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Posted 30 August 2013. PMN Crop News.

So Much Disease

Weather conditions have been ideal for many soybean diseases this year

Source: Penn State University Press Release.


University Park, Pennsylvania (August 20, 2013)--When we have good weather for crops, we have good weather for pathogens. In many areas of the state, this year has brought humid and rainy conditions with mild temperatures. It’s making for a good corn and soybean crop, but this is also the perfect environment for most foliar diseases. Last week, we shared with you some information on corn foliar diseases. This week, let’s consider the major issues in soybeans.

Frogeye leaf spot

Frogeye leaf spot can be problematic for yield and quality if seen in significant amounts prior to R3. There are several options for fungicides, and these are most effective if applied at the R3 growth stage. Because there is widespread resistance to the strobilurin mode of action, be sure to choose a product with multiple modes of action, like a strobilurin + triazole or other combination product.

A fungicide efficacy table for frogeye leaf spot, and other soybean diseases, is available here.

Downy mildew

Most of us see some level of downy mildew in our fields every year when we have high humidity. Usually a drier spell of weather that we typically have in the middle of the season will clear up the problem. Some in our state still have not had a hot dry spell since the beans have canopied, and they are experiencing high levels of downy mildew.

This disease is characterized by angular, bright yellow spots on the upper surface of the leaf with fluffy tan growth on the underside of the spots. As they age, the upper leaf spots will become darker and resemble those of brown spot, but the fluffy undergrowth will reveal the true identity of this disease.

We normally don’t consider downy mildew a major yield-robber of beans, and because of that little information is available about fungicide efficacy. Only in very serious cases will it cause severe defoliation and infect the seed within the pod.

White mold

Like I said, conditions are great if you’re a fungus this year and this is especially true for the fungus that causes white mold, Sclerotinia sclerotiorum. This is a soilborne pathogen that will cause white fluffy growth on the lower stems of beans, and ultimately form hard, black structures that may survive for years. Under a thick canopy with little air flow is where this fungus thrives, so get down low and scout as many locations in your field as possible. It spreads slowly on its own, but once you’ve got it in a field it is incredibly difficult to get rid of it.

Anne Dorrance, Plant Pathologist at Ohio State, advises us to: “Monitor fields. First, make note of the variety that was planted, if infections are above 20% incidence (more than 20 plants out of 100 are infected), then drop this variety from your lineup. Second, harvest these fields after all of the other fields are harvested…the approach here is to prevent contaminating additional fields.”

There are some fungicides labeled for white mold, however it is challenging to get a spray to the lower stem where it is needed past the R3 growth stage.

Alyssa Collins