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When should you apply fungicide to soybeans?

Tom J. Bechman hands holding soybean plant
KEEP BEANS HEALTHY: Your goal is to have soybean plants in September like this one, without disease on the leaves or plant. Do you need fungicides to reach that goal?
Soybean Corner: The jury is split over applying fungicide as a standard practice vs. only if conditions for disease warrant it.

I applied fungicides on soybeans. They made 64 bushels per acre with a dry August, but the neighbor’s beans made 60 bushels with no fungicide. Should I skip fungicide this year?

The panel of Indiana certified crop advisers answering this question includes Betsy Bower, agronomist for Ceres Solutions, Lafayette; Shaun Casteel, Purdue Extension soybean specialist; Greg Kneubuhler, G&K Concepts Inc., Harlan; and Bryan Overstreet, Purdue Extension educator, Jasper County.

Bower: Did you scout for diseases before ordering fungicide? Did you leave an unsprayed check? There still could have been value to fungicide. Maybe the stage wasn’t set for high yield due to environment.

Don’t skip fungicide for 2022 based on one year’s data. What is the year like weather-wise? Are you seeing some disease on leaves? Are you finding insects in fields? Have you marketed any 2022 soybeans? What is the current market price? All these questions should help you make a more informed decision. Visit with your agronomic professionals. Have them look at a couple of your fields.

Kneubuhler: We get side benefits that include delayed maturity and increased seed size from fungicide. Delayed maturity allows plants to capture more growing degree days and later rains, which often increase seed size. In the presence of leaf diseases, fungicides offer a good yield response. Couple that with the side benefits, and we often see nice yield responses.

Soybeans are made or lost in August for most of Indiana. Rainfall late in reproductive stages results in a large percentage of yield gain. Fungicides must be applied well ahead of when disease begins and when we need late rains. In essence, you’re making an application in anticipation of what’s to come. So, to ask “Should we skip fungicide?” is a difficult question unless you can predict the weather for the next 30 days post application. In the long run, response to fungicide alone is a 2- to 3-bushel increase, on average.

Overstreet: Check to see if there is any evidence of disease problems. If there are signs of disease, determine what it is. Follow with the correct treatment. If not, check the long-term weather forecast. Will conditions be right for disease? Putting out fungicide just to put it out could cause some long-term effects of resistance in the future when you need it. Have a justified reason to apply the fungicide.

Casteel: Fungicides in soybeans are a great management tool to prevent and limit disease development if the application is made at the proper time. Not all soybean diseases are created equal, nor are all fungicides. Fungicide applications to control white mold should be made during flowering and early reproductive development. This should not be an automatic application, but if the field has a history of white mold and conditions are conducive for infection and disease development, fungicide application may be warranted.

The same approach needs to be taken during pod development for leaf diseases like frogeye leaf spot, which can cause yield loss. Scout for lesions, and assess disease pressure and threat of further development based on field conditions. Prophylactic use of fungicides can be a waste of money and threaten longevity of fungicides. However, fungicides have a place to protect soybeans and save or even add bushels that might be lost if disease goes unchecked.

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