Leaf rust is developing in Kansas wheat fields, according to Kansas State University wheat breeder Allan Frtiz. If you see white spots on the flag leaf, it means the rust pustule is beginning to form.
"It's going to be a leaf rust year," Fritz says. "There are a lot of disease issues out there. Leaf rust is heaviest in south central Kansas, but has been showing up all over the state. Western Kansas may also have issues with stripe rust. Powdery mildew is really going to be an issue."
Leaf rust is caused by a parasitic fungus (called Puccinia recondita f. sp. Tritici). Conditions are most favorable for leaf rust in the eastern two thirds of Kansas, but significant losses can also occur in western Kansas under irrigation or during years with high precipitation. Powdery mildew thrives in wet weather, as has been prevalent throughout the state this week
Fritz recommends that wheat producers all over the state begin scouting their fields immediately for signs of disease. He says variety selection and fungicides are the two ways to control leaf rust.
"Farmers need to make decisions now," says Fritz. "Look at your fields and make the decision now."
Leaf rust is especially concerning this year because of damage caused by the early April deep-freeze, which killed many primary tillers, allowing the secondary tillers to begin growing.
"Delayed tillers have more susceptibility to rust," said KSU Extension Agronomist Jim Shroyer. The earlier rust appears in the life cycle of wheat, the greater the potential for substantial yield losses. Shroyer also emphasized that producers should start applying fungicide immediately to minimize losses from leaf rust.
Leaf rust causes very small orange pustules that erupt through the leaf surface. Leaf rust pustules occur randomly across the leaf; this distinguishes leaf rust from stripe rust, which has narrow yellow stripes of pustules.
Weather is a major factor in how rust progresses. Warmer weather (65 to 80 degrees Fahrenheit) with humidity promotes leaf rust development. Light rain, heavy dew, and six to eight hour continuous leaf wetness promotes germination and infection.
Wheat variety also has considerable influence on yield loss. Susceptible varieties easily succumb to rust infections and lose flag leaf area. Overley, which was planted on more acres in Kansas this year than any other variety, does have some resistance to leaf rust.
Leaf rust can be controlled with fungicides, but there are a number of economic and yield potential factors to consider. For example, a 20 percent loss in yield on 20 bushel per acre wheat at $3 per bushel is a loss of $12 per acre, which does not cover the cost of a fungicide application. However, a 20 percent loss on 70 bushel per acre wheat at $3 per bushel is a loss of $42 per acre. Farmers need to consider the fungicide and application cost along with the yield potential and expected price per bushel when deciding to apply a fungicide.
Fritz and Shroyer agreed that if fields are already heavily damaged by the freeze, applying a fungicide may not be economically feasible, but recommended applying fungicides to "good wheat out there."