Preparing for Rust Diseases in Winter Wheat
April 13, 2007
|Figure 1. Leaf rust.
|Figure 2. Stem rust.
|Figure 3. Stripe rust. (IANR Photo credit: Tamra Jackson)
Three rust diseases occur on winter wheat in Nebraska - leaf, stem and stripe rust.
Leaf rust (Figure 1) is the most common. It occurs every year and can cause up to 5% yield loss. Stem rust (Figure 2) occurs rarely because most of the winter wheat cultivars grown in Nebraska have good resistance to it. Stem rust can cause up to 100% yield loss on susceptible cultivars. Stripe rust (Figure 3) has occurred sporadically since the year 2000. It can cause up to 40% yield loss. All three rust diseases are favored by wet weather. Stripe rust is favored by cool temperatures (50-59°F) whereas leaf rust is favored by intermediate temperatures (60-72°F). Stem rust has the warmest temperature requirement (75-85°F). If all three rusts occur, we would expect to see stripe rust first, followed by leaf rust and then stem rust.
Whether the pressure of rust diseases will be high enough to cause economic yield loss will depend on the weather. The risk of disease will be high if we get above average rainfall in May and June. Disease risk generally is higher in irrigated than in rain-fed wheat. Monitoring weather forecasts and reports of rust development in southern states will be helpful in estimating the risk of rust diseases in Nebraska. If leaf or stripe rust or both diseases develop rapidly in Texas and Oklahoma in April or in Kansas in May, the risk of either or both diseases developing in Nebraska will be high.
There are two major strategies for managing rust diseases in Nebraska: cultivar resistance and foliar fungicide applications. If the cultivars planted have moderate to good resistance to stripe or leaf rust, foliar fungicide application may not be necessary unless infections are heavy or other diseases such as powdery mildew and leaf spots are present at levels that can cause economic yield loss. Susceptible cultivars should be monitored closely for rust development and should be sprayed if rust occurs and conditions are favorable for disease development. For foliar fungicide sprays to be profitable, the yield potential of wheat produced for grain should be 40-50 bu/ac in rain-fed fields and 75-85 bu/ac in irrigated fields. See the March 16, 2007 CropWatch article, Managing Foliar Diseases of Wheat with Fungicides for a list of foliar fungicides registered for control of foliar diseases of wheat.
In general, a fungicide application will be cost effective if rust, powdery mildew or leaf spot diseases are present in trace amounts on the flag leaf in the boot stage of development and infection below the flag leaf is moderate to severe. Regular scouting of fields for disease will help determine when to spray, based on disease level.
Extension Plant Pathologist, Lincoln