Wheat stripe rust disease caused major crop losses last season to much of California's 675,000-acres winter wheat crop, and University of California farm advisors are advising producers to be alert for another possible outbreak in the 600,000-acres crop this season.
Yields were reduced by as much as 75 percent in the Sacramento and San Joaquin valleys due to wheat strip rust and more than 40 wheat-producing counties in Northern California were declared disaster areas.
Now is the time to start checking for wheat stripe rust, according to Doug Munier, Glenn County field crops farm advisor.
Last year, Lee Jackson, UC Davis cereals specialist, found wheat stripe rust very early. By March, wheat stripe rust in California was severe and widespread. Several new races were present last year, so some varieties previously resistant were greatly affected by this important wheat disease.
At this early stage of wheat development, stripe rust is difficult to see, according to Munier.
“It develops slowly and will not show up in the upper leaves until later. It will often appear on the bottom leaves, possibly a leaf laying on the soil surface, and will appear as necrotic blotches surrounded by a few small pustules. These pustules may have the spores washed off of them by any recent rain,” explained Munier.
The new wheat variety Summit was still very resistant to last year's strains of the wheat stripe rust fungus disease. Much of the acreage in the Sacramento Valley this year is planted to Summit, according to Munier. This should result in normal yields unless new strains develop which overcome this resistance. The wheat stripe rust fungus has been rapidly developing new strains over the past few years.
If milling quality wheat is the goal, Munier said Summit may need nitrogen applied during the boot to flowering stage. Thirty to 40 pounds of actual nitrogen usually raises grain protein by 1-2 percent.
Munier said many wheat growers have responded to the increase in wheat stripe rust by planting barley. “There are two newer University of California varieties, UC 937 and UC 933 which have yielded very well with good tolerance to barley leaf diseases,” said Munier.
Colusa County Farm Advisor Jerry Schmierer warned that even though stripe rust-resistant strains were identified last year, that does not mean a new strain cannot develop on these new varieties.
“Resistant varieties are the first line of defense against this disease, but we must keep watch so that we don't let a new strain catch us off guard,” said Schmierer.
Rust spores, he said, are spread by wind to initiate infection. Disease development is most rapid at temperatures of 50-60 degrees with intermittent rain and dew. Secondary cycles occur at seven to 10 days intervals.
If wheat stripe rust is found this early in the growing season some growers may consider using a fungicide, which may be economical considering the current weather pattern and amount of time left for grain-fill.
Jerry Schmierer, Colusa County farm advisor, and Munier conducted several fungicide trials last year and will do several more this year to get more information on the economical effectiveness of fungicides for wheat stripe rust.
The two better fungicide options are Tilt and Quadris. Tilt may be applied no later than full flag leaf emergence. It cannot be applied to forage wheat. Quadris can be applied up to late head emergence. The Quadris label does not address wheat for silage, but says it can be used on wheat for hay (defined as dried to 20 percent moisture before being fed).
Schmierer said to “positively identify the rust before spraying. Consider spraying if rust is found on 10 percent of any leaf on 10 percent of the plants in a stand.”
General yellowing of lower leaves without any evidence of pustules cannot be attributed to the rust pathogen.
Dry, windy conditions should slow the disease's progress.
Tulare County Farm Advisor Steve Wright said multiple factors contributed to the severe yield loss last season to the central valley wheat crop from stripe rust. The most often cited factor is the increasing acreage of early-planted susceptible wheat varieties grown for silage.
“This early-planted acreage served as a source of inoculum for spreading the disease,” he said.
Next was the near perfect weather conditions during the 2002-03 growing season that was ideal for the development and spread of the disease. And finally, the genetic adaptability of the stripe rust organism to develop new races capable of infecting a wide range of host plants.
A study done in Kings County last year indicated that for every unit increase in stripe rust resulted in a 794-pound yield loss.
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