USDA research plant pathologist Xianming Chen says that stripe rust has a low potential to damage spring wheat this year. (Capital Press file photo)

Stripe rust season is “generally over” for winter wheat, and has low potential to cause damage to spring wheat due to dry weather, USDA research plant pathologist Xianming Chen said.

Fungicide application is not recommended except for fields planted with susceptible or moderately susceptible varieties under irrigation or in areas with high moisture.

“This year definitely we had more stripe rust and a little bit higher pressure than 2021,” which was so hot and dry the fungus was not an issue, Chen told the Capital Press. “This year has been a little bit less than last year, because the rust started late.”

The rust began in April. Since then, the weather has been “pretty much dry,” and not too hot, except for brief showers, so the disease didn’t develop very quickly.

“This year is a relatively less rust year, but we have a good rust level for germplasm screening,” Chen said. “That’s what we like to have.”

Rust was lower in commercial fields due to farmers applying fungicide, he added. Most varieties grown commercially have some level of resistance.

Bobtail

Chen’s research team did not notice obvious differences in stripe rust reactions except for the Oregon State University semi-dwarf soft white winter wheat variety Bobtail, which had “distinct reactions in different nurseries even at the same locations,” Chen said.

“Based on this year’s observations, Bobtail may have two different sources of seeds — one is still highly resistant to stripe rust while the other is or has become moderately susceptible,” Chen said. “From next year and forward, growers need to pay attention to stripe rust if planting this variety.”

‘An alarm to the world’

The wheat genes “Yr5” and “Yr15” are among the few that offer effective all-stage resistance against stripe rust pathogen populations worldwide, Chen said. The two genes have been used “quite intensively” in breeding programs within the last 20 years.
Stripe rust isolates virulent to Yr5 were reported in Australia and India many years ago, and in China and Turkey in recent years, but have not spread.
Virulence to Yr15 was also reported in several countries many years ago, but “has not caused much concern until this year,” Chen writes.

Stripe rust was first found on wheat varieties carrying Yr15 in the United Kingdom and later found in several European countries, including Denmark.

“Apparently the rust has caused damage in commercial fields grown with wheat varieties carrying Yr15,” Chen wrote in his latest stripe rust update. “This is an alarm to the world.”

No stripe rust isolates have been found in the U.S. with virulence to either Yr5 or Yr15.
So far, about 15 to 20 varieties, mostly spring wheat, are grown commercially with the two genes together, including Washington State University’s Seahawk and Patwin 515 in California. About 80% of California wheat acreage is currently grown with various 515 varieties carrying both Yr5 and Yr15, Chen said.

Because of the combination of Yr5 and Yr15 in U.S. wheat varieties, the Yr15 race from Europe or the Yr5 race from Asia will not be an immediate threat to U.S. varieties even if the races spread to the U.S., Chen said.

But virulent mutants may arise from the rust population within the country and the pathogen is able to combine different virulent genes.

“We all need to pay attention to rust in the fields,” Chen said. “When growers find anything unexpected, collect samples, or let me know and we can look at the situation.”
Send samples to Chen via USDA and WSU, 410 S.E. Dairy Road, 114B-101, Pullman, WA 99164.