Severity of rust depends on winter conditions

By MATTHEW WEAVER

Capital Press

Growers are being advised this fall to plant wheat varieties that are not susceptible to stripe and stem rust.

Stem rust was more significant in the Palouse region this year compared to last, particularly in Washington's Whitman County, said USDA Agricultural Research Service plant geneticist Xianming Chen.

That's due to rain in late June, he said. Stem rust requires dew formation on leaf surfaces to infect plants, generally between late May to late June.

The source of stem rust is barberry bushes, and Potlatch, Idaho, was heavily infected this year, Chen said. The rust spreads to wheat from the bushes.

More locations had stem rust this year, but some did not have a lot, he said.

Unlike stripe rust, stem rust has less chance to cause a widespread problem in Eastern Washington and Northern Idaho, but significant damage can occur in isolated spots, which may result in a significant hit for some growers, Chen said.

Fungicide application for controlling stem rust is not as effective as for controlling stripe rust, as stem rust occurs in very late stages of growth, typically after the flowering stage.

Avoiding a late crop helps reduce risk, Chen said. But the most effective approach is to grow resistant varieties or avoid susceptible varieties.

Most Pacific Northwest wheat varieties are resistant to stem rust, but not Ug99, the race of black stem rust threatening wheat in the Middle East and Africa that has yet to come to the U.S., Chen said.

Chen and Washington State University plant pathologist Tim Murray recently looked at two winter wheat fields between Palouse and Colfax, Wash., that had significant stem rust. Varieties AP 700CL and ORCF 102 were grown in the fields, and there were many big hotspots, up to 50 meters in diameter, with plants in the center having more than 80 percent severity. Chen said a rough estimate of yield loss for entire fields was 3 to 5 percent.

If susceptible or moderately susceptible varieties are already planted, Chen said, farmers cannot do anything until next spring.

Whether stripe or stem rust will show up depends on the winter and spring weather, he said. The first forecast for next season will come after December, he said.

"If this winter is cooler than normal, then the rust will be late and very likely will be lower level," he said. "If this winter is warmer, then we will have an early rust and likely a very severe rust."

The likelihood of stem rust next year will depend on how much barberry infestation there is, Chen said.

He is calling for new wheat cultivars that are resistant to stem and stripe rust, such as soft white winter wheat Lambert or hard red winter wheats Eddy and Declo.

If all winter wheat fields had the resistance level of the variety Madsen, the standard of stripe rust resistance, Chen said yield loss would have been less than 1 percent without fungicide application.