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Cure for sick cornfields

Gil Gullickson 12/12/2013 @ 3:00pm Crops Technology Editor for Successful Farming magazine/Agriculture.com

Soybeans may get more attention on in-season fungicides, but there’s good evidence that it can often help corn yields, too. 

Carl Bradley, University of Illinois Extension plant pathologist, says the price of corn is driving the interest and the economic payoff of spraying a fungicide on corn at or near tasseling time. 

“A fungicide application is generally going to cost around $24 to $32 an acre,” he says. “With corn over $7 a bushel, a 3- or 4-bushel (per acre) yield response can make that pay.”

Much depends, though, on corn prices. 

Payoff odds are steeper at lower corn prices. At $4 per bushel and a $28-per-acre application cost, 7 bushels of corn per acre are required to break even.

Responses can vary. “In all of our Illinois trials from 2008 and 2012 at several locations, we’ve had an average yield increase of 5.8 bushels an acre compared to no fungicide. In some trials, the response has been as high as 40 bushels (per acre),” Bradley says.

The two biggest midseason corn disease threats in the Midwest are gray leaf spot and northern corn leaf blight. Gray leaf spot is noted for its oval or rectangular brown lesions starting on lower leaves prior to tasseling. 

“It can explode after tasseling in high humidity and warm temperatures,” says Bradley. 

Northern corn leaf blight has shinier lesions; they are more cigar-shaped. The lesions get bigger after tasseling, and the disease can spread fast if conditions are favorable.

Bradley says your own assessment of the need to apply a fungicide to growing corn starts with the following risk factors.

- Hybrid susceptibility. Some are more resistant to disease, and seed companies will often label them in a multistep ranking from susceptible to resistant.

- Corn residue on the soil surface. With continuous corn, plant diseases more easily carry over from one season to the next on corn residue and attack the next crop.

- Planting date. Late planting may mean that more pathogen inoculum is present when corn is not as fully developed. This could lead to greater disease severity and reduced yields. 

- In-season weather. Frequent rains, high relative humidity, and warm temperatures lead to pathogen infection and accelerated midseason disease spread.

“The more of these risk factors, the higher probability for disease to develop,” says Bradley. 

Interpretation tips  

Season-long scouting is the most important thing you can do to determine early if you have a developing problem. Here are his tips for interpreting your scouting observations.

- If the hybrid is susceptible or moderately susceptible to plant diseases, look closely to see if the disease lesions (spots) are present on the third leaf below the ear on half or more of the plants just prior to tasseling. If less than half of the plants show disease that low on the plant, fungicide treatment will be less likely to pay for itself.

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