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Make preliminary disease ID in field

Tom J. Bechman hand holding the Purdue Corn & Soybean Field Guide to identify downy mildew
DIAGNOSING DISEASE: Agronomist Steve Gauck makes a preliminary identification of downy mildew in this soybean field using the Purdue Corn & Soybean Field Guide.
Soybean Watch: Identify which diseases are in your fields, even if their impact is minimal.

Walking soybean fields this time of year can be a challenge, but there are rewards for those who put in the effort. One advantage is knowing which diseases appear in your fields. It isn’t always necessary to treat them now, but it can help make future management decisions easier.

Steve Gauck, a regional agronomy manager for Beck’s, Greensburg, Ind., recognized small, yellowish spots on a few leaves as he visited the Soybean Watch ’22 field earlier this summer. Beck’s sponsors Soybean Watch ’22. The purpose of the project is to make frequent observations in one typical field so you might know what to look for in your fields.

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“The spots which I saw looked like downy mildew, but I wanted to check for myself,” Gauck says. “I carry the Purdue University Corn & Soybean Field Guide with me. It has a section on soybean diseases, featuring colored pictures.”

Looking at the picture of downy mildew on a soybean leaf in the guide, Gauck was convinced the disease he spotted in the field was downy mildew. The leaf in his hand and the leaf in the book looked identical.

To be 100% certain, he could have sent the leaf to the Purdue Plant and Pest Diagnostic Laboratory. For a small fee, the lab would make a diagnosis, using plant pathologists and agronomists on campus. Most land-grant universities offer a similar service. In this case, since the infection was minor, Gauck didn’t pursue sending the sample on for verification.

Tracking disease

“Downy mildew seldom causes economic loss in soybeans here,” Gauck says. “Yield likely won’t be impacted.

“However, it’s still good to know which diseases are in the field. If you find a disease like downy mildew, then you know inoculum for the disease is present. It still may not be a large enough reason to change any cultural practices, but you want to have all the information you can when making decisions.”

According to the Purdue guide, downy mildew is a fungal disease favored by cool, humid weather. Older lesions turn brown as tissue dies. Under humid conditions, tufts of spores can appear on the underside of lesions. Since older leaves are more resistant, you’re more likely to find downy mildew higher in the plant on newer growth. If the disease gets a strong foothold, some seed may be affected with white mold. It can develop without visible symptoms on the outside of soybean pods.

Soybean varieties differ in resistance to downy mildew. Should it become a larger problem, rotation out of soybeans temporarily or tillage to help destroy infected residue is an option for control.

“It’s usually not going to be a problem, and it typically doesn’t affect yield,” Gauck says. “But anytime you see symptoms in a soybean field, it’s worth determining the cause. Other diseases could have greater impact.”

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