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Emerging Soybean Pathogens

By Emily Unglesbee
DTN Staff Reporter

ROCKVILLE, Md. (DTN) — University of Illinois Extension plant pathologist Nathan Kleczewski wants Midwest farmers to add two emerging soybean diseases to their lexicon this year: taproot decline and red crown rot.

Taproot decline has been on the rise in the South since it was first spotted in a few states in 2007. Its territory is expanding northward, and Kleczewski suspects some Midwest growers may already have it. Red crown rot is caused by a common pathogen in peanut production, where it causes a devastating disease called Black Rot. It’s less prevalent in soybean fields, but it cropped up last year in central Illinois, Kleczewski said.

Both diseases can be easily misidentified with other more common soybean diseases, particularly SDS. Taproot decline and red crown rot can both cause interveinal chlorosis during the reproductive stages of growth.

“How much of this is being confused with SDS, I’m not sure,” Kleczewski said in a webinar presentation hosted by the Illinois Soybean Association. “It’s another reason to get out of your truck, pay attention to your plants and really dig in and look for these symptoms and signs.”

Here’s the dirt on these newest soil-borne pathogens invading soybean fields:

TAPROOT DECLINE

Soybean taproot decline is caused by a group of fungi called Xylaria — which are normally a wood-rotting family of pathogens. But scientists officially confirmed that this fungal group was causing soybean root and stem rot in 2017. Taproot decline has already surfaced in Arkansas, Alabama, Louisiana, Mississippi and Tennessee.

The fungus first infects the roots of a soybean plant and its symptoms can surface at any time during the growing season, Kleczewski noted. It can cause wilting plants, interveinal necrosis, plant death and spotty stands from poor emergence.

Taproot decline has a distinctively scattered pattern in fields. “The affected plants are very sporadic, so you’ll see one plant that’s looking sick next to a healthy plant,” Kleczewski explained. “Then the higher the incidence, the more overlapping symptoms you see.”

To diagnose taproot decline in the field, growers must split the stem. Like SDS, the inner pith will be white, but the roots will be a striking jet black, with very few fine roots attached.

“What you also can see sometimes are these little, fingerlike projections on the base of the stem or the soil surrounding the stem,” Kleczewski said. “These are actually fungal structures called stromata.” When these pathogens infect trees, they also produce these stromata, which are called “dead man’s fingers” or “devil’s horns.”

Yield loss is possible as disease severity increases. “In Louisiana, they’re looking at anywhere between 700,000 bushels to 3.5 million bushels being lost annually since they first started seeing this disease cause problems in 2014,” Kleczewksi said. “That’s one of reasons why I want people to keep their eyes out for this disease, and if we do see it, let’s nip it in the bud.”

The fungus overwinters in the soil, so the disease can do well in fields with continuous soybean fields with lots of soybean residue and plant debris. Scientists have also found that precision planting practices that allow farmers to place soybean seeds in exactly the same slot year after year, can increase infection rates. “You’ve basically inoculated your seeds with this pathogen,” Kleczewski noted.

Scientists and companies are working to identify soybean varieties with tolerance to this disease, as well as seed treatments that protect against it, Kleczewski said.

Click on these links to read more about taproot decline:

http://news.utcrops.com/…

https://sites.aces.edu/…

https://www.dtnpf.com/…

RED CROWN ROT

Red crown rot in soybeans is caused by a fungus called Cylindrocladium crotalariae, which infects roots early in the season after planting, but plays the long game. Symptoms often don’t emerge until R3 to R5 growth stages, at which point, it can cause interveinal chlorosis, wilting, dying plants and rotting roots.

As its name suggests, red crown rot is distinguished by the tiny red structures called perithecia that cover the stem surface near the soil, giving it a reddish tinge.

“When you split those stems, you’ll notice there’s a little bit of that browning in the lower part of the crown, but instead of that healthy white center pith, it will have a gray look to it just in the lower part of the crown root — unlike in charcoal rot, where that color can extend up the stem,” Kleczewski added. The disease does well in warm, wet soils and it can overwinter and survive for two to three years in the absence of a soybean host. As a result, management recommendations mostly involve rotating away from legumes for at least two years, avoiding heavy residue and improving drainage in wet spots in the field.

The disease’s sudden appearance in Illinois in 2018 remains a bit of a mystery, Kleczewski noted. “It shouldn’t be here in Illinois!” he said. “It probably came over on some equipment and established itself. We want to look and do some surveys this year, so if anybody sees red crown rot, we’ll be taking samples.”

Click on these links to read more about red crown rot:

http://www.aganytime.com/…

http://www.mississippi-crops.com/…

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow her on Twitter @Emily_Unglesbee

(PS/AG)

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