The past decade has seen a rise in foliar diseases in Midwest soybeans, from white mold to charcoal rot. An arsenal of tools emerged as U.S. farmers geared up to battle Asian soybean rust — and learned in the meantime how to handle pathogens that turned out to be a more persistent threat.

The challenge today is determining when to call in the power of foliar fungicides, and how to keep these valuable tools viable in the face of resistance.

White mold

Among the earliest foliar threats is white mold, also called sclerotinia stem rot. Water-soaked lesions at stem nodes are the first symptoms, followed by the formation of cottony, white mycelia on the stems.

Treat infected crops at R1, the very early stage of flowering. That’s when it may still be possible to suppress — though not control — an existing outbreak, allowing the small handful of labeled fungicides to penetrate the canopy to reach the infection sites on the stems.

For fields with a history of white mold, plant resistant varieties, step up your weed control to minimize the number of alternate hosts in the field, dial back populations to increase airflow through the canopy, and scout carefully if temperatures are cool and wet as flowering begins.

Frogeye leaf spot

Long considered a “Southern disease,” frogeye leaf spot has proven its capability to overwinter in northern climates and establish itself in the Midwest. It can significantly impact yield as its lesions destroy photosynthetic area and inhibit the crop’s ability to fill pods.

Resistant varieties are still effective — the Rcs3 gene works against all U.S. populations, according to Anne Dorrance at Ohio State University.

Scout susceptible varieties carefully, and be ready to treat with a labeled fungicide. Dorrance and her colleague Dennis Mills note that a 2008 study determined that a fungicide application was highly effective and economical when one or two lesions were found per 25 feet of row.

The pathogen that causes frogeye leaf spot, Cercospora sojina, overwinters on crop residue. Burying residue is an effective control, note Dorrance and Mills. If your tillage system doesn’t allow burying residue, rotate away from soybeans for at least a year, they suggest.

Other threats emerge

A cousin of frogeye leaf spot’s pathogen, Cercospora kukuchii, causes sunburn-like discoloration that interferes with photosynthesis and pod fill. The disease can also cause purple seed stain, lowering the value
of the crop.

The small, irregularly shaped lesions of brown leaf spot (Septoria glycines) look a lot like those caused by bacterial blight, but Septoria affects older leaves while bacterial blight occurs on younger, upper leaves.

Several fungicides are labeled for control of brown leaf spot, but the disease doesn’t threaten yields unless more than 25% to 50% of the canopy defoliates prematurely, according to Iowa State University’s write-up
on the pathogen.

Charcoal rot can kill plants outright, attacking in hot, dry conditions during the crop’s reproductive phase. According to agronomists at Asgrow, little can be done to manage charcoal rot right now, other than planting non-host crops for three years or more, or minimizing stress and planting lower populations in soybeans.