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Stripe rust muddles wheat variety selection

Selecting the right wheat variety for North Texas growing conditions used to be fairly simple. Pioneer 2571 consistently produced good yields and a late fungicide application was adequate to take care of leaf rust disease.

Consequently, most growers put a lot of acres in 2571.

Then stripe rust changed the equation and created a variety selection dilemma. Pioneer 2571 proved to be susceptible to stripe rust and late fungicide applications proved ineffective in limiting damage. Stripe rust attacks early and if a farmer waited to spray until he would treat leaf rust, it was too late.

“Until the 2000 crop, we had seen little stripe rust in North Texas,” says Jim Swart, Texas Extension integrated pest management specialist at Commerce. “After 2000, we had to regroup,” Swart told growers recently at the annual Agricultural Technology Conference, held on the Texas A&M-Commerce campus.

“We got hammered by stripe rust in 2000. Fortunately, we had variety trials out that year so we could document those that appeared to be tolerant of the disease. Growers responded the next year and planted less susceptible varieties.”

Part of the dilemma, Swart says, is stripe rust occurs when early conditions are good for wheat.

“Stripe rust develops in early season, around mid-March, and becomes a problem in cool, moist conditions. Those conditions are good for wheat, as well. By mid-April, the disease subsides.”

He says stripe rust is recognizable by the linear pustules on the leaf surface. Leaf rust displays darker orange pustules in a scattergun pattern and comes on later in the season.

Leaf rust has caused more trouble for North Texas wheat in the past and has attracted more attention from researchers.

“Wheat breeders have spent a lot of time over the years developing wheat resistant to leaf rust.” Stripe rust had occurred so rarely that breeders did little to develop tolerant strains.

The variety and fungicide trials Swart had out in 2000 and in subsequent planting seasons have identified key differences growers have begun to use.

Swart looked at three varieties, Pioneer 2571, Pioneer 25R57 and Agripro Patton, and a fungicide, Folicur, applied as an early preventive treatment and a late therapeutic treatment. He also had an untreated check. He evaluated presence of both stripe and leaf rust, yields and pounds per bushel.

“We tested three varieties, two locations and three fungicide treatments,” Swart said. “Most of our acreage has switched to Pioneer 25R57. It's resistant to stripe rust and produces good yields. It is susceptible to leaf rust, however.

“Agripro has heavy stripe rust infestations but is resistant to leaf rust. Pioneer 2571, the old standard, is moderately susceptible to both rusts.”

Swart cautions that results come from a relatively short trial period, but he's made some observations that should help in developing a variety and a fungicide program. Since stripe rust occurs early, a preventive treatment is best with susceptible varieties, Pioneer 2571 and Agripro Patton. With varieties more susceptible to leaf rust, a therapeutic, late application would be best.

In one test Pioneer 25R57 yielded so well the difference between treated and un-treated plots was hard to identify, and stripe rust was not a factor.

Some leaf rust showed up: 9 percent with an early treatment, 15 percent with no treatment and only 1 percent with a late application. Yields for all three topped 70 bushels per acre: 70.6 for non-treated, 78.3 for early and 81.1 for late.

“If cool, moist conditions in mid-March make stripe rust likely, an early, preventive treatment is best,” Swart said. “If leaf rust is more likely, I'd recommend a late, therapeutic application.”

He offers what he calls a “knee jerk” recommendation, based on limited research. “We can't live with stripe rust, so plant a resistant variety, Pioneer 25R57. Unfortunately, we have no variety that's resistant to both stripe and leaf rust, but 70 percent of our acreage has gone to 25R57.

“So, we have to be watchful, scout for leaf rust and spray as needed.

“If leaf rust is more likely, consider selecting another variety and in any case, plant more than one variety.

“Scout, beginning in mid-March.

“Time fungicide applications for the specific disease.

“If it's not broke, you can't fix it. If you select a resistant variety, don't spray it to prevent stripe rust. We've seen no response to fungicide if no disease is present.”

Swart said the Texas Wheat Board has aided his research efforts. “They've provided excellent cooperation with this effort.”

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