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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 4 - Disease Control, No. 5, April-May 1986


STEEP Researcher Leading Cereal Rust Control Effort

Roger Veseth

Rusts are the most destructive foliar diseases of cereals in the Pacific Northwest. Stripe rust and leaf rust are the predominant rust diseases, although stem rust can also be severe in some areas. The rusts are difficult to control because the spores of the fungus are spread by the wind and the disease level is greatly influenced by yearly variations in weather conditions.

STEEP researcher Roland Line, USDA-Agricultural Research Service (ARS) plant pathologist at Pullman, WA, has concentrated much of his research effort on rust control. His research involves a cooperative effort with several pathologists, plant breeders and other research and extension people throughout the region. In addition to ARS support, funding for his rust research comes from the Washington Wheat Commission. Line's research covers several areas including: assessing losses caused by rusts; monitoring and predicting rust epidemics; cooperating in varietal studies of the characteristics of resistance; use of fungicides; and other strategies for integrated control systems.

Life Cycle Review

Stripe and Leaf Rust

Stripe and leaf rust have similar life cycles. In the fall, fungal spores called urediospores, produced on the previous wheat crop or on volunteer wheat, are carried by the wind to newly emerging wheat. If adequate moisture is present, the spores germinate and infect the seedlings. The primary source of stripe and leaf rust inoculum in the Northwest is from wheat growing in the region. Spores do not move from the warm Southwest region into the Northwest.

The rusts usually increase slightly in the fall after infection occurs and then overwinter on the living wheat plants, ready for increased spore production in the spring. Stripe rust, which grows best at low temperatures, begins to rapidly increase in the early spring at the same time winter wheat begins rapid growth (March to April). Leaf rust is later, increasing very slowly until temperatures increase in May when winter wheat plants are in the late jointing to boot stages of growth. Then, if moisture is available, leaf rust can increase even more rapidly than stripe rust.

Stem Rust

In contrast to stripe rust and leaf rust, stem rust does not overwinter on living wheat plants. As the infected crop matures, stem rust produces black spores called teliospores which remain dormant in the straw during the winter. In late spring, usually May and June in the Northwest, the teliospores germinate and produce basidiospores which cannot infect wheat or barley, but can only infect young leaves of the common barberry bush. The basidiospores are short lived so the barberry must be within a few miles of the field for the spores to survive and continue the life cycle. From the barberry leaf, the fungus produces aeciospores which can only infect wheat and barley, but not the barberry. Once established in the wheat or barley field, stem rust can increase very rapidly through successive productions of the red urediospores under moist conditions.

The barberry is not only essential for completion of the stem rust life cycle but is also the major source of new races since that is where the only sexual reproductive stage occurs. Barberry eradication programs in the Northwest, initiated in the 1940's, decreased the frequency of stem rust epidemics as well as the number of new races. The eradication program terminated in the late 1970's. Since then, the incidence and severity of stem rust has increased significantly.

Loss Assessment

For 16 years, Line and other researchers have collected data on crop losses caused by stripe rust and leaf rust. Loss assessments were made using selective fungicides and by comparing yields of susceptible and resistant varieties. They determined relationships of yield to rust intensity. In general, when stripe rust was less than 20 percent or leaf rust was less than 30 percent at the soft dough stage, yield reductions were less than 10 percent. When stripe rust was greater than 60 percent or leaf rust greater than 55 percent, yield loss exceeded 20 percent.

The two pathogens have frequently reduced yields by 20 percent or more in the Northwest during years when higher than normal yields were expected. Yield losses of 60 percent have been measured on susceptible varieties. Destructive epidemics of stripe rust have occurred in 15 of the last 22 years.

Leaf rust has become increasingly more important over the past two decades. In 1974, losses in susceptible spring wheat varieties exceeded 50 percent. Since then, leaf rust has caused losses in 1975, 1976, 1978, 1980, 1981 and 1984.

Because stem rust appears late in the season, the typically dry weather of June and July in the Northwest often limits stem rust development. Therefore major losses from stem rust are less frequent. However, the incidence and severity of stem rust has increased since 1980, especially on barley. In 1984, stem rust reduced yields of barley and wheat by more than 30 percent in some fields of eastern Washington and northern Idaho.

Predicting Rust

In order to monitor the severity of stripe rust and leaf rust and the development of new races, Line and other researchers have utilized a series of 15 to 25 trap plots in Western U.S. The trap plots consist of several past and current varieties, and new experimental lines, as well as lines that differentiate races of rust. They are periodically examined to determine rust intensity and infection types. Commercial fields are also monitored between sites. Rust samples from the trap plots and commercial fields are then evaluated on seedlings in greenhouse experiments. From this information, Line and other researchers are able to monitor rust developments in the region and forewarn growers and plant breeders of the vulnerability of wheat varieties and new lines to the races of rust present that year.

Line has developed a successful model for predicting stripe rust, It is based on: the number of negative degree days in December and January; defining the date when rapid winter wheat growth and rust development occurs in early spring; and then calculating the subsequent positive degree days for 80 days after the beginning of spring regrowth. Using the model in combination with monitoring data, Line has accurately predicted the severity of stripe rust each year since 1979. The negative degree day winter component of the model has also been useful in predicting leaf rust.

In developing the model, winter and spring temperatures were found to be the most important variables affecting stripe rust epidemics. Stripe rust was most severe in years with warm winters and cool springs. Precipitation, both frequency and amount, was not a limiting factor except in late spring, and then infection correlated with cool temperatures. Since 1958, warmer than normal

winters and cooler than normal springs have been the trend in the Northwest. These conditions have contributed to rust epidemics. Unlike stripe rust, precipitation in late May through July appears to be important for leaf rust and stem rust.

Stem rust is a warm temperature rust which appears later in the season than leaf rust, usually in late June and July. Frequent rain in May through July is necessary for severe stem rust epidemics. These conditions also delay crop maturity which allows more time for stem rust development, Except for 1985 the last 6 years have been unusually favorable for stem rust.

Resistance

Varietal resistance has been the main focus of rust research through a cooperative effort between Line and many other plant breeders in the Northwest. A major problem is the rapid development of new races of rust. Some soft white wheat varieties such as Paha, Yamhill, Fielder, Moro, Jacmar and Tyee have good race-specific resistance to stripe rust at all growth stages, However, within 3 years after release of most varieties with this type of resistance, new races have developed. Thirty-two races of stripe rust have now been identified. Two additional races were collected in 1985.

Other types of resistance to stripe rust have been identified. One is high-temperature, adult-plant resistance (HTAP). Seedlings of cultivars with HTAP resistance are very susceptible at a wide range of temperatures, but as the plant ages, it becomes more resistant at high temperatures. This type of resistance has remained durable for more than 20 years, and all evidence indicates this resistance is nonspecific to races, HTAP resistance is currently found in several soft white winter wheat varieties such as Luke, Daws and Stephens.

Line and other researchers are working to determine the characteristics and inheritance of stripe rust resistance, to develop methods of screening for resistance, and to identify resistant breeding lines. More than 7,000 winter wheat and 6,000 spring wheat crosses, commercial varieties and advanced lines have been evaluated for resistance to stripe rust in the field in 1985. Almost 5,000 spring selections were evaluated for seedling resistance to three important races of stripe rust in 1985 as well.

A high percent of the Northwest wheat varieties have some resistance to stripe rust. However as stripe rust resistance has improved, leaf rust has become more important. The researchers have identified a slow rusting type of resistance to leaf rust which appears to remain durable even though it is race-specific. They are working to improve the understanding of the nature and heritability of the resistance.

As with stripe and leaf rust, the preferred control for stem rust is also through resistance. Although several spring wheat varieties have good resistance, most barley and winter wheat varieties are susceptible to stem rust. Building resistance to stem rust is difficult because of the numerous races that can be produced during the barberry stage. However, if funding is available, Line and other researchers plan to determine the degree and type of resistance of local varieties and new breeding lines. With that information, selection for stem rust resistance could be made. Growers would also know the vulnerability of varieties and could decide which variety would be best to grow in areas where rust may be severe.

Fungicides

Line and associates have been testing systemic fungicides for rust control since 1969. Triademefon (Bayleton) has been tested since 1973 and currently is one of the most effective fungicides. The researchers have developed guidelines for use of Bayleton to control rust. These are based on several factors including: prediction of stripe rust and/or leaf rust levels; intensity of rust at different stages of growth; type and degree of plant resistance; potential yield of the crop; and expected economic loss. An emergency use permit for the chemical was granted in 1981. This was the first extensive use of a fungicide for control of rust in the U.S. Line estimates that the fungicide increased Washington wheat production by over one million bushels in 1981 alone. Since 1981, Bayleton has been registered and interest in development of chemicals for control of cereal diseases has increased. Five additional commercial and experimental fungicides have been identified as showing promising control of rust as foliar sprays or seed treatments.

Chemicals that are effective in controlling strip and leaf rust may also be effective against stem rust but they have not yet been fully evaluated. Line reports that test plot fungicide applications for leaf rust near the boot stage also covered the critical stage for stem rust development, controlling both diseases in several cases. Rust development after the early dough stage of cereals has little effect on yield.

Cultural Management

Because of current crop production practices, stripe and leaf rust have year-round hosts in the Northwest. Both winter and spring wheats — early and late seedings - are grown under irrigated and dryland conditions. In addition, wheat is occasionally planted in irrigated areas in the late summer as a cover crop or livestock feed after harvest of row crops. Also, volunteer wheat is often present in other crops and on field margins. All of these conditions provide sufficient inoculum to initiate stripe and leaf rust epidemics.

Line points out that practices which tend to delay crop maturity can increase the chance of rust damage, particularly for stem rust which develops late in the season. These include late seeding, both in the fall and spring, and high rates of nitrogen fertilizer. In addition to slowing maturity, high nitrogen fertilizer rates can create a more favorable rust environment through increased lodging, as well as increase the susceptibility of the plant. Elimination of the common barberry is a critical management factor for stem rust control. Stem rust infested cereal residue cannot contribute to infection of the following year's crop unless the common barberry is present to complete the life cycle.

Use of Trade Names

Research results are given for information only and are not to be construed as a recommendation for an unregistered use of a pesticide. Always read and follow label instructions carefully. To simplify the information, trade names have been used. Neither endorsement of named products is intended nor criticism implied of similar products not mentioned.

     
 

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