Rust Diseases of Wheat: Leaf, Stem, and Stripe
This NebGuide provides information on the three rust diseases of wheat. Information is present that will assist producers with rust control decisions.
Rusts remain the most important diseases of wheat worldwide because of their wide distribution, their capacity to form new races that can attack previously resistant cultivars, their ability to move long distances and their potential to develop rapidly under optimal environmental conditions, resulting in serious yield losses. Leaf rust, stem rust, and stripe rust comprise the three rust diseases of wheat. In the central plains of North America, leaf rust is the most common and important from a crop production aspect.
Leaf Rust
Leaf rust, caused by the fungus Puccinia recondita, is generally distributed throughout eastern and central Nebraska and occurs sporadically in western Nebraska. The disease is most damaging when the upper leaves of infected plants become severely rusted before mid-June. Heavy rusting causes early loss of these leaves, which reduces the grain filling period and results in smaller kernel size. Since 1990, leaf rust has cost Nebraska wheat growers an average of $5.5 million annually. When combined with the leaf diseases tan spot or Septoria leaf blotch, leaf rust becomes an even more serious threat to wheat production.
Figure 1. (100K GIF) Susceptible and resistant reaction of leaf rust.
The Disease
Leaf rust is generally found on the leaves, but occasionally is found on glumes and awns. Symptoms are circular or oval, orange pustules on the upper surface of infected leaves. The spores within these pustules are easily dislodged and cover hands and clothing with an orange "dust". Each pustule produces about 1000 spores daily, each of which is capable of reinfecting wheat. As wheat ripens, this orange stage turns into a new stage of the fungus which is black, and often a mixture of orange and black stages will be intermingled on a leaf.
Like the other rusts, the leaf rust fungus is an airborne pathogen that may be spread by the wind over long distances. In the central Great Plains of North America, the fungus can overwinter on fall-infected leaves of winter wheat as far north as central Kansas. Most of the leaf rust that affects Nebraska probably overwinters in Texas and northern Mexico. The spores blow north from these overwintering sites, usually reaching Nebraska in mid-May. Once established in a field, the rust spores spread locally within that field as well as to nearby wheat fields. About every 7-10 days new pustules are formed, and under the right conditions severe rusting occurs 30-40 days after the initial infection.
Figure 2. (113K GIF) Severe leaf rust on flag leaf.
Weather plays a key role in the development of leaf rust. Cool nights followed by warm (60-80F) days that create long dew periods in the wheat canopy are ideal for rust development. The rate at which leaf rust develops in April and May in Oklahoma and Kansas often determines the severity in Nebraska in June. If rust develops rapidly in these states, it will most likely be a problem in Nebraska.
Control
Since many races of the fungus exist, it is difficult to produce commercial wheat varieties that are resistant to all races. The Nebraska wheat breeding program attempts to incorporate leaf rust resistant genes into varieties that provide an adequate level of resistance to the most common races. The leaf rust resistance is often overcome five to six years after release of a variety because races develop that can attack the resistance genes in that variety.
Figure 3. Seasonal movement of leaf rust in the Great Plains.
However, growing resistant varieties is still the best and most economical method of minimizing rust losses. When selecting wheat varieties, growers should use the concept of variety complementation. Variety complementation encourages the producer to select adapted varieties that differ in parentage, maturity and disease reaction. Because there is no single perfect variety, complementation allows the producer to counterbalance potential weaknesses in each variety. This compensation improves the opportunity for yield stability of the entire production system. To select complementary varieties:
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Identify your 'workhorse' varieties, ones that have a history of good performance on your acres.
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Select varieties that differ in parentage from your workhorse varieties and other varieties grown.
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Select varieties that bloom either earlier or later than your workhorse varieties to spread the risk from weather and disease, and to spread out harvest.
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Select varieties that have specific characteristics needed for your production conditions or areas. For example, leaf rust susceptible and wheat streak mosaic tolerant varieties fit well in the Panhandle where leaf rust is not a serious problem, but wheat streak mosaic can be devastating. In eastern and central Nebraska leaf rust often reduces yields, so leaf rust resistant varieties are strongly encouraged.
Fungicides currently registered for leaf rust control on wheat include: Bayleton 50WP (triadimefon); Dithane M-45, Dithane F-45, Dithane DF, and Penncozeb (mancozeb); and Tilt (propiconozole). Product labels list application rates and methods. Tilt is applied at flag leaf emergence. Bayleton provides more effective control when applied in combination with mancozeb at boot stage. The mancozeb products are applied at boot stage and again in 7-10 days. Also, the addition of a spreader sticker may improve the level of rust control. The cost of aerially applying fungicides to winter wheat during the growing season will be $15-$18 per acre, depending on the product and application costs. At current wheat prices, a grower must realize at least a consistent five to six bushel per acre yield increase to break even. Leaf rust, in most years, does not become severe enough to justify routine spraying of winter wheat. In these years the cost of treatment would not pay for itself. However, when the potential for severe damage from rust is high, the cost-benefit ratio usually favors chemical control.
Growers needing to decide whether to treat their wheat fields can follow certain guidelines. Monitoring the rust situation in the southern Great Plains and the local leaf spot situation in early May gives a good indication of the potential for foliar disease development in Nebraska. For example, in 1994 leaf rust was slow to develop in Texas, Oklahoma, and Kansas, and as a result, leaf rust severities in Nebraska were light. The earliness or lateness of the winter wheat crop, current weather conditions, and long range forecasts are important factors in determining if spraying is advisable. If trace amounts of rust and leaf spots are present on the flag leaf in the early boot stage of development, and infection below the flag leaf is moderate to severe, the likelihood is good that severe infection of the flag leaf will occur and applying a fungicide is advised. Based on these criteria, a reasonably accurate decision as to whether or not to spray can be made during the early boot stage, while the flag leaf is still relatively free of infection. In western Nebraska, sprinkler irrigated fields should definitely be checked weekly in May and June so that the buildup of leaf rust and/or tan spot can be detected early.
Stem Rust
"The greatest pest of crops" said Pliny, a Roman philosopher, when speaking of stem rust. So it has been, and no doubt it will continue to plague wheat wherever it is grown. Stem rust, caused by Puccinia graminis, has been the cause of at least eight major rust epidemics in the United States since 1916. The last widespread epidemic in Nebraska was 1962. However, in the mid-1980s it decimated susceptible varieties in commercial wheat fields in eastern and central Nebraska, which exemplifies the ever present threat of this serious disease. Other than these sporadic outbreaks, the disease has been controlled by growing resistant varieties.
Figure 4. (125K GIF) Stem rust showing ragged edges of the pustule.
The Disease
Stem rust, as the name implies, infects the stems but is not always confined to the stems. It can infect leaves, sheaths, glumes, and awns. Symptoms are elongate, reddish brown pustules with ragged margins. The host epidermis is ruptured by the pustule, giving the stems and sheaths a rough-textured surface. Stem rust pustules are larger than those of leaf rust and often erupt on both leaf surfaces. As the wheat matures the pustules are replaced by the black spore stage.
Because of their later seasonal development, spring wheats in North America have a longer infection window than winter wheats. Temperatures that favor stem rust development are about 5 degrees warmer (65-85F) than they are for leaf rust. As with leaf rust, free moisture on the wheat canopy is necessary for infection. Under these conditions, new pustules are produced every 7-10 days during the growing season. Unlike leaf rust, stem rust rarely overwinters as far north as Kansas.
Figure 5. (133K GIF) Stem rust on leaves and stem of seedlings. (Photo courtesy P.S. Baenziger)
Control
Effective control has been achieved by a combination of early variety maturity, which escapes severe infection, and by incorporating effective stem rust resistance into Nebraska's commercial wheat varieties. Although the fungicides registered for leaf rust will control stem rust, their use is generally unnecessary because of the extensive use of resistant varieties.
Stripe Rust
Stripe rust occurs only in trace amounts in Nebraska, usually during cool periods in early June. It is more important in the higher elevations of the western mountain states and in the Pacific Northwest. It is included here only for comparison.
Figure 6.(99K) Stripe rust on wheat flag leaves.
The Disease
Pustules are light yellow and occur on leaves in distinct, straight-sided stripes about 1/16" wide and of irregular length. Pustules also may develop on the heads. Stripe rust develops at slightly cooler temperatures (55-70F) than leaf rust. Once temperatures exceed 70F, stripe rust develops very slowly, which is why its occurs only in trace amounts in Nebraska.
Control
Since stripe rust is not a serious threat to Nebraska's wheat production, control is not an important factor. Where it is an important disease, losses are minimized by growing resistant varieties or treating with fungicides.
Table I. A comparison of rust characteristics
Leaf rust Stem rust Stripe rust Pustule location Leaf Stem and leaf Leaf and head Pustule color Orange Dark red Yellow Pustule arrangement Single Single Stripes Temperature range for infection 60-80F 65-85F 55-70F Control methods Resistance and Resistance and Resistance and fungicides fungicides fungicides Occurrence in Nebraska Yearly Trace yearly Occasionally
Acknowledgement
The author wishes to acknowledge Lenis Nelson, professor of agronomy, University of Nebraska-Lincoln, and Roger Hammons, associate manager, Nebraska Crop Improvement Association, for the information on variety complimentation
