What’s Up With Crop Diseases

06/30/2011 @ 10:03pm

This year’s soggy weather in many locations has set the stage for future foliar diseases this growing season. Mark Carlton, an Iowa State University Extension agronomist, outlined the situation in Iowa at the ISU Southeast Iowa Research Farm near Crawfordsville, earlier this month.


In soybeans, watch for these foliar diseases: 

1.    Brown spot

2.    Frogeye leaf spot

3.    Cerospora leaf spot

Fungicides can help protect soybeans from these diseases. However, there are no thresholds developed. Start scouting at R1 (beginning flowering) and watch for disease movement and rainy cold weather. These factors can increase the odds of a positive payback from fungicides. Carlton notes there were cases last year where timely fungicide applications at R3 (beginning pod) had an 8 to 10 bushel per acre payoff.

Fields with continuous no-till beans, particularly with frogeye leafspot, also favor disease development.


In corn, the decision is more clear-cut. “We need to get out in the fields and scout,” he says. When corn is at the V14 to V15 stage, when corn is up around eye height, scout up to tasseling.”

In the case of gray leaf spot (GLS), the threshold developed in the mid-1990s are lesions on the third leaf below the ear leaf or higher on 50% of the plants at tasseling. This threshold is being reevaluated due to new hybrids and fungicide products that have been developed since them.

For now, though, the threshold stands. It’s also important to consider high-risk cases, such as fields in continuous corn with high residue in river bottom areas. Hybrid ratings for GLS resistance should also be taken into account.

“Timing is important,” says Carlton. The optimum time to apply fungicides is between tasseling and the R1 (silking) stage.