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Leaf rust poses a serious threat in 2008Kansas The risk of significant yield losses from leaf rust and other foliar diseases has increased dramatically this past week. This alert comes from K-State Extension Wheat Plant Pathologist Erick DeWolf. Leaf rust was discovered in commercial fields and variety demonstration plots in Sumner County on April 24th. The leaf rust was present on the F-1 and F-2 leaves (the two leaves just below the flag leaf), with an incidence of less than 10 percent and severity less than 2 percent, in both Jagger and Jagalene. Leaf rust has also been observed on Jagalene and Jagger in commercial fields and variety evaluations in northern Oklahoma. The disease was slightly more advanced in Oklahoma. Leaf rust was observed at trace levels on Overley near Stillwater, but the disease has not yet been detected on Overley in Kansas. Do not be fooled by this slight delay in the arrival leaf rust races that can overcome the type of resistance in Overley. Overley is very susceptible to certain races of leaf rust, and this variety will likely become diseased soon. Resistance in Fuller, Santa Fe, and Postrock appears to be holding. Powdery mildew has also increased significantly during this past week. Jagalene, Overley, Jagger, and Postrock are all susceptible to powdery mildew. The severity of powdery mildew was greater than 25 percent on the F-1 leaves in commercial fields of Jagalene and Overley in Sumner County. These levels of mildew can also result in significant yield losses. The growth stages of wheat in the southern tier of counties ranges from jointing to early stages of heading, depending on the planting date last fall. The current presence of even low levels of leaf rust and severe powdery mildew suggests that potential for yield loss is significant. Yield losses of greater than 35 percent can be expected if the leaf rust and powdery mildew move onto the flag leaves of susceptible varieties prior to flowering. Varieties that are resistant to leaf rust will not have this much yield loss from leaf rust, but may still be vulnerable to powdery mildew or other diseases. Wheat fields in Southcentral and Southeast Kansas should be checked for symptoms of disease and to verify growth stage as soon as possible. Wheat in the central and northcentral districts of Kansas will be at critical growth stages within two weeks. The most effective timing for fungicide applications is between flag leaf emergence and heading. It is important not to apply the fungicide too early. Producers should wait until the wheat has reached the optimal window for application. The yield response for foliar fungicides in Kansas will typically fall between 4 and 13 percent with the yield average response of 10 percent. Yield responses can be greater than 20 percent in fields where leaf rust arrives prior to flowering. The cost of a fungicide and application ranges between $20 and $27 per acre. The price of wheat is still very strong (above $8). Fields of Jagalene, Overley, and Jagger with a yield potential of greater than 30 bu/acre are excellent candidates for a fungicide application. DeWolf strongly recommends that seed production fields be sprayed. 5/5/08 Date: 5/1/08
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