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Cure for sick cornfields

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.

- On hybrids that are intermediate between susceptible and resistant to plant diseases, add in the factor of field history to the previous assessment. If the field was in corn the prior year and overwintered with at least 35% residue on the soil surface, that would give another reason to spray a fungicide labeled for corn.

- If the air is very warm and humid at tasseling in midsummer, that’s one more reason to consider a fungicide treatment. Still, both high temperatures and high humidity must be present.

“It was warm in 2012 but also very dry,” says Bradley. “Generally, under dry environments, the fungal pathogens that cause foliar diseases of corn will not be very active.”

- If a hybrid is rated as resistant to plant disease, Bradley says it is generally not recommended to treat. However, those fields still should be scouted. If nothing else, it will ensure those disease ratings are accurate.

“Some people may encourage you to spray a fungicide on corn – whether you can see a disease problem or not – for general yield improvement,” says Bradley. “Results from my program’s field research trials show that yield responses will be greater and more consistent when diseases are targeted rather than making preventive applications. Disease risk and scouting observations should be considered.”

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Timing Is Key 

Bradley says timing of spraying a fungicide is important. If you decide to spray, sometime between tassel emergence and silking is usually best. “If you do it earlier than that, you may not get control of the disease through the end of the season,” he says.  

In looking at all of their trials with fungicides on corn, Bradley says they get a yield improvement of at least 5 bushels per acre 58% of the time. 

“Variability in response to treatment is usually because of the severity of the disease,” he says. “When less than 10% of the total leaf area is affected by disease lesions, we get at least a 5-bushel-an-acre yield response less than half the time. When 10% or more of the leaf area is diseased, we get at least a 5-bushel-an-acre yield response from the fungicide 85% of the time.”

If you have a field or fields that are problems for plant disease and you plant continuous corn, your first line of defense is to plant hybrids that are rated resistant to disease. 

“Depending on the disease, few of our hybrids are 100% resistant,” says Bradley. “Some are certainly better than others. In one trial we did with a susceptible hybrid, we got a response of 40 bushels an acre by treating with a fungicide. In the same trial with a less susceptible hybrid, the treatment only gave a couple-bushel response.”

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