Strawbreaker Foot Rot: New Research Considerations
Chapter 4 -Disease Control, No. 6, April-May 1986
Roger Veseth
Infection and Symptoms
Winter wheat infection can occur from late fall through early spring when the spores contact the lower leaf sheaths at the base of the plant near the soil line. Fall infection generally accounts for most of the yield loss. Spore production in winter and in spring from fall infected plants can cause “secondary infection” in the crop. This secondary infection has little impact on crop yield. It does, however, increase inoculum carryover for infection of future winter wheat crops. Severity of strawbreaker foot rot varies from year to year depending on climatic conditions. Cool, wet conditions and lush fall growth of the winter wheat tend to favor the disease.
The initial symptoms occur at or near the soil line. An elliptical ”eyespot” lesion, with a brown margin and pale center, is diagnostic. A dark gray-black spot maybe visible at the center of the lesion. Yield loss results from a reduced number of tillers, incomplete grain filling, reduced kernel weight, premature dying of tillers (whiteheads) and lodging which can further reduce quality and increase harvesting losses. Yield losses can exceed 50 percent under severe disease conditions.
Several STEEP researchers are involved in developing the technology for control of strawbreaker foot rot. This is one of the research priorities of Tim Murray, Washington State University plant pathologist at Pullman. His research, funded by STEEP, the Washington Wheat Commission and other sources, covers several aspects of control including cultural practices, fungicides and developing varietal resistance.
Cultural Practices
Delayed Seeding Date
Delaying the seeding date in the fall has been a common practice used by Northwest producers in an attempt to reduce levels of strawbreaker foot rot. Other diseases reduced by late-seeding include Cephalosporium stripe and barley yellow dwarf. One possible reason that delayed seeding provides some protection is that the strawbreaker fungus may penetrate and infect older, dying leaf sheaths more easily than young leaf sheaths. The early leaves of larger, early-seeded winter wheat plants begin dying in the late fall and through the winter just as the cool, wet weather begins to favor development of the fungus.
Laboratory research results by Murray demonstrate the winter wheat plant’s reduced resistance to infection by the fungus as the plant develops (Fig. 1). Time between inoculation and infection of the first true leaf (after the coleoptile) was progressively reduced as age of the plants increased from the 1-leaf stage to the 4-leaf/l-tiller stage.
Unfortunately, delaying the seeding date to reduce straw breaker foot rot infection can have several important disadvantages. One is increased damage from Pythium root rot. The Pythium fungus is favored by cool, wet conditions of the late fall. It is much more damaging to lateseeded germinating seeds and young seedlings than to large, vigorous plants with well established root systems typical of early-seeded wheat. Late-seeded wheat also typically has a lower yield potential because of reduced vigor and winter survival. Late-seeded wheat plants also begin spring growth from an earlier (smaller) development stage. Under conventional tillage, with little or no surface residue, late-seeded wheat creates one of the highest soil erosion potentials in the Northwest. In addition, loss of water by runoff and evaporation can also decrease yield potential.
Besides reduced resistance to strawbreaker foot rot infection with increasing leaf tissue age, another factor may help explain the higher level of infection in larger, earlyseeded plants. The primary mechanism responsible for transfer of the fungus spores from the soil to the plant is through raindrop splash. Larger, more vigorous wheat plants resulting from early seeding have a larger surface area for infection in the fall than small, late-seeded wheat plants.
Surface Residue
Surface residue levels also appear to influence the incidence of strawbreaker foot rot. Researchers and producers have reported reduced levels of foot rot under conservation farming systems. Murray points out that the reason for this is still uncertain. Several possibilities have been suggested. One reason could be the reduction in raindrop splash of spores onto the plant with greater amounts of surface residue. Higher levels of microbial activity near the surface associated with residue decomposition may also compete with the foot rot fungus and reduce its inoculation production potential. A third possible reason could be that the slower fall emergence and growth rate often associated with no-till and minimum tillage seedings may have the same effect as delayed seeding. Whatever the reason, these conservation farming systems offer an additional management tool for control of strawbreaker foot rot.
Burning wheat stubble after harvest is not effective in controlling strawbreaker. Enough infected crowns are protected by the soil or otherwise escape burning to provide inoculum to cause serious infections in future crops.
Crop Rotation
Although a 3-year rotation (2 years out of winter wheat or winter barley) does control some diseases, such as Cephalosporium stripe, crop rotations have not been shown to be effective in controlling foot rot. Crop rotation does reduce inoculum carryover. However, only a small amount of inoculum is needed for significant yield losses. Winter barley is only slightly affected by foot rot, but it does help maintain the fungus in the field. Spring grain crops are not affected.
Fungicides
Fungicides have been and continue to be one of the most widely used methods of control for strawbreaker foot rot. Three fungicides — Benlate PNW, Mertect 340-F and Topsin 70W — are now registered. Two disadvantages of relying on fungicides for control are increased production costs and the potential for development of strains of the fungus that would be resistant to the fungicides. Resistant strains have been found in Europe and may eventually occur in the Northwest. Murray and other researchers have been sampling for disease resistant strains over the past several years. So far, 132 isolates of the fungus have been collected from seven eastern Washington counties but none have shown resistance to fungicides.
The purpose of the fungicide is to stop the fungus lesion development at the leaf sheath before it penetrates into the stem. A difficult question for producers is what level of infection makes a fungicide application economically feasible. Murray and Otis Malloy, WSU Extension pathologist, have outlined some guidelines in a new Washington Extension Bulletin (EB 1378), which should be available in early May. Two considerations are stressed: (1) Has strawbreaker foot rot caused previous losses in the field? (2) Was winter wheat growth better than average going into the winter?
They suggest making the final evaluation of the field in the spring just before jointing when the winter wheat is well tillered. Collect 50 tillers from throughout representative areas of the field and spray them clean with water so they can be examined for lesions. If at least 5 of the 50 tillers have identifiable lesions (10 percent infection), a fungicide should be considered. They point out that if 10 percent of the tillers have lesions, there are typically about another 10 to 20 percent that are infected, though lesions are not yet apparent. The decision whether to use a fungicide must also consider the yield potential and price of the crop.
Plant Resistance
Natural disease resistance would be the least expensive and most reliable control for strawbreaker foot rot in winter wheat. Cooperative plant breeding research efforts are underway with agronomist Clarence Peterson and geneticist Robert Allan with the USDA-ARS at Pullman and other STEEP researchers. Some European varieties have high resistance and crosses are being made with varieties adapted to the Northwest. Several new experimental lines now exhibit good resistance and yield potential and may be released as new varieties in the future. Currently however, there are no commercial varieties available with high levels of resistance to strawbreaker foot rot.
The use of field research trials to evaluate strawbreaker foot rot resistance is difficult because of the yearly variations in weather conditions and the limit of one comparison per year. One focus of Murray’s research has been to develop a rapid seedling test which would allow screening of large numbers of plants to identify resistant lines.
Earlier work by Murray has shown that resistant varieties have stems which contain a wide band of mechanical support tissue called hypodermic and a high concentration of lignin. In susceptible varieties, the hypodermic is narrow and lignin concentration is low. It is believed that these two factors are responsible for resistance. The fungus is not able to decompose the lignin and penetration of the wide hypodermic layer is greatly reduced. Unfortunately the measurement of these two characteristics is very time consuming and expensive.
Murray’s more recent laboratory work with seedlings has revealed another possible means of evaluating resistance. Resistant varieties react differently to infection than susceptible varieties. Wheat seedlings are inoculated at the 1.5- to 2-leaf stage. After 18 days, microscopic evaluations of the “resistant reaction” in the first true leaf sheath are used to estimate resistance. Resistant varieties were found to have increased lignin concentration at the site of infection which blocked further movement of the fungus into the leaf sheath. Typically, resistant varieties had about 2 percent infection rates compared to 80 percent infection rates for susceptible varieties. To date, six varieties have been tested extensively and results correlate closely with reaction in the field.
If the seedling test continues to prove reliable, it offers the potential to greatly accelerate the screening of large numbers of varieties and experimental lines in a very short period of time. Studies are also underway to evaluate the genetic heritability of the resistance reactions evaluated in the seedling tests.
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.