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Stripe Rust Continues to Threaten Winter Wheat Fields

Recent rain events have led to severe infestations of stripe rust in winter wheat fields.

If rain makes grain, the last few days did wonders for the 2015 hard red winter wheat crop.

Acres from Texas to Nebraska received much-needed moisture to hopefully get the wheat crop to the finish line. However, the weekend rains, coupled with subsequent damp, cool weather also are providing a perfect environment for stripe and leaf rust.

Stripe rust infestations are hit-and-miss throughout the High Plains, says Bob Hunger, Extension plant pathologist at Oklahoma State University. In his weekly Wheat Disease Update, Hunger says some fields have had infestations of stripe rust already; those that didn't are likely to see infestations coming along in the wake of rain, cool temperatures, and dew. Dampness increases the risk that the disease will move to the upper leaves of the wheat plant. "This is important because the upper leaves contribute most of the energy used by the plant to make grain," says Erick DeWolf, plant pathologist at Kansas State University.

Should You Spray a Fungicide?

Wheat growers will have to make some key management decisions. The most important thing: get out and scout those fields. Where stripe rust is currently present on the flag leaves at low levels and most of the leaf’s green area is still intact, a fungicide application will still be beneficial in most cases. "There is a point of diminishing return however," reports K-State's DeWolf. "If the disease has already destroyed more than 25% of the upper leaves, the crop will be less likely to benefit fully from the fungicide application.

"In this case, the disease has already damaged a good portion of the leaf area and has likely already begun to infect much of the remaining green tissue. The remaining green tissue may still die even after the fungus has been suppressed by the fungicide," he adds.  

Where stripe rust was already present on lower leaves before the weekend of April 16-17, it may now quickly spread to the more critical upper leaves. A fungicide application now would likely be beneficial in those situations, depending on the yield potential of the crop.

However, producers need to know at what stage of development their wheat is at before deciding whether to spray a fungicide.  "All wheat fungicides (except for Prosaro and Caramba) must be applied prior to the start of flowering," Hunger says. "Prosaro can be applied through Feekes Growth Stage 10.5.1, which is when flowering is just starting." Caramba can also be applied from flagleaf to flowering and has a 30-day PHI. Reading fungicide labels is a must, and be sure to pay attention to preharvest intervals (PHIs) required for the various products.

Wheat farmers have a lot of fungicide options to choose from although product availability may vary regionally in the state. Most of the products are rated very good to excellent on stripe rust. In general, the largest reductions in disease severity and greatest increases in wheat yield or grain quality occur when fungicides are applied between full extension of the flag leaves and anthesis (when the male flower parts have just begin to emerge). Applications intended for the management of glume blotch or head scab should be made between the beginning of anthesis and 50% flowering. Always consult the product label for specific growth stage restrictions and preharvest intervals (PHI) before making a fungicide application.

Products, their active ingredients, and PHIs can be found here: Foliar Fungicide Efficacy Ratings for Wheat Disease Management

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