Cool, wet conditions this spring, reminiscent of 2003, set off severe stripe rust outbreaks in wheat up and down California's San Joaquin and Sacramento valleys, but growers who applied fungicides in time gained some control over the disease.
Prior to harvest, Steve Wright, Tulare County farm advisor, said he expected to see yield losses of up to 10 percent to 15 percent. While significant, that is short of the disastrous losses of 40 percent to 60 percent that occurred in 2003, when the rust struck a number of susceptible varieties in early spring.
In 2006 it overcame Summit and Blanca Grande varieties, previously thought to be resistant. Wright noted the disease is pumping out new races at an increasingly rapid rate.
Yecora Rojo stood off the rust for 25 years, but Summit was resistant for only three seasons. Even some varieties of triticale, previously resistant, showed infections.
Gene Aksland, agronomist with Resource Seeds Inc. at Goshen, Calif., said stripe rust has reached epidemic proportions on a worldwide basis.
He noted that 14 races of stripe rust are known to occur in wheat, multiple races often appearing a single field. The spring weather was blamed for most infections, but some races have become tolerant of warmer temperatures.
Aksland said the pathogen is somehow able to survive between wheat crops in a “green bridge,” about which little is known.
The wheat breeders' dilemma, he said, is how to combine resistant genes with the agronomic traits necessary for a successful variety. The solution will be an integrated management program to manage the rust.
“We can rapidly run out of genes to work with, and the disease can also develop resistance to fungicides. It will have to be a combination of resistant varieties and fungicides.”
Seed companies are aggressively continuing selection and evaluation of new plant material, and some is promising, but the process is slow. Perhaps only three selections are made from among 1,000, and then it takes about 10 years of development between the first cross of promising material to a new variety ready for market.
Jerry Schmierer, farm advisor in Colusa, Sutter and Yuba counties, said the majority of the wheat acreage there was planted to Summit, one of the latest in a string of varieties initially resistant but later very susceptible to the mutating fungal disease.
Some fields, he said, were a complete loss anyway because of water-logged conditions after spring rains, regardless of infections, which were the worst since 2003. The rust caught many growers unaware that year and their fungicide treatments were too late.
However, he added, those growers who responded in time to yellowed “hot spots” of infection this spring with fungicides were able to control the rust.
Fungicides with propiconazole as the active ingredient, such as Tilt, and strobilurins, such as Quadris, were equally successful, provided they were applied at their respective recommended stage of crop development.
Schmierer said his trials in 2004 indicated that a fungicide treatment costing about $25 an acre translates to a grain yield gain approaching 500 pounds. He explained, however, the bushel weight and wheat price still influence whether a fungicide treatment is economical.
The return on investing in a treatment also hinges on the severity of the disease. In some seasons, such as 2005, he said, the disease may be present in a field but not spread to a treatable level.
“Resistance,” Schmierer said, “is still our first line of defense, and hopefully there are some materials in the pipeline. The problem is resistance breaks down fairly fast, so growers need to assume that a new variety can get stripe rust, inspect fields for it, monitor the weather for temperatures below 70 degrees F with high moisture, and treat with fungicides as necessary. Generally, I recommend growers just plan to factor in a treatment.”
Ideally, he said, growers would have their acreage divided up between three or four varieties with different levels and sources of resistance. “Otherwise, we are really asking for trouble when we plant a single variety.”
In Glenn County, stripe rust “hammered” Summit fields severely this year, according to farm advisor Doug Munier, who agreed that genetic resistance is preferred, but complete resistance to the disease's stream of new strains has not been achieved thus far.
And although fungicides have proved to be highly effective, the trick remains in determining whether to treat. The key time is fairly early-on in expression of the disease, so it's a gamble on how much the disease will develop that particular season.
“We do know that the wetter the year,” Munier said, “the greater the chances of disease. And another thing is the earlier the planting, the earlier the onset of disease. Here in the northern Sacramento Valley an early planting would be in late-October or early-November.
“With a late-November or early December planting, we generally see less of the disease. But because of our rain patterns it's not that simple. If you wait that late, you may not be able to plant all your acreage before it gets too wet.”
Munier went on to say that continuing variety trials are identifying materials having improved resistance.
Prior to harvest this season, Munier said, inspection of the crop showed kernel shriveling and reduced growth caused by the airborne fungus, pointing to severe reductions in both yield and bushel weight.
Significant for growers in his county, he said, is the current wheat price range of $5 to $7 per hundredweight needed to offset a $25 per acre fungicide treatment. The treatment, however, may be combined with a nitrogen application to increase grain protein content for some economy.
For 2007, he suspects most growers growing wheat will probably again plant Summit or Blanca Grande and budget for a fungicide treatment. “That is, unless we have a severely dry year.”
Others will be planting less wheat and looking at alternatives, including barley or garbanzos, or summer crops, such as cotton or corn.
Lee Jackson, Cooperative Extension agronomist at the University of California, Davis, said his main thrust is to work with breeders across the United States to turn out new resistant material.
As part of a continual process, he said, a new generation of resistant varieties will be available next season and will hopefully have stable resistance through different combinations of genes.
In the meantime, he noted, it is important growers be aware that if their wheat becomes susceptible to the rust, the only option is using fungicides to minimize damage. “If they treat before the disease has done a lot of damage, they can nearly fully control the disease.”
But, Jackson warned, “one constraint is the continual weather patterns we have that promote new infections, while most of the available fungicides have a residual activity of only about three or four weeks.”
According to the National Agricultural Statistics Service, California winter wheat production for 2006 was forecast at 396,000 tons, or 39 percent less than 2005. Harvested acreage was expected to be 220,000, for an average yield of 1.8 tons per acre.