Bean Fungus on the Rise

By Emily Unglesbee
DTN Staff Reporter

ST. LOUIS (DTN) — It isn’t easy to fool a plant pathologist, but one soybean disease nearly pulled a fast one on Daren Mueller last summer.

The Iowa State University scientist was on the hunt for sudden death syndrome (SDS) in Iowa soybean fields. Every time he spotted the tell-tale yellow patches of SDS, he pulled the truck over to collect samples.

"But the first four fields I drove into to grab a plant with SDS turned out to be infected with stem canker," Mueller recalled. "It got me thinking that a lot of the SDS we thought we had last year was maybe stem canker."

Stem canker is part of a somewhat mysterious fungal disease complex on the rise called Diaporthe, which can also produce another soybean disease called pod and stem blight. At a spring 2015 meeting, plant pathologists from the north-central Midwest consistently reported seeing more Diaporthe disease in soybeans in 2014 than they had in years, even decades, Mueller said.

"It’s now definitely something we’re all aware of, and I really don’t want people to mistake it for SDS," he told DTN. "We need to figure out why it’s showing up so much."

Much is still unknown about how the Diaporthe fungus complex works, said University of Wisconsin plant pathologist Damon Smith. Researchers are still trying to get a handle on when plants are infected, why the diseases become active, and what treatments can control them.

While Mueller and Smith delve into research projects this year to try to clear up some of the mystery, one thing remains certain: The disease is on the rise and farmers should be on the lookout for it in 2015, Smith said.

"We need to keep this on our radar because there will likely be more soybeans out there this year," Smith pointed out. Wisconsin growers saw widespread stem canker damage last year, and the disease’s inoculum survives on soybean residue and in the soil, he noted.

SORTING THE FUNGUS OUT IN THE FIELD

Of the two Diaporthe fungal diseases, stem canker can be the most difficult to correctly identify in the field, because its symptoms mirror those of other diseases, such as SDS, Phytophthora root rot, white mold, and charcoal rot.

Stem canker produces rust-colored lesions that begin at a soybean node and work their way up or down the plant, Smith explained. These lesions can be distinguished from the lesions of Phytophthora root rot, which generally begin at the soil line of a soybean stem. As they age, stem canker lesions darken and can eventually cause the plant to die prematurely.

From the road, patches of yellow soybean canopies dying from stem canker look a great deal like SDS. Generally, the leaves will not show as much of the interveinal chlorosis — when the leaves turn yellow but the veins remain green — that is so striking in plants infected with SDS, Mueller said.

Growers who suspect white mold will find that stem canker-infected plants lack the hard, black structures called sclerotia, as well as the mold’s characteristic white tufts lining the soybean plant, Smith said.

Finally, growers can usually distinguish between charcoal rot and stem canker based on the weather conditions. Charcoal rot prefers dry, hot weather, and stem canker appears to favor soggier growing conditions, particularly early in the season, Smith explained.

Pod and stem blight of soybeans can be easier to identify, thanks to lesions that give the stem a bleached appearance and the black, pepper-like structures that dot the stems and pods of an infected plant. Seeds infected by the disease will have a wrinkled, chalky appearance and can actually transmit the fungus to future plants, Smith said.

Pod and stem blight also requires wet conditions, particularly during the pod-filling stage.

PREVENTION AND CAUSES

Smith describes the current knowledge on the prevention and management of Diaporthe diseases as "full of black holes."

Very little data exists to recommend specific fungicide applications and their timing or seed treatments, Mueller said.

Crop rotation to corn or small grains would likely help reduce inoculum in a field, but alfalfa has been shown to be a good disease host, particularly for stem canker, Smith said. The disease can also live on weeds such as black nightshade, curly dock and morning glory.

Reducing your soybean plant population might help, as crowded plants have been associated with the disease, as well, Mueller added. Growers can also look for soybean varieties that have a good resistance rating for the diseases.

Both Mueller and Smith hope future research will shed light on both the management and the causes of the Diaporthe disease complex. For now, Mueller believes plant stress, particularly from recent weather extremes, might be prompting the Diaporthe fungus to become activated in soybean plants and more visible to growers.

"We’ve seen a steady increase of it in Iowa the past three years," he noted. "So it could be coinciding with climatic extremes we’ve had the last several years."

Smith agreed, calling environment "a big driver," and added that genetic changes might also have played a role.

Diaporthe diseases were a major problem for bean farmers in the mid-20th century, but germplasm improvements in the 1960s and 1970s helped farmers control it. As the disease’s agricultural impact receded, good genetic resistance against the disease may have slowly slipped out of modern cultivars, Smith explained.

To learn more about the Diaporthe fungal disease complex, see two new factsheets from the Crop Protection Network on stem canker: http://bit.ly/…, and pod and stem blight: http://bit.ly/….

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

(PS/AG)