Stripe rust presents in county wheat

WHITMAN COUNTY – Stripe rust has been discovered in varieties of winter wheat on the Palouse and actively producing spores.

The rust was first spotted in Washington State University's experiment fields around Pullman. Dr. Xianming Chen, WSU research plant pathologist said the appearance of the rust is normal for the Palouse and is about a week earlier than last year.

"Stripe rust occurs in the Pacific Northwest, especially the Palouse region, every year," Chen said. "Significant damage occurs three of every five years. If growing highly susceptible varieties and without using fungicide, yield losses can be as low as three percent and as high as about 90 percent with an average of about 43 percent over the last 40 years."

He said the currently grown wheat varieties have an average yield loss of eight percent if fungicide is not used.

Chen said for fields with moderately susceptible varieties, ratings between five to nine in the Seed Buying Guide, but have not been sprayed with fungicide, the field should be sprayed within the next two weeks. He said if the wheat is between one to four in susceptibility, fungicide is generally not needed.

"For spring wheat, the first application of fungicide is needed at the time for herbicide application for fields planted with moderately susceptible or susceptible varieties," Chen said. "Second application is generally not necessary as the summer is predicted to be quite hot."

Contributing factors

The three main factors that contribute to stripe rust are the host plants, pathogen inoculum and the environment.

"The stripe rust fungus can only infect and grow on susceptible host plants including wheat and some wild grasses such as goatgrass and foxtail," Chen said. "Every commercially grown wheat variety in our region has some level of resistance to stripe rust, but not every variety has a high-level resistance that can completely avoid stripe rust infection or damage."

Wheat varieties are rated for stripe rust resistance and susceptibility on a scale from one to nine. One being the most resistant and nine being the most susceptible. Chen said the scale was formed on past testing data.

He said based on the current stripe rust situation, if the wheat variety is in the five to nine range it is encouraged to use fungicide for those fields.

An inoculum is considered any part of the pathogen fungus, such as the spore, for example. Chen said spores of the stripe rust fungus, produced from infected plants, can infect the same plants and nearby plants of wheat and grasses, producing more spores to re-infect and infect new plants.

"The spores can be spread by wind to infect other fields even hundreds of miles away," Chen said. "How many cycles of infection and how much amount of spores produced depend upon the susceptibility of varieties and acreage of susceptible varieties, some characteristics of the pathogen population such as virulence to host plants, aggressiveness for growth and fitness to the environment."

Chen said since the rust pathogen is now in the Palouse region, additional spores can be blown to the region from other regions where stripe rust has been occurring, such as Walla Walla and different areas in central Washington.

The major environmental factors that contribute to stripe rust are moisture and temperature.

"Rust spores need dew formation to germinate and grow into plants. Under high moist conditions, more infections occur, leading to more severe rust," Chen said.

He added that the recent showers in the region have created ideal conditions of dew formation during nights.

When there is no dew, stripe rust spores cannot germinate and infect other plants and will die off.

"The more and longer the high moisture conditions, the faster the rust develops, more severe rust, and more damage to wheat crops," Chen said.

Temperature conditions need to be ideal for stripe rust to survive. The fungus thrives in a cool environment but doesn't survive in too cold or too hot temperatures. Wheat plants can be infected with stripe rust in the fall and will be able to survive if the winter is mild.

"When the weather becomes warm in March to May, stripe rust shows up, producing new spores," Chen said. "Once rust enters plants, it can stay there alive for as long as six months, infecting in September to November and reappearing in March to May of the following spring."

He said the infection cycle can be as short as two weeks under optimum temperature conditions. These conditions would be 45-55 degrees at night and 65-75 degrees in the daytime.

Under favorable condition, of cool and wet, one rusted leaf in early May could lead to an acre of completely rusted in roughly a month.

Chen said if the weather continues as dry and hot, the rust may stop.

"Growers should check their fields once a week, if they see active rust, producing yellow-orange colored powders, rust spores," Chen said. "Fungicides can protect crops for 20-40 days depending upon chemicals. Therefore, growers should check their fields about three weeks after the first fungicide application."