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Soybean Fungicides: Are they worth it?

08/05/2014 @ 3:50pm

We are approaching or are at the R3 stage of soybeans in many fields (see the picture above of a soybean plant at the R3 stage). 

The R3 stage is when there are pods developing on the lower nodes (there still will be flowers on the upper nodes as the stages in soybeans overlap). As a general rule, most fungicides are recommended at the R3 stage of growth. The question is, are they worth it? This is a difficult question to answer and will depend on many factors. Let’s take a look at a few of those factors:

Disease presence - Most farmers apply a fungicide to control diseases. Fungicides do an excellent job of this as long as the disease is on the label. The following link will take you to a website that rates the different fungicides and the diseases they control http://extension.udel.edu/factsheet/wp-content/uploads/2014/04/2013_Soybean_Fungicide_Efficacy_Table.pdf

It is important to remember that fungicides will not control SDS, Phytophthora, or any bacterial diseases, and there are only a few fungicides that will help with white mold.

Fungicide choice - This is probably a touchy subject and many people have their idea of the proper product choice. In my experience the strobilurin class of fungicides have provided the most consistent yield responses (Quadris, Aproach, Headline, and premixes with these active ingredients). This same class of fungicides will keep your soybeans greener for a longer period of time increasing the chance for larger soybean seed size and therefore higher yield potential. It may also make soybeans tougher to cut in the fall.

Timing - Several studies have shown that the R3 stage provides higher, more consistent yield responses than the R1 (beginning flower) stage. There are few studies (or none) comparing fungicide applications at later stages. It would be interesting to know if applications closer to R4 or R5 could increase yields greater than the R3 stage.

Weather - Continuous wet weather will increase the chances for disease and the possibility of a response. We don’t know what the weather will be like so this is difficult to predict.If you think the prospects are good for a good yield, then you may want to protect your investment with a fungicide.

Variety - If you have a variety that is more susceptible to diseases such as Septoria brown spot, Frogeye, or Cercospora, then a fungicide may increase the likelihood of response.

Crop Rotation - Due to the wet weather this year there may be more fields that have a soybean/soybean rotation. In these situations disease pressure will be higher and have a greater probability of response.

The average yield response of University studies is around 2 Bu./A.  If you decide to use a fungicide be sure to leave a check strip and note the stage of the soybeans to determine if it was worth it. Some farmers may decide to throw in an insecticide to go along with the fungicide and the results have been variable. The only feeders I have noticed this year are Japanese beetles and a few bean leaf beetles.

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