infection in the time period of 2 weeks before tasseling to 2 weeks after tasseling. Chlorothalonil (Bravo) and Propiconozole (Tilt) are recommended for common rust, and Tilt is recommended for gray leaf spot.

Anthracnose. I have had numerous reports of anthracnose leaf blight on seedling corn, mainly from the northern part of the state, but the crop seems to have gotten ahead of it now.

Phytopthora troubleshoot. Dave Feltes, Extension IPM Educator of the Quad Cities Extension Center, reports some soybean fields being replanted for the third time due to Phytopthora infection.

So the question becomes, "Should a fungicide seed treatment be applied?" The variety is resistant to one race of Phytopthora. It is not really an easy question to answer at this point in the season; actually, it should have been asked on the first replant or, even better, at planting.

Seedling resistance doesn't really kick in until about 5 days after germination, so fungicide seed treatments can be a tremendous aid in getting the seedling through this time period in a conducive environment. The real question remains, though, of whether the damping off was due to Phytopthora or Pythium seedling blight (see issue no. 5, April 23, 1999, of the Bulletin for a discussion of symptomology and treatment for these two diseases). If the damping off on all of these occasions was due to Pythium, then a seed treatment would likely be unnecessary because the Pythium species that cause us trouble are not too active when the soil temperatures get this warm. If the disease that caused the damping off on each occasion was Phytopthora, then fungicide seed treatment may still be beneficial, although at this point in the season I would opt for a variety with field tolerance to the disease if a seed treatment was used, or certainly a variety with resistance to more than one race. Varieties with field tolerance to Phytopthora are active against Phytopthora infection of mature plants rather than seedlings.--Suzanne Bissonnette

Author: Suzanne Bissonnette