General Characteristics. Three rusts (stripe rust, leaf rust, and stem rust) occur on wheat in the Pacific Northwest. Stripe rust appears as golden-yellow, long, narrow stripes on the leaf surface and glumes; leaf rust appears as small, red pustules on the leaf surface and leaf sheath; and stem rust appears on the stems and as large, red-brown, diamond-shaped pustules on the leaf surface. Stripe rust and stem rust can also occur on the heads. Stripe rust and leaf rust overwinter on wheat and rapidly increase during the spring. Stripe rust develops during the cool temperatures of early spring. Leaf rust develops at warmer temperatures later in the spring. The stem rust pathogen does not usually survive on living plants during the winter. It survives as dormant spores on straw and depends upon the common barberry for completion of its life cycle. In the spring, the dormant spores germinate and produce another type of spore that infects barberry leaves but not wheat. Spores produced on the barberry are the source of inoculum for the wheat in the spring. Therefore, elimination of the barberry would eliminate or reduce stem rust.

Historical Importance. In the late 1950's and early 1960's stripe rust caused losses in excess of 70 percent. Since then, destructive epidemics of stripe rust that cause losses of more than 20% have occurred in fields of susceptible cultivars in three out of four years and every year in western Washington. Stripe rust reduced yields in the Pacific Northwest by more than 20 percent in 1981 and more than 15 percent in several other years. Without development of resistant cultivars and emergency registration of a fungicide (Bayleton) for rust control, losses caused by stripe rust in Washington would have exceeded 50 percent in 1981. Omar, was completely destroyed in 1981. A new form of Puccinia striiformis that attacks barley (barley stripe rust) is now present in the Pacific Northwest. Barley stripe rust looks like wheat stripe rust but is a different pathogen that is more severe on barley and is not a destructive disease of wheat. We expect barley stripe rust to damage barley in the same manner that wheat stripe rust has damaged wheat in the past.

As we develop wheat cultivars with better stripe rust resistance, leaf rust becomes more important because tissue not damaged by stripe rust is damaged by the later developing leaf rust. Consequently, leaf rust has become increasingly more important since 1962. Losses caused by leaf rust in susceptible varieties have exceeded 50 percent in some years. When not controlled, leaf rust has reduced yields by more than 20% in one out of two years since 1974. When not controlled in irrigated fields, leaf rust can cause severe losses almost every year. Those losses have exceeded 60 percent in some fields.

Stem rust is less frequently severe, but when present, it can cause major damage to both wheat and barley in specific areas. In 1980 to 1984, stem rust significantly damaged both wheat and barley in eastern Washington and Oregon and northern Idaho, especially in late maturing fields. In 1993, because of late planting of spring wheat and barley and unusually favorable weather, stem rust was very severe in those same regions. Spring crops were again planted late in 1996. Severity of the disease will depend upon weather in late spring and early summer.

Monitoring Rust. Races of Puccinia striiformis, the pathogen that causes stripe rust, are identified by the cultivars that they attack, and new races of the pathogen frequently evolve to attack cultivars that were previously resistant. Figure 1 lists the races of Puccinia striiformis that have been detected in North America and their virulence on differential cultivars. Fifty-six stripe rust races have been identified of which 45 have been detected in eastern Washington.

In 1995, the most prevalent races in the Pacific Northwest were those virulent on Hatton, Tres, Tyee, Moro, Jacmar, Weston, Paha, Yamhill, Fielder, Owens, and seedlings of Hyak, Madsen, Stephens, and Daws (Races CDL-5, CDL-20, CDL-22, CDL-25, CDL-26, CDL-27, CDL-29, CDL-33, CDL-38, CDL-41, CDL-42, CDL-43, and CDL45 to CDL-55). In addition to the wheat stripe rust races, at least 14 barley stripe rust races were identified in the United States in 1994, and more than 35 races have been identified since then. All major barley cultivars are susceptible to the barley stripe rust.

Most winter wheat varieties currently grown in the region are very susceptible to leaf rust. Since the current races can attack most winter wheat varieties and the resistance in spring wheat is now race specific, there is no major selective pressure for new races of leaf rust. Consequently, no significant new races of Puccinia recondita, the pathogen causing leaf rust, were detected in 1995.

Stem rust attacks both barley and wheat. We have more races of stem rust in the Pacific Northwest than in all of the other regions of North America, and races of Puccinia graminis, the pathogen causing stem rust, are uniquely different from races in the other regions. This is because new races more easily arise from the stage of rust that occurs on the common barberry. All barley cultivars and some wheat cultivars are susceptible to stem rust .

Effect of Weather. The rusts are obligate parasites and must have a living host to grow on. The continual presence of living plants (wheat, barley, and some grasses depending upon the rust) throughout the year provides hosts for the rusts and adequate inoculum for initiation of new stripe rust and leaf rust epidemics. Therefore, the factor that is most limiting for rust development is often the weather. When used in combination with monitoring data, a model for predicting stripe rust, based on temperature and precipitation, has proved to be reliable since 1979. When that information is used with precipitation data in the late spring, it has also enabled prediction of leaf rust and stem rust. Severe stripe rust epidemics and leaf rust have been predicted for 1996. As of early May 1996, stripe rust and leaf rust epidemics were already developing in fields of hard red and club wheats in central Washington.

Resistance. High-temperature, adult-plant resistance to stripe rust, which has now been incorporated into all major soft white winter wheats and most spring wheats (see Figure 2), has continued to be durable against all races in the Pacific Northwest. In contrast, the high resistance that is expressed in both seedling and adult plant stages at all temperatures has been effective for three years or less.

Information on the characteristics and inheritance of high-temperature, adult-plant resistance has been obtained, and that resistance has been or is being used in development of new resistant cultivars. New information on the inheritance of race-specific resistance and location of genes for resistance has been obtained, and that information and material should be useful in developing new disease control programs, identifying races, and understanding how resistance works.

We are currently evaluating the national germplasm collection for resistance to stripe rust in the field at Mt. Vernon and Pullman, WA and for specific resistance to the five most virulent stripe rust races in the greenhouse. The five races include all of the identified virulences in North America. As of this date more than 40,000 germplasm entries for regions throughout the world have been evaluated in the field and most of those have been evaluated for resistance to the stripe rust races.

Studies of the inheritance of slow rusting resistance to leaf rust, have resulted in new information and germplasm for resistance to leaf rust. That germplasm has been useful in developing more durable leaf rust resistant wheats.

We annually evaluate commercial cultivars, advanced breeding lines from breeders in the Pacific Northwest, and differential cultivars for resistance to stripe rust, leaf rust and stem rust. The information on resistance of germplasm and advanced lines has made possible the development and release of rust resistant cultivars and has provided breeders with new sources of resistance, lead to a better understanding of how to use rust resistance, and improved the resistance of wheat cultivars.

Use of Fungicides. Resistance to all disease problems may be difficult or impossible to incorporate into a single cultivar, and new races of the pathogens are a frequent problem. Therefore, additional control measures are necessary. We have an ongoing program to study the use of fungicides for control of the diseases as part of an integrated disease control program. Results of those studies show that foliar applications of Bayleton, Tilt, Folicur, Spotless, Punch, ASC-66811, SAN-619 (Alto), RH-7592 (Govern), LS86263, and BRC-519 (Quadris) control stripe rust, leaf rust and stem rust, especially when applied at jointing to heading stages of growth, and they have effectively prevented major wheat losses. Some of the chemicals also control leaf spotting fungi such as septoria. Baytan, Raxil, and several new experimental fungicides applied as seed treatments control early rust and mildew development, and if they are, manage to prevent a delay in emergence, their use significantly improves yields. Bayleton has been used as a foliar spray since 1981 to control stripe rust and leaf rust when existing cultivars become susceptible to new races and in combination with various types of resistance. Tilt was registered for rust control in 1988 and has also been

highly effective as a control for the diseases. Baytan was registered as a seed treatment in 1989. The remaining chemicals are still in the experimental stage and are not yet cleared for use on a commercial basis. However, several are expected to be registered in the near future. Guidelines for the use of the chemical seed treatments and sprays have been developed based on type of rust, type of resistance, intensity of rust, stage of growth, potential yield, and economic return. A new expert system, MoreCrop, has been developed for managing wheat diseases and is explained in another section.