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Rust impacts more than soybeans
By Randy Buhler
Colorado State University Cooperative Extension
Logan County Agent (Agronomy)
Rust seems to be everywhere this fall. We have rust in wheat, soybeans, and maybe even your grain drill.
The latest agriculture news headlines detail the discovery of Asiatic soybean rust in Louisiana. Our main concern is that dry beans and garden beans are also susceptible to this rust disease. Kudzu is an alternate host for soybean rust. With abundant kudzu in some areas of the south, we may see soybean rust develop rapidly next season. The only prevention is application of approved fungicides. We do not have resistant varieties available for commercial production.
Local crop consultants and farmers are paying attention to the yellowing Wheat fields. The yellowing is the result of a fall infection of leaf rust. This event is rare for our area. South Texas and Oklahoma often see this sort of fall infection.
Most of the varieties of Wheat we grow in northeastern Colorado are very susceptible to the prevalent rust races. Leaf rust is notorious for changing its virulence as we change resistance genes in our commercial Wheat varieties. It is common for a new variety to have very good resistance when first grown, but within four or five years, it becomes susceptible.
Most varieties of winter Wheat do not have any leaf rust resistance during the seedling growth stage, even though the maturing plant shows significant rust resistance. Most damage from rust occurs after boot stage and during grain fill. Yield losses from leaf rust are usually in the 4 to 10 percent range. Severe infection early in the season can cause yield losses exceeding 40 percent.
Cold winters cause the leaf rust fungus to die. Re-infection comes from the over-wintered fungus in mild winter areas producing spores that blow north to our wheat. The fungus can reproduce itself in 7 to 10 days. It grows best when temperatures are between 60 and 75 degrees Fahrenheit. Leaf rust can survive the winter, if Wheat leaves remain green through the winter.
You need to take no action at this time regarding leaf rust in wheat, other than to know if it is present. Winter weather conditions will determine the chance of winter survival and prevalence of new infections next spring.
Fungicides are available to protect Wheat from yield loss. The products are expensive and application at specific times is necessary to get economic protection. Rust researchers in Nebraska recommend fungicide applications only if the yield potential for dryland Wheat is 45 bushels or more per acre. They recommend treatment for irrigated Wheat when yield potential is 75 bushels or greater. The exception would be seed Wheat production. Treating with fungicide produces greater seed size and lower cleanout. With soybean rust creating a major market for the same fungicides, costs could escalate by next spring.
The local custom of applying dry fertilizer mixed with the seed Wheat works fine. Just realize that fertilizer materials can attract moisture and are somewhat corrosive in nature. A failure to thoroughly clean the drill and coat the shiny parts with rust inhibitor could leave your drill seized up by next spring. That rusting could prove expensive to fix.
Date: 11/23/04
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