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Leaf samples confirm 4 diseases hit cornfield

Tom J. Bechman gray leaf spot, southern rust and tar spot on one corn leaf
LOTS OF DISEASE: Leaves from the Corn Watch ’21 field in late September sent to the lab before harvest confirmed that several diseases were present, including gray leaf spot, southern rust and tar spot.
Corn Watch: Diseases deliver a late-season attack despite fungicide application.

Why did 20% to 30% of the stalks in the Corn Watch ’21 field fail the pinch or push test before the end of September? Here’s a clue. Leaf samples, which also included one ear with husks but no stalks, sent to the Purdue University Plant and Pest Diagnostic Lab confirmed the presence of not one, not two, but four diseases.

The samples, pulled from rows near the outside and still showing some green tissue, were positive for gray leaf spot, southern rust, tar spot and anthracnose. The latter disease comes in two forms: One affects leaves and the other affects stalks.

Related: Stalk rots show up in cornfields

“We’re also positive that there was anthracnose stalk rot, because the symptoms are very clear to diagnose,” says Dave Nanda, director of genetics for Seed Genetics Direct, sponsor of Corn Watch ’21

Nanda also found a stalk with a pinkish discoloration inside the pith, usually associated with fusarium or gibberella stalk rot. Earlier in September, Dan Quinn, Purdue Extension corn specialist, issued a report noting that he also found what he believes was likely gibberella stalk rot in a field infected heavily with tar spot.

Environmental ties

The samples were sent to Purdue because the grower wanted to know with certainty if tar spot was in the field — and one of the culprits in bringing about quick deterioration of leaf and stalk tissue. “We found the tiny black spots, but we wanted to be sure,” Nanda says.

Sending a sample is relatively simple, inexpensive and painless. Unless other tests are needed, the cost for in-state analysis is only $11, plus shipping. The sample was shipped late on a Thursday, and results were online by Monday afternoon. You do need to download a submission form and include it with the sample. For more information, visit ppdl.purdue.edu.

The field was sprayed with fungicide soon after tasseling, at an appropriate time. Nanda diagnosed gray leaf spot in the field and notified the grower, and he sprayed the field.

“We visited in mid-August, and the fungicide was doing a good job of holding the gray leaf spot in check,” Nanda says. “It had not advanced more at that point, and we didn’t notice other diseases either.”

So, what happened to the field? And why did stalk rot set up shop so early?

Nanda notes that after a heavy rain in mid-July, it was dry with virtually no rain for more than five weeks, likely stressing plants. Then significant rain returned, followed by warmer-than-normal weather for late August and early September.

“There were good conditions for disease, and a fungicide is only effective for so long,” Nanda says. “Disease pressure mounted late in the season. Foliar diseases weakened plants already stressed by earlier drought. That sets the table for stalk rots to come in quickly. Apparently, that’s what happened this year.”

The combine will be the deciding factor in determining how much stress and disease combined to hurt yield, Nanda concludes.

TAGS: Crop Disease
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