About Leaf Rust

Rust diseases on wheat, especially stripe rust, have caused significant yield losses in recent years. Planting resistant varieties is a key management decision. However, new races of rusts evolve and overcome resistance. A wheat variety that has been resistant in the past may not remain resistant to new races of rusts. Stem rust on wheat has not caused significant losses to Kansas wheat in many years. However, new races detected in Africa — if introduced to the United States — could damage Kansas wheat varieties.

Stripe rust and stem rust infestations can cause higher yield losses than leaf rust. Stripe and stem rust induce more cell disruption, which leads to more water loss through evaporation and reduced photosynthetic leaf area. The increased water loss results in more stress, which in turn causes a higher yield loss when compared with a leaf rust infestation.

Management Decisions

1. The earlier rust appears in the life cycle of wheat, the greater the potential for substantial yield losses. Losses are greater when leaf rust infects the flag leaf at flowering (30 percent loss with 65 percent flag leaf infestation) than when leaf rust infects the flag leaf at the soft dough stage (7 percent loss with 65 percent flag leaf infestation). If rust develops at an earlier growth stage, as is common with stripe rust, losses have the potential to be greater than 50 percent. Stem rust causes the highest losses since the stem can be completely girdled by the rust pustules and prevent grain fill.

2. Weather is a major factor in how rust progresses. Mild weather (50 to 75 degrees Fahrenheit) with some humidity is favorable to increasing stripe rust, but hot, windy conditions impede its development. Warmer weather (65 to 80 degrees Fahrenheit) with humidity promotes leaf rust development.

3. Wheat variety also has considerable influence on yield loss. Susceptible varieties easily succumb to rust infections and lose flag leaf area. Resistant varieties may exhibit some rust pustules, but these pustules develop slowly and do not dramatically increase in number. Stripe rust-susceptible varieties include 2137, Trego, and Above. Stripe rust resistant varieties include Jagger, Jagalene, Overley, and TAM 111. In 2005, evidence of limited susceptibility to stripe rust occurred in some fields of resistant varieties; therefore, all varieties should be scouted. For a complete listing of wheat varieties susceptible to leaf and stripe rust, see the disease rating table on page 25 in the 2005 Kansas Wheat Seed Book.

4. Economics and yield potential are the final 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.

For more information, visit WHEAT LEAF RUST by Robert L. Bowden, Extension Specialist, Plant Pathology, KSU