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AgriLife Research cotton pathologist pursues a mystery

by Steve Byrnes, Texas A&M AgriLife

Dr. Terry Wheeler and her colleagues are embroiled in a mystery.

Wheeler, a Texas A&M AgriLife Research plant pathologist at Lubbock, and her cohorts Dr. Jason Woodward, Texas A&M AgriLife Extension Service plant pathologist at Lubbock, and Dr. Tom Isakeit, AgriLife Extension plant pathologist, College Station, have been getting calls from cotton growers across the Southern Rolling Plains and High Plains about a problem thought to have been resolved 40 years ago.

Since 2015, a growing number of cotton acres across the region have succumbed to what appears to be classical bacterial blight.

The problem though is the blight, which Wheeler said is caused by Xanthomonas citri subspecies malvacearum, known as Xcm, was found in cotton varieties normally considered highly resistant to the disease. 

The malady is a serious yield-limiting disease of cotton. It survives and can be spread in cottonseed or in plant debris on the soil. To make matters worse, most of today’s producers have no experience with it, Wheeler said.

 Bacterial blight has become a major problem in cotton producing areas across much of the south since 2011, Wheeler said. In severe outbreaks, it can quickly defoliate and decimate an entire crop within a matter of days. The worst losses happen when it rots the bolls.

“This problem must be eliminated for the sustainable production of cotton,” she said. “It’s control is especially critical in our region since its ability to be spread through infected planting seed could result in its spread literally worldwide.“

 A generation ago when Paymaster HS-26 was the primary cotton variety in the High Plains, there was no real long-term solution until resistant cotton cultivars were developed. The popular Fibermax cultivars were among these. 

“The concern is further compounded because this disease was found in highly resistant cotton cultivars in Amherst, Halfway, Lubbock, Plains and Victoria in 2015,” Wheeler said. “But this was the first time since resistant Fibermax cultivars were first planted in the region in 2000 that any bacterial blight had been seen on resistant cultivars in the U.S.

What’s causing this outbreak? That’s the mystery Wheeler and her colleagues are actively seeking to solve. She has her suspicions. One is that the age-old problem morphed into a new race. Or, as Wheeler suspects is more likely but has yet to prove, there are two types of bacteria cooperating to become a “super blight.”

“We just don’t know yet,” she said. “If two different bacteria have formed a symbiotic relationship, it will have been the first time it’s ever happened in cotton and, to my knowledge, it would also be the first time such a relationship has ever happened in any plants.”

To test the two-bacteria theory, Wheeler is inoculating hundreds of resistant and non-resistant cotton seedlings in her greenhouse with the malady and with other bacteria, called Pseudomonas syringae, which was isolated in close proximity to the Xcm.

The process involves dipping a toothpick into a petri dish containing the bacteria and using it to scratch an “x” on the underside of the seedling leaves. Affected plants soon start showing a reaction ranging from “water soaking” of the leaves — in which the leaves exude moisture along the x-mark — to yellowing and eventual leaf loss.  

 “The goal of this project is to determine if and why the situation with bacterial blight in cotton has changed,” Wheeler said. “To date, we are in the midst of gathering clues, but have no definitive answer. But that’s what researchers do, you give us a problem and we strive to find a solution and then ways to cope with it.”

In the meantime, Wheeler said Woodward, Isakeit and other Extension specialists in Georgia and Mississippi, two other states with the problem, are developing materials for producers to manage the disease through integrated pest management principles.

These include identifying current cotton cultivars and their various levels of susceptibility to the disease. Wheeler said this is important because while some of the cultivars formerly thought to be resistant now can contract the disease, they do not suffer the damage to the extent that non-resistant cultivars do.

“It will also be very important to plow in infected plant material and to only plant seed that is clean of the bacteria,” Wheeler said.

These changes in production practices, particularly the choice of cultivars, will be monitored by AgriLife Extension personnel through surveys.

“If through our greenhouse testing efforts or through the monitoring of the coming season’s cotton crop a new race of Xcm is found, it will be made available to cotton breeding programs, so resistant cultivars can be created to combat the problem,” Wheeler said. “However, after testing hundreds of Xcm isolates, we have not found a new race of Xcm. They are all the standard race 18 that have been present since the late 1970’s.”

But for now it, whatever “it” is, be it a new race, an unheard-of dual bacterial cooperation or something else not remotely related, remains a mystery that hopefully will be solved before too many more cotton crops pass.

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