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.

Health benefits?

The past several years have seen a lot of excitement — and debate — over the label for plant health benefits of strobilurin fungicides. Many farmers swear they’ve seen healthier plants and longer-lasting foliage after an R3 application of a strobilurin, even in the absence of disease.

University plant pathologists like Kiersten Wise at Purdue aren’t sold on the practice, though.

“What we’ve found is that when we don’t have disease pressure there — foliar diseases such as frogeye leaf spot or Cercospora leaf blight — we don’t often see an economic benefit from a fungicide application,” Wise says.

“One of the big drawbacks to using fungicides for these plant health benefits is that when we use the same mode of action over and over again, we select for fungicide-resistant strains of the fungus,” she adds, noting that frogeye leaf spot is of particular concern. “Our standard recommendation is that fungicides should be applied only when foliar disease pressure is potentially yield-limiting.”

 

Disease controls take one-two punch approach

Growers looking at any of the major disease control products for the soybean market will find something interesting in 2014 — most are combinations with two active ingredients. The strobilurin class of fungicides is the foundation of most soybean disease control programs, but for 2014, each version will often be packaged with another active ingredient.

This resistance-management strategy makes sense, especially since cases of resistant disease have already been discovered in some locations. For example, frogeye leaf spot resistance has been found in some parts of the country, which means those familiar names like Quadris, Stratego and Headline more than likely carry an add-on. Quadris becomes Quilt Xcel, Stratego becomes Stratego YLD, and Headline becomes Headline AMP.

Each of those strobilurins carries a triazole with it to prevent resistance and heat up its disease control action. DuPont’s Aproach fungicide, introduced in 2013, will add Aproach Prima to the portfolio in 2014, if regulators approve. This premix includes a triazole as well.

And there’s a new soybean fungicide that rolled out in 2013 — Priaxor — that includes a strobilurin combined with another active ingredient, Xemium. This is a carboxamide, which is a new mode of action for the soybean market and another enhancement
in the move to prevent disease resistance in the disease control business.

As you select your soybean disease control program for 2014, consider the active ingredients you’re choosing and make sure you’re hitting the crop with two modes of action. It’s an investment today to prevent resistance in the future.