When Is Right Time to Spray Fungicides?

ISU plant disease specialist answers questions from farmers regarding fungicide application on corn and soybeans.

It's near the end of July, a common time for fungicide applications. And there are many questions coming from farmers about using fungicides to control corn and soybean diseases. Iowa State University plant pathologist X.B. Yang provides the following answers.

What diseases are prevalent in Iowa? The most common soybean diseases are brown spot, bacterial blights, Rhizoctonia root rot, and a few cases of frog eye leaf spot. Soybean rust, which is light in the southern U.S., should not be a concern in Iowa this season.

Regarding corn, common corn rust is found throughout Iowa with one area reporting high incidence. Southern corn rust also has been found in southern Iowa but at a low level. There are also reported findings of grey leaf spot.

When to spray? If you determine that fungicide use is needed to control soybean diseases, now (late July) is the time to spray for many soybean fields. Over the years, we have found that sprays at R3-R4 growth stage provide the highest returns if one application is made.

Because fungicides only protect plants for two to three weeks, in a rainy season like this year, one spray will not protect the plants from disease attacks after three weeks. For those farmers who mixed a fungicide with the Roundup application, the applications were likely too early to achieve the best control results.

For most growers, spraying fungicide after R4 growth stage of the soybean plant should provide better coverage for the month of August - which should be the time when later season foliar diseases develop. However, we do not encourage people to spray after R5 or later because the soybean plants grow high and dense, which increases the damage done by the tractor. Tractor damage to the soybean plants may minimize the gains made from disease protection.

It is important to note that fungicides have no effect on bacterial blight.

Can you differentiate bacterial blight from brown spot? Yes, here's how to tell the difference. ISU Extension plant disease specialists have received more reports of bacterial blight this season. Abundant rainfall promotes development of this disease which is spread by splashing rain and favors cool temperatures.

One of the common questions is how to tell the difference between bacterial blight and brown spot. This is an important question because symptoms of the two diseases are similar, and fungicides have no effects on bacterial blight.

Brown spot starts in the bottom leaves. The lesions are numerous irregular, dark brown spots on both upper and lower leaf surfaces. Adjacent lesions frequently merge to form irregularly shaped blotches.

Bacterial blight is found in the upper portion of the plants, mostly in young and tender leaves. Lesions of bacterial blight have a yellow halo that is lacking in brown spot. Infected leaves often have ridged appearance, unique for bacterial blight. When temperatures get hotter and leaves become old in early August, severity of bacterial blight should decrease. But the disease may reemerge later in the summer when the temperature gets cooler again.

There is no measure to control bacterial blight except for selecting a better soybean variety, one that is less susceptible to bacterial blight. If you find high levels of bacterial blight this season, you should not plant the same soybean variety for next year.

Which fungicides to use? It is natural that a company representative will promote his or her own products. Basically, almost all fungicides on the market can provide good control of foliar diseases found this season. Their protection periods are not greatly different. Therefore, you should select a fungicide based on the price and availability.

Fungicides for hail injured corn? There are discussions about applying fungicides to corn injured by hail. Keep in mind that fungicides cannot help plants recover from hail injury or cure the damaged plants. However, if the foliar disease risk is high for the rest of season, use of fungicides can protect undamaged or functional corn leaves from disease attacks. Such applications act as insurance for yield production. In a situation or season when disease risk is low, such insurance is unnecessary. In this wet season, the potential benefit from disease protection seems greater than the cost in terms of dollars.

For photos showing difference between bacterial blight and brown spot, go to www.extension.iastate.edu/CropNews/2008/fungicide+application+questions.htm

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