Score your cornfield for disease risks

Should you plan on corn fungicide treatments in 2010? The answer isn’t as simple as “How much does it cost?”

It has to be more than a “spray and pray” decision, points out Del Voight, Penn State Extension crops agronomist. Yield responses reported in university and seed company test plots have been highly variable.

“In Pennsylvania, results were inconsistent,” he adds. “Some plots have had a 2-bushel response; some had 15-bushel responses, suggesting a hybrid effect on the response.”

Greg Roth, Penn State Extension corn agronomist, agrees. He urges checking the company ratings for a variety’s susceptibility to diseases.

Hybrids with greater susceptibility to foliar diseases, such as gray leaf spot, tend to have larger yield responses to foliar fungicides, adds Carl Bradley, University of Illinois plant pathologist. And some research shows that disease risks can increase when corn is planted later than normal.

Key Points

• Corn fungicides are more than a “spray and pray” decision.

• Erratic yield responses suggest hybrid susceptibility.

• A simple score card will help you assess the risk and reward.

Score corn vulnerability

Key decisions need to be made when considering a fungicide. To assist in that decision, the accompanying corn score card will give you a better idea of how numerous factors relate to a specific field and the probability of return from an application, adds Voight. It’s a start to making an informed decision.

The basic research hasn’t been done to develop a clear relationship to the yield response, contends Roth. But it’ll give you an idea of the level of disease risk factors present in a field.

There are other benefits, such as standability and harvest ease, that could be considered as well for certain fungicides. Teaming this score with assessing forecasted weather for 45 days after application will also be helpful.

Yes, forecasts that far out are iffy. But this gives you a better handle to assess the risks and rewards of making a fungicide application. 

Corn fungicide score guide

Disease history: Low-lying fields with a history of disease are more likely to respond to a fungicide.

1. No

2. Yes

High yield history: High-yield fields are more likely to show an economic response. 

1. Poor

2. Average

3. High yielding

Hybrid resistance: The lower the genetic resistance to gray leaf spot, northern corn leaf blight and anthracnose, the more potential for an economic response.

1. Highly resistant

2. Average

3. Poor resistance

Crop rotation: Corn following corn tends to harbor more disease inoculum.

1. Following other crops

2. Following corn

IPM: Corn diseases are just starting to appear, especially in no-till corn on cornfields.

1. Less than 5% visible disease

2. 5%-10% visible disease

3. 10% or greater disease

Fertility: Low K levels and compacted soils may exacerbate disease effects on lodging and yield.                                                                 

1. High fertility

2. Average

3. Poor fertility

What’s your score? If you rated a field high risk in every category, it would score 16 points in this scale, or 6 with all low-risk ratings. Next, assess the weather outlook for the 45 days following the application window. Foliar diseases tend to thrive under wet and humid conditions.

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WARNING SIGNS: Gray leaf spot lesions on 25% to 75% of plant leaf area by early dent may spell 10% to 20% yield losses.

This article published in the March, 2010 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.