Soybean South December 2021

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DECEMBER 2021

‘Dead man’s fingers’ Scientists work on taproot decline

Start planning for herbicide shortages

PHOTOS BY MATTHEW RAINWATERS/ARKANSAS AGRICULTURAL EXPERIMENT STATION

The stromata of an emerging soybean pathogen resembles those of other fungi that are also known as “dead man’s fingers.”

‘Dead man’s fingers’ Scientists work to get a handle on taproot decline

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.”

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

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 SOYBEANSOUTH.COM

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, which some call “dead man’s fingers,” on debris from previous harvests, Spurlock said. Sometimes easy to overlook, the spiky stromata are usually white at the tip and at times 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.

“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 TWITTER: @SOYBEANSOUTH

University of Arkansas researchers are growing different varieties of soybean plants in the laboratory to study the impacts of Xylaria necrophora on taproot decline.

to explore the interaction of Xylaria necrophora with different soil types and evaluate pathogen detection method, Rojas said.

Qiurong Fan, research associate on the project, has been collecting Xylaria samples from soybean fields around Arkansas to find out which soybean varieties are the most susceptible and most 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.

Cover photo courtesy United Soybean Board DECEMBER 2021

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Start planning for herbicide shortages now

By Larry Steckel

R

etailers and basic suppliers are all concerned about herbicide shortages in 2022. We can recall similar concerns this past spring.

In most cases, applicators were able to get what they needed. Retailers this past spring were often able to fill shortages by accessing carryover from 2020. I have been told there will be no carryover herbicides to fill holes in 2022, so the probability of herbicides not being available is much more likely.

The two herbicides everyone is expecting will most likely be in short supply in Tennessee are glyphosate and glufosinate. Several retailers have said they are only being allocated at best 80% of the glyphosate and glufosinate they sold in 2021.

Moreover, delivery of those herbicides may not occur until July in some cases. As such, here are some thoughts on how to effectively stretch the 2022 glyphosate and glufosinate supply.

Not just any glyphosate will do

Please remember that not just any glyphosate can be tankmixed with XtendiMax or Engenia — only the K-salt glyphosates listed on the XtendiMax or Engenia tankmix websites may be used. If you have other glyphosate formulations (i.e. DMA or IPA salts of glyphosate), be sure to use them for weed control in burndowns or in corn, etc.

This will save the approved K-salt glyphosate formulations to be tankmixed with XtendiMax and Engenia post in Xtend crops.

Be flexible

Also, be flexible about how to access glyphosate in 2022.

There are a number of premixes that contain glyphosate and would be an option for that herbicide. Products like Acuron GT, Enlist Duo, Flexstar GT, Halex GT, Sequence or maybe even some Extreme that might be on the back shelf would all be a way to acquire glyphosate.

Finally, if glyphosate cannot be found, then a good substitute would be clethodim for grasses. Remember if ryegrass, goosegrass, junglerice, barnyardgrass or fall panicum are the targets, then use at least 16 oz/A of a 2 lb clethodim to get the best results.

No good substitute for glufosinate

Unfortunately, there will not be a good herbicide substitute for glufosinate. One has to make every gallon count. A way to do that is to apply it when it has the best chance to work well.

This would include applying it on hot days with highs well into the 80s. Also apply it between 9 a.m. and 4 p.m. to get the most consistent weed control. If applying to Enlist crops, be sure to tankmix it with Enlist One. In Extend crops, apply it no later than 10 days after a dicamba application to have the best chance of killing the Palmer amaranth that is dicamba resistant. Finally, Cheetah Max (glufosinate + fomesafen) is the only premix that contains glufosinate that comes to mind as a possible option to access glufosinate.

Dr. Larry Steckel is a University of Tennessee Extension weed specialist. He may be reached at lsteckel@utk.edu

VICKY BOYD

Glyphosate and glufosinate herbicides are expected to be in short supply in 2022. Start developing alternative programs now.

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