Should You Apply Fungicides In Your Soybeans?

Aug 11, 2016

By Alyssa Collins

Some of the diseases we’ve been seeing won’t be helped by a fungicide, so it is important to know what disease you are dealing with. For instance, bacterial blight resembles brown spot, but a fungicide will have no effect on its growth or spread. As you make your final disease management decisions for your beans this summer, Anne Dorrance, plant pathologist at Ohio State University, wants you to bear in mind the following:

Is brown spot or frogeye present? Both of these foliar diseases are caused by fungi that can survive on soybean residue from the previous year crop. In the case of frogeye, there were some highly susceptible varieties that were grown in the past, and we had a rapid build-up of disease. Those varieties are now gone and even this year, due to the cold winter as well as the heat we have had, build-up of this disease is very slow. In contrast, brown spot is typically present in every field, but this usually will only contribute to 3 to 4 bu/acre yield loss in any given year. If brown spot is present and the lower 1/3 of the plants have lots of brown spot, with today’s prices this might be one option to consider to apply a fungicide.
Fields that are planted continuously to soybean and no-till soybeans. These fields have the highest levels of residue on the soil surface and are the best for getting foliar diseases started early.
Susceptible varieties to either disease. Some varieties have little resistance and these need to be watched. If disease is present, then fungicide applications are warranted. If temperatures and dry weather have kept disease from developing, best not to spray.
Yield potential of soybean and field. We have some fields that can’t get out of the 30’s for various reasons. Another input cost is not going to help move that margin enough. Soybean cyst nematode, poor drainage, big holes in the field—all of these reduce the chances of getting the inputs back from the dollars spent.
Soybeans that have experienced hot dry, weather – drought like conditions- don’t spray. When fungicides have been applied under these conditions in the past, the yields were 2 to 4 bushels lower in the sprayed vs the unsprayed passes. This doesn’t take into account the additional costs of the application. In addition, we have seen spider mite flare-ups occur under these conditions.
What are your soybeans contracted for? If the price is not locked in and we have bumper crop, the price per bushel often will drop. So when you cost out the charge of the fungicide and application costs, be sure to compare it to the price you expect.
Finally, the last tip. If you do spray, leave at least 3 unsprayed passes placed across the field (not at the borders). A comparison between the sprayed and unsprayed under this year’s unusual season will give you the best information for making this decision in the future.

Source: psu.edu