Rust Update 7/2/10

July 2, 2010

Xianming Chen
Stripe Rust of Wheat

With the majority of fields have been sprayed with fungicides, winter wheat crops have reached stages by which fungicides may no longer be applied. Head infection is more common this year than in past years due to the much more rust-favorable weather conditions and heavy inoculum. Rust damage, including loss of grain yield and reduction of grain quality (smaller grain size, shrivel grain, and lower test weight and flour rate), is mainly caused by leaf infection. Head infection has a much less effect on grain yield and quality than leaf infection. Rusts do not produce known toxins and are not seedborne, and therefore, head infection will not cause problems of toxins in food products and carrying the pathogen to next season. Unlike smuts and bunts, rust on head should not change grain color as rust spores will be blow away during harvesting and threshing. However, it is a good idea to get grain as clean as possible.

Stripe rust has been and will continue increasing in spring wheat crops. Crop growth stages range from late stem elongation to flowering. Rust severity (leaf areas infected) reaches up to 100% in some nurseries. Many fields have been sprayed with fungicides. If not, fields of crops with a susceptible reaction should be sprayed, those with an intermediate reaction may or may not need to be sprayed depending upon yield potential, and those with a resistant reaction should not be sprayed.

According to Dr. Juliet Windes, stripe rust of wheat is developing in southern Idaho. Dr. Mary Burrows has reported that stripe rust is occurring in Montana. Based on reports by Drs. Marcia McMullen, Marricells Acevedo, and others, stripe rust has developed to its highest level of the recorded history in North Dakota. According to Dr. Gary Bergstrom, stripe rust of wheat showed up, for the first time, in the state of New York. Dr. Wayne Temple reported severe stripe rust on both winter and spring wheat in British Columbia, Canada. So far, wheat stripe rust has been reported in 23 states of the U.S. (California, Oregon, Washington, Idaho, Texas, Louisiana, Arkansas, Oklahoma, Mississippi, Kansas, Nebraska, Illinois, Maryland, South Dakota, Indiana, Wisconsin, Colorado, Virginia, Delaware, Minnesota, North Dakota, New York, and Montana) and 3 provinces of Canada (Ontario, Manitoba, and British Columbia).

Stripe Rust of Barley

Stripe rust of barley has been found in the Palouse region (Whitman County, Washington). In our experimental nurseries, rust reached 15% of severity and 40% of incidence (percentage of plants infected). Rust of up to 2% of severity and <1% of incidence was found in the McGregor variety testing nursery west of Colfax yesterday (July 1). With weather conditions still favorable to stripe rust, barley stripe rust is expected to develop to severity levels significantly higher than the very low levels in the past several years, but not as a big problem as wheat stripe rust. Please check your field within the next week or so. If rust incidence reaches to 10%, consider fungicide application.

Leaf Rust of Wheat

Leaf rust of wheat is occurring in the Palouse region. On June 29, Dr. Michael Pumphrey found few pustules of leaf rust in his breeding nursery near Pullman. Yesterday (July 1), we found leaf rust (1-2% of severity, <1% incidence) on several winter wheat varieties in the McGregor’s variety trial field west of Colfax.

As quite severe leaf rust was found in central Washington two weeks ago and the disease is now occurring in eastern Washington, we expect to see significant levels of leaf rust this year. Fungicides that have been applied to control stripe rust also have protected crops from leaf rust infection. However, leaf rust may develop on crops about one month after fungicide application. Nevertheless, leaf rust will not be as severe and widespread as stripe rust.

Stem Rust of Wheat

On June 28, stem rust was found in the winter wheat field near the barberry bushes near Potlatch (Latah County, Idaho). The severity level (<1% severity, <1% incidence) was much lower than that of the winter wheat field this time of last year. No stem rust was found in the spring wheat field, which is closer than the winter wheat filed to the bushes. The relatively low level of stem rust this year is due to the facts that both spring and winter wheat fields have been sprayed to control stripe rust and that the spring wheat variety is likely resistant to stem rust based on our variety tests with the isolate from the same field last year. We expect that stem rust will develop to significant levels, but should be localized as in the last two years.