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Wheat Rusts - Another Moving Target? February, 1999 |
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The wheat rusts are some of the oldest known plant diseases, with reports of rust epidemics found in the writings of the ancient Greeks and Romans, and in the Bible. Why are the rusts so widespread and devastating, and how can they be controlled? In this article we will try to answer these questions, and describe efforts by the Wheat Improvement Team to limit losses from the rusts. First, it is important to realize there are three rust diseases of wheat: leaf rust, stem rust, and stripe rust. All are caused by different but closely related fungi, and can occur in Oklahoma, but only leaf rust can cause significant losses. Primarily, this is because the leaf rust fungus can overwinter in Oklahoma in most years, and conditions that typically exist during the spring favor leaf rust development over development of stem rust or stripe rust. The two most common methods to control leaf rust are: (1) application of a fungicide in the spring or, (2) planting a leaf rust resistant variety. Of these two methods, planting resistant varieties is preferred because of the cost associated with chemical control in relation to the profit obtained from producing wheat. Unfortunately, resistance to leaf rust in wheat varieties frequently is not durable and lasts only a few years after the release of a variety. For example, varieties such as Siouxland, Cimarron, and Karl were resistant to leaf rust at the time of their release, but the leaf rust fungus quickly "adapted" to and overcame the resistance in these varieties. Thus, breeding for leaf rust resistance is a continual race between incorporating new and effective resistance genes into varieties, and the leaf rust fungus adapting to and overcoming those genes. Historically, many varieties released as leaf rust resistant had a type of resistance called "seedling resistance." Seedling resistance, which provides a very high or complete level of resistance, is expressed from seedling through adult plants but nearly always is short-lived, i.e., the pathogen is able to quickly adapt to and overcome seedling resistance. Cimarron, Siouxland, and Karl are examples of varieties with seedling resistance. In contrast to developing varieties with seedling resistance, the approach taken in recent years at Oklahoma State University is to develop improved wheat varieties that have a more "durable" type of leaf rust resistance known as adult plant resistance (APR). APR is not expressed at all or only is weakly expressed in seedlings, but is more strongly expressed as plants mature. Low levels of leaf rust may occur on varieties with APR, but these levels usually do not result in significant yield losses. Additionally, the leaf rust fungus usually is not able to quickly adapt to and overcome APR, which is a tremendous bonus regarding the longevity of the resistance in the variety. Varieties recently released from Oklahoma State such as Custer, Tonkawa, and 2174 all have adult plant resistance to leaf rust. To date, levels of leaf rust that cause significant losses have not been observed on these varieties. This does not mean that the resistance in these varieties will last forever, and some losses from rust may occur. However, because APR tends to be more durable, an agronomically desirable variety should last for a longer period of time than would a variety with only the seedling type of resistance. In summary, WIT is continuing to strive
for the release of agronomically superior varieties that have resistance to
the diseases that pose significant constraints to wheat production. This is
especially true with leaf rust, where the incorporation of durable, adult
plant resistance has a high priority and may help to extend the life of a
variety by resisting leaf rust for a longer time.
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