August is time to scout for corn, soybean diseases

August is time to scout for corn, soybean diseases

Fields in Iowa aren't showing much incidence of corn disease so far this summer, but keep an eye out.

Fields across Iowa are not showing much incidence of corn diseases this summer—not yet anyway. Except for common rust, according to reports as of early August from Iowa State University Extension field agronomists around the state.

“We are still NOT finding much for leaf diseases in corn with the exception of for common rust disease,” says ISU’s Brian Lang. Based at Decorah, he covers a wide area of northeast Iowa. “But now, as we move into August, you need to keep scouting fields at least once a week to be on the lookout for symptoms of disease development.”

Keep an eye out for symptoms of corn leaf diseases

SYMPTOMS: Northern Corn Leaf Blight is a fungal disease producing long, cigar-shaped grayish or tan lesions. NCLB likes cool, wet weather and was prevalent in Iowa the past two years. With this summer’s hotter weather, gray leaf spot and Southern Rust disease may be more likely to show up.

Northern Corn Leaf Blight is a disease that requires cool weather (daytime temps in the high 60, low 70 degrees F). Eyespot also prefers 70 degree daytime temps, thus not much eyespot is being found either.  Gray Leaf Spot (GLS) requires very humid warm weather (80 degree daytime and greater than 90% humidity). “Historically, significant problems with GLS are most commonly found in southeast Iowa, but we have seen this disease at times in susceptible hybrids north of Hwy. 20 in northern Iowa,” says Lang.

Common Rust favors night temperatures of 65 to 70 degrees, moderate daytime temperatures, and high humidity. “Our current weather pattern matches up well with this, but in general common rust disease is not considered a very serious threat to yield reductions in hybrid corn compared with more serious threats to seed corn and sweet corn. Check fields and check your hybrid corn disease resistance ratings,” he advises.

Follow these scouting tips to watch for leaf diseases in corn

Scout now for leaf disease presence using the following check list for risk assessment and judgement whether to treat or not with a foliar fungicide:

1)  Scout for ANY disease presence found on the third leaf below the ear leaf on up the plant on greater than 50% of the plants.  NCLB and GLS are potentially more serious diseases than eyespot and common rust.  Southern Iowa also needs to be concerned with southern corn rust (80 degree daytime and high humidity), a much more serious disease than Common Rust which is most commonly found in our part of the state.

2)  Is the field continuous corn with overwintering corn residue on the soil surface (reduce till or no-till)?

3)  Is the hybrid susceptible to the disease(s) in question being found on the leaves?  If the hybrid is moderately susceptible to the disease(s) in question, consider disease history in the field, crop residue and weather forecasting into August for conditions favorable or not for disease development. For photo ID of corn leaf diseases, go to: pages 28-33.

What’s going on with soybean diseases this summer?

Bacterial blight has picked up, but this is not a fungal disease.  This website compares and contrasts Bacterial Blight to Septoria Brown Spot

Septoria Brown Spot appears to some degree every year in every field.  It starts in the lower canopy. As long as it does not advance up into the mid-canopy or higher in July, it’s not considered a problem.

Cercospora Leaf Blight, starting in August, scout for this disease, particularly in the northern-most counties in Iowa. Fungicide treatments in R4 to early R5 can be effective on this disease, however, many varieties also have good resistance to this disease.  Here’s some photos and additional information about this disease from University of Wisconsin

Sudden Death Syndrome is showing up in some bean fields

The continuation of a wetter than normal summer should enhance SDS disease development in soybeans. There are no treatments that can be applied at this time, but take notes and adjust management accordingly for future years of soybean production in these fields (resistant variety, seed treatment, crop rotation without corn or soybeans for at least two years).

New ‘Farmer’s Guide’ Books on Corn & Soybean Diseases: If you are looking for a helpful, easy to understand resource on corn and soybean diseases, here are two new publications specifically for farmers. They provide easy-to-read disease listings and overviews, basic instructions for using diagnostic and scouting tools and non-technical management recommendations. Each book is about 150 pages with color photos. Visit Click on the links within to order.

TAGS: Extension
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