First column: Soybean seedlings. Second column: Soybean seedlings inoculated with Xylaria necrophora at the time of planting. Third column: Cotton seedlings. Fourth column: Cotton seedlings inoculated with Xylaria necrophora. Photos provided by Vinson Doyle

Scientists today know much more about taproot decline, a disease primarily seen in soybeans in Louisiana, than they did just a few years ago. Once called a “mystery disease,” it interferes with root development, often killing crops before they emerge and sharply limiting yields of infected plants that survive.

Researchers have learned a lot about Xylaria necrophora, the pathogen that causes the disease — but many questions remain. LSU AgCenter plant pathologist Vinson Doyle is focusing on refining inoculation methods so he can better understand the disease.

“One of the big challenges that we've had is reproducing the disease consistently in greenhouse studies and growth chamber studies,” Doyle said. “The pathogen doesn't always cause disease, so when you add the pathogen to the soil, sometimes you see plants that show no evidence of symptoms. Other times, the seedlings don't even emerge. You get the two ends of the spectrum. We want plants that are still alive but exhibiting symptoms.”

Being able to produce symptomatic plants more consistently will aid cultivar screening and other research.

Doyle also is studying the mRNA of soybeans, cotton and peanuts with taproot decline to understand how each plant responds to — and in some cases, resists — the disease. Peanuts don’t seem to be affected by taproot decline, and cotton is being evaluated because the disease has begun showing up in that crop.

Interestingly, though, taproot decline probably is not a truly new problem in cotton. The disease has been around for a long time and likely was misdiagnosed as other diseases or abiotic stresses in years past.

“Some pathologists have speculated that maybe cotton was the first host — that this pathogen made its way into agricultural populations on cotton and then jumped over on the soybean — because cotton's been grown in this area a lot longer,” Doyle said.

He hopes researching how taproot decline shifted from one crop to the next will offer clues about how the disease functions and possibly can be controlled. He has turned to historical collections of samples from Louisiana and Florida in the 1930s and ’40s for insight. Back then, a similar pathogen existed mostly in wooded areas and on woody debris.

“Now we're trying to find this pathogen in forested populations so we can find isolates that may not be particularly well adapted to soybean or might show characteristics that give us an insight into how this pathogen shifted over onto soybean,” Doyle said.

While there is still much work to do, Doyle is pleased with the progress that’s been made.

“I think we've made a lot of progress on understanding this pathogen, and we've also developed some insight into how you can actually manage the disease,” Doyle said. “We know that debris is probably the primary source of the disease, if not the only source.”

Managing plant debris in fields can help prevent taproot decline.

Doyle has been studying taproot decline since 2017, when very little was known about it. He has enjoyed the process of scientific discovery.

“It’s an incredible opportunity,” Doyle said. “When you work with an emerging disease, it’s exciting from a scientific perspective but also critically important to all of the stakeholders that are involved. When you get to work on something like this, it’s rewarding.”