Time to Consider Fungicides on Soybeans

Photo from http://www.kygrains.info/

Soybeans are susceptible to a number of fungal and bacterial pathogens. These pathogens cause leaf spots and blights and are generally common in Kentucky; however, few fields in any given year are seriously damaged by foliar diseases. Crop rotation and weather that is unfavorable to disease development typically keeps foliar diseases at low levels, but this growing season has been anything but typical.

I never thought I’d complain about rain in July, but again, this growing season has not been typical.  The unusual, wet conditions that we have been dealing with are very favorable for the development and spread of fungal and bacterial pathogens.  This year may be the year to take a serious look at your spray schedule and fungicide use in soybeans.

The best chance that a fungicide treatment will result in a net economic gain for soybeans occurs when disease conditions exist which justify making an application, such as when your bean fields become an environment highly favorable to disease development.  Common soybean diseases found in Kentucky that warrant fungicide applications include Brown Spot, Frogeye Leaf Spot, Cercospora Leaf Blight, Anthracnose and Soybean Rust.  All of which thrive in wet conditions and all can cause significant yield loss.

Achieving good results with fungicides requires excellent spray coverage, regardless of the crop. Both aerial and ground applications have produced good results in corn and soybean.  Timing is also critical. Fungi have certain life stages that are vulnerable to fungicides. Similarly, plants have vulnerable stages which might require protection when the risk of target diseases is high. Either case, if a fungicide is applied too early or too late, it will not produce the desired results.  The most effective applications are usually made at the early pod (R3) growth stage.  Later applications may be needed should soybean rust ever need to be controlled.

There are a great number of other disease organisms and diseases that manifest symptoms on soybean foliage that are not mentioned above. For example, many root and stem diseases ultimately cause foliar symptoms to be expressed, but the pathogens that cause those diseases do not infect foliage directly. Rather, symptoms are usually the result of imbalances in nutrients and water in foliage. Soybean cyst nematode and stem canker are examples. In the case of soybean sudden death syndrome, foliar symptoms are the result of one or more foliar toxins that are produced by the causal fungus as it grows in diseased root tissue.

Virus diseases, such as soybean mosaic and bean pod mottle, reproduce systemically and, thus, can readily be found throughout infected foliage with the aid of an electron microscope. The principal symptoms of virus diseases are expressed in crop foliage.

For more information on Integrated Pest Management in soybeans and other crops or for a copy of the University of Kentucky’s Soybean Fungicide Efficiency Table, contact the McLean County Cooperative Extension Office at (270)273-3690. Educational programs of the University of Kentucky Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability or national origin.


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