Wednesday, January 26, 2022

‘Dead man’s fingers’

Scientists work to get a handle on taproot decline

taproot decline deadman fingers
The stromata of an emerging soybean pathogen resembles those of other fungi also known as “dead man’s fingers” — photos by Matthew Rainwaters/Arkansas Agricultural Experiment Station

A monster that lives by eating the dead is hiding underground, and it has developed a taste for soybeans.

For the past two years, Arkansas Agricultural Experiment Station researchers have worked in the lab and in the field to learn more about the emerging pathogen identified in 2014 as the prime culprit of soybean taproot decline — Xylaria necrophora.

They are studying its genetics and testing for resistance and susceptibility in different soybean varieties. They have also conducted crop cover tests to develop better recommendations for limiting fungal development in one of the state’s top three cash crops.

Terry Spurlock, associate professor and Extension plant pathologist for the Division of Agriculture, said Xylaria necrophora so far has not been a serious problem for Arkansas soybean farmers outside of Chicot, Desha and Jefferson counties. But the pathogen has been found in other areas of Arkansas and has caused a stir in Mississippi, Louisiana, Alabama and Tennessee.

“The severity at the moment is highest for growers in the southeast part of the state,” Spurlock said. “I’ve been in a field in Chicot County where the entirety of the field had it.”
He added that the pathogen had been misidentified for years, creating confusion because it looked like something else.

We knew in 2014 it was something different. We pulled a fungus out of the ground that wasn’t on record for this. A lot of people thought it was black root rot, but that is caused by a different fungus,” Spurlock said.

He, along with fellow experiment station plant pathologist Burt Bluhm, was one of 10 authors of a paper published by the American Phytopathological Society in 2017 that gave the first description of the causal agent. The malady has since become known as soybean taproot decline. Other authors included researchers from Louisiana State University, Mississippi State University and Alabama Cooperative Extension.

Symptoms

The Arkansas cover crop study provides more insight into this plant disease. Symptoms caused by Xylaria necrophora include rotten taproot, yellow foliage and rusty leaves on the soybean plant. The fungus starts out white in the pith center and turns black after it matures, Spurlock said. Since it is a relatively new pathogen, research on management options has been limited.

Signs of Xylaria necrophora presence are small, fingerlike stromata that some call “dead man’s fingers” on debris from previous harvests, Spurlock said. Sometimes easy to overlook, the spikey stromata are usually white at the tip and sometimes pinkish when young. They turn black at maturity.

If the pathogen is correctly identified, a farmer can work with Extension agents to develop a control plan, he said. It could include ways to prevent the infection of disease-free fields and reduce the amount of inoculum from a previous season so the land can be replanted with a more tolerant variety.

In his observations so far, Spurlock has noticed Xylaria necrophora more in fields that have been continuously planted with soybeans with no crop rotation.

However, he said he has also seen severe taproot decline in fields that have been in a corn-soybean crop rotation. This was a “frustrating” observation, because a corn-soybean rotation can increase yield by 3 to 4 bushels of soybeans per acre.

Division of Agriculture pathologist Alejandro Rojas said the origin of Xylaria necrophora in Arkansas is not clear yet. However, some historical isolates associated with the same species were collected in Louisiana in gardenia and sugarcane in the late 1930s.

The research

In the field, Spurlock said studies have been conducted with rotations from soybeans to cotton, soybeans to corn, and soybeans to soybeans, with till and no-till scenarios and fungicide applications. His team is also researching the use of chemicals for seed treatment and in-furrow or at-planting application.

taproot decline fungus in petri dish
Xylaria necrophora, the causal agent of soybean taproot decline, is sometimes called “dead man’s fingers” because of its reproductive stromata.

“We’re taking a multi-faceted, multi-state approach to manage it,” Spurlock said of the ongoing research.

In addition to microbiome studies, researchers have used remotely piloted aircraft to document the pathogen’s impact compared to previous soybean yields.
In the lab, Rojas and his team are performing cultivar screenings and evaluating seed treatments to find soybeans that are the most resistant and susceptible to Xylaria necrophora. They also conducted a cover crop study to see what best outcompetes the fungus.

Eight cover crops were tested: wheat, barley, rye, mustard, black oat, vetch, winter pea and blue lupine, along with Hutcheson, a known susceptible soybean cultivar.
Field and greenhouse assays are also going to be conducted to explore the interaction of Xylaria necrophora with different soil types and evaluate pathogen detection method, Rojas said.

Qiurong Fan, research associate on the Xylaria necrophora project, has been collecting Xylaria samples from soybean fields around Arkansas to find out which soybean varieties are the most susceptible and tolerant to the pathogen. So far, she has found 10 non-pathogenic and 30 pathogenic isolates of Xylaria in the state.

The results

In the experiment station cover crop study, barley and oats were the most susceptible at the seed level to Xylaria necrophora. The study also found that cover crops at the seedling level were less susceptible, but mustard and radish had the greatest reduction of root mass.

Some of the cover crops like rye and winter pea had reduced root damage, Rojas said. The cover crop study provides insight into how the fungus survives from one growing season to the next.

Spurlock said he is “almost always going to recommend crop rotation,” but he realizes that not every farmer can afford to rotate. Each farmer’s situation is different, he noted, and they must decide the financial impact of taproot decline for them personally.

Although the fungus could ride on the wind in the dust during harvest, Rojas and Fan said its spores are more likely to be dispersed by tractor tires, boots or some other direct contact with the soil.

The University of Arkansas contributed this article.

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