Taproot Decline Nibbles Away Yield

Is a mystery soybean disease lurking? It might take a hands-and-knees approach to find a new malady in Midsouth soybean fields, but plant pathologists are keeping a close eye on taproot decline (TRD) and hoping it remains relatively mild in virulence.

In 2005, Arkansas and Mississippi crop consultants began spotting isolated soybean plants with moderate chlorosis, dead nodules and degraded taproots. Two years later, Tom Allen, a plant pathologist with Mississippi State University Extension, knew he was dealing with a soybean disease unreported in literature.

Many soybean diseases behave in a deceptively similar fashion. To some, TRD looks similar to sudden death syndrome (SDS) found in the Midwest. SDS will often manifest in a pickup-truck-sized area, or larger areas when soybean cyst nematode (SCN) is present. Conversely, TRD appears to be limited to a small group of plants or single plants scattered across fields.

TRD produces interveinal chlorosis. A degraded taproot on a plant removed from the soil profile is a telltale TRD sign. Wash away the soil and portions of the root appear to have been dipped in ink. There is often tremendous black fungal growth at the base of the stem and on the taproot below the soil line. In some cases, thick, ropey-like, white mycelium is present. Cutting into the stem with a pocketknife reveals discolored vascular tissue. 

“When you grab and jerk, if you leave the taproot behind, it’s TRD,” Allen says. “Typically, SDS doesn’t remove the entire root system by jerking unless something else is at play like hardpan or drought issues, at least in the southern U.S.”

TRD-infected plants can look like runts obscured by the upper canopy. In infected plants, there is usually enough absorption of nutrients and moisture for the plant to complete the season, but it won’t produce normal yield. To date, TRD has been found in Arkansas, Louisiana and Mississippi on dryland and irrigated ground. 

Yield losses from TRD are typically minimal, Allen says. However, in 2014, a Mississippi field with severe TRD suffered an 18% yield reduction. 

Will TRD increase in severity? “Right now, we just don’t know much about TRD’s disease cycle,” says Terry Spurlock, University of Arkansas Extension plant pathologist. “The data from the past two years indicates it is behaving like a soilborne disease. Soilborne diseases spread short distances from their point of origin, unless contaminated plants, plant residue or soil are transported elsewhere. It’s possible TRD could get worse in fields where it already occurs or we may begin finding it on more acreage.”

In 2015, the genus of the fungus was identified and labeled TRD. However, the species of fungus causing TRD remains unknown, and research isn’t far enough along to offer answers. Allen and his colleagues have isolated the fungus, reinfected plants and produced similar disease symptoms, but he doesn’t expect a final species answer until 2017 at the earliest.

Right now rotation seems to be the only means of blocking TRD symptoms. “The fungus isn’t listed on any fungicide label and is rare to find on the Internet,” Allen says. “There is no treatment for TRD in 2016.”

A task force of six researchers from TRD-affected states has been formed. Allen hopes to begin TRD field trials in 2016. “Yield losses have been minimal in almost all cases,” he adds. “If growers spot TRD, they should take note but not panic.”  

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