Vinson Doyle, a mycologist and associate professor in the LSU AgCenter Department of Plant Pathology and Crop Physiology, is pictured with soybean plants that are part of a project studying the effects of nitrogen on the incidence of the taproot decline disease. Photo by Olivia McClure
Taproot decline was once called a “mystery disease.” It wasn’t clear what caused it or how to go about controlling it.
LSU AgCenter plant pathologists Vinson Doyle and Trey Price have spent the past several years trying to demystify the disease, which impedes development of soybean roots and severely hampers yields. They now know that taproot decline is caused by a pathogen — a newly identified species called Xylaria necrophora. The pathogen is unique in that it’s the only member of the Xylaria genus that attacks an annual crop; all others are pathogens affecting woody shrubs and trees or saprophytes that feed on dead wood.
It probably was in Louisiana soils decades ago, long before soybeans were grown here. And because it triggers foliar symptoms similar to those produced by other diseases and abiotic stresses, it was likely misdiagnosed many times over the years.
No fungicides seem to stop taproot decline, which often kills plants before emergence but can affect soybeans in later stages of development, too. It can be found in Louisiana and a handful of other southern states.
So, what’s next in unraveling this mystery?
“What we’re trying to understand is how this pathogen causes disease. We’re looking at the secondary metabolites that this fungus produces,” Doyle said. “We’ve identified two compounds that are in relatively high concentrations that are known to be phytotoxic, and when the root cells are exposed to these compounds, the cells die. It inhibits root development.”
Pinpointing these two compounds marks an important turning point in learning about how taproot decline functions and how it may be controlled.
“These compounds are nitrogen-based compounds, and so we want to understand whether variation in soil nutrient availability, particularly available nitrogen, has an impact on disease severity,” Doyle said.
Soybeans typically don’t require nitrogen fertilizer. But many farmers grow soybeans
in rotation with other crops that need additional nitrogen.
“If we can determine that nitrogen has an impact on disease severity,” Doyle said, “then we can start to talk to farmers about soil nutrient management.”
Scientists also are studying whether stressors such as high temperatures play a role in disease development.
“Secondary metabolite production is often a response to stress,” Doyle said.
When it comes to finding control solutions, he and his colleagues have made some useful findings — like the fact that populations of the pathogen responsible for taproot decline tend to show a low level of diversity.
“In a pathogen where there’s little diversity, the likelihood of them overcoming resistance or evolving resistance to fungicides is low,” Doyle said. “So that’s a good thing. We are trying to find cultivars that are resistant to taproot decline, and once we find them, they’re likely to be more stable.”
That work is taking place at the AgCenter Macon Ridge Research Station in Winnsboro, where Price is based.
“We have ‘copies’ of Arkansas, Louisiana and Mississippi official variety trials that are inoculated in the field to try and identify resistant varieties for farmers,” Price said.
The scientists also are searching for chemical control options, having installed seed treatment and in-furrow fungicide trials in six locations in Arkansas, Louisiana and Mississippi. In addition, they are working with field pathologists in those states to gather more data on yield losses caused by taproot decline.
As for what farmers can do in the meantime to reduce their chances of taproot decline-related problems, cultural practices are the answer.
“Past research has shown that tillage and crop rotation are effective in reducing taproot decline incidence,” Price said. “Anything that reduces the accumulation of debris reduces the inoculum load, thereby reducing taproot decline incidence. Altering seed placement from year to year also may reduce seed and seedling contact with infested debris.”