Understanding of soybean taproot decline disease evolving

Kyle Peveto, Gould, Frances I., Blanchard, Tobie M.  |  9/12/2018 6:56:19 PM

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Taproot decline begins with orange discolorations in the leaves. Then the disease causes interveinal chlorosis. Photo by Trey Price

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The stroma of a Xylaria fungus is attached to soybean debris in a field where taproot decline disease was found. Photo by Teddy Garcia-Aroca

Scientists are gaining new understanding of a disease that has killed soybean plants in several states.

Initially called a “mystery disease” more than a decade ago, soybean taproot decline has become common in Louisiana.

Taproot decline has also been found in Alabama, Arkansas, Mississippi, Missouri and Tennessee, said Trey Price, a plant pathologist at the LSU AgCenter Macon Ridge Research Station in Winnsboro who expects the disease’s presence to be confirmed in a few other soybean-growing states soon.

“Since we’ve taught people how to identify the problem, reports have gone up in recent years,” Price said.

Taproot decline begins with orange discolorations in the leaves, and then the pathogen causes interveinal chlorosis. It leads to blackened taproots and lateral root sections. In some instances, most of the taproot is missing when the plant is inspected.

Researchers have learned the disease is caused by a species of Xylaria, fungi commonly found growing on dead wood, Price said. While most in the genus are “wood rotters,” he said, there are some disease-causing species.

Vinson Doyle, a mycologist in the LSU AgCenter Department of Plant Pathology and Crop Physiology, is studying the fungus along with Teddy Garcia-Aroca, a graduate student in the department.

The fungus seems to initiate new infections from soybean debris — mainly stems — left over from the previous season, Price said. Because seedlings are often killed, Price assumes the fungus affects soybeans soon after planting.

“If you dig up symptomatic plants, more often than not you will find a blackened soybean stem from the previous season in contact with soybean roots from the current season,” he said.

The pathogen’s origin is unknown, but research is ongoing.

“There are many unanswered questions concerning taproot decline, and it takes a considerable amount of time and resources to research specific topics,” Price said.

While the disease has attracted attention and caused discussion in the past 15 years or so, soybean taproot decline likely isn’t a new threat, said Garcia-Aroca.

“Even though this disease was recently reported, we think it has been present in soybean fields for a long time, maybe decades,” he said. “We think it probably went unnoticed or was misdiagnosed for similar-looking diseases, such as sudden death syndrome and Thielaviopsis.”

The best way to avoid taproot decline, Price said, is to practice crop rotation. Soybean monoculture and reduced tillage practices lead to large amounts of soybean debris in fields, so tillage operations that speed up soybean debris decomposition will lessen the incidence and severity of taproot decline. But Price still recommends reduced tillage practices to maintain soil health, reduce erosion and prevent contamination of water sources.

While he has not specifically researched weather’s role in the disease, Price said hot and dry weather appears to lessen the severity of taproot decline incidences.

Price and others are seeking several other solutions to taproot decline. Varieties that are resistant to taproot decline may be available, but further study is needed.

“We continue to look at seed treatments and in-furrow fungicides but have not identified anything that works well yet,” Price said.

Kyle Peveto

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