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Does Your Corn Have Gray Leaf Spot?

Does Your Corn Have Gray Leaf Spot?
Severity varies with planting date, hybrid.

Over half of the 22 samples of corn, three stalks each, brought to the Franklin County 4-H fair for judging last week had at least a few gray leaf spot lesions on leaves. In some cases there were only a few. In a couple cases, there were several, and they were moving up the plant toward the ear lead. It was the teachable moment for me, as the judge, and the 4-H'er, as we discussed their project and their specimens in one-on-one conversations.

The more people who know what gray leaf spot looks like, especially in the younger generation who help mom and dad on the farm, the better job of scouting we can expect in the future. Or at least that's the theory I'm banking on.

The reason many of the exhibits had at least a few lesions, albeit normally small lesions, was that gray leaf spot threatens to run rampant right now, notes Dave Nanda, crops consultant, Indianapolis. Nanda writes Corn Illustrated articles for Indiana Prairie Farmer  magazine and also is research director for 1stChocie hybrids, Milton, Ind.

He's spent the last two weeks walking and scouting Indiana cornfields. Some were planted to hybrids he sold. Many were competitor's hybrids. His philosophy is to help customers in general.

"The disease is certainly showing up on hybrids that are susceptible," Nanda says. "If the forecast for hot, humid weather over the next week or more holds, the disease will likely be off to the races."

It's not too late to spray fungicide in some cases, especially in later planted corn. However, so far Nanda has not found as much of the disease in corn planted in late May or very early June. He recommends that customers with those fields continue to scout. He also recommends that anyone who planted relatively susceptible hybrids to gray leaf spot scout.

It may no longer be economical to spray if the silks are all brown, indicating pollination is over and the ears are formed, especially if the disease is still below the ear lead. From here on out, adjustments that affect yield by the plant involve kernel size and test weight. By the time the gray lead spot shuts down the plant, those decisions should be made. The biggest threat would probably be that premature death would open up the plant for secondary infections of stalk rot.

Scout, scout, scout, Nanda says. That stakes are high. It's $30 or more per acre to spray a fungicide. Tha's 10 bushels of corn at $3 per bushel. But it;'s alos big-time losses if gray leaf spot knock out plants during or near pollination. The biggest thres tnow is likelyt to late-planted corn of susceptible hybrids.

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