Stripe rust distribution update
Stripe rust has continued to show up in Kansas, with additional observations made in the western portion of the state (Figure 1). It should be noted that although stripe rust was observed on flag leaves, incidence remains very low in many locations. As stripe rust infections favor cool weather, the warmer days forecasted over the next week should help mitigate the risk of major spread. Wheat in Kansas ranges currently ranges from heading to well into the grain fill period.
Figure 1. Distribution of stripe rust in Kansas as of May 29, 2020. Map is based on observations of K-State Research and Extension, crop consultants, and wheat producers in the state. Map created by Kelsey Andersen Onofre and Erick DeWolf, K-State Research and Extension.
Evaluating the need for fungicide applications
As we have mentioned in previous eUpdate articles, many fungicides labeled to manage wheat diseases cannot be applied after Feekes 10.5.4 or within the 30-day window prior to harvest (https://bookstore.ksre.ksu.edu/pubs/EP130.pdf). This will be important to remember as wheat in Kansas moves into those final stages of crop development. Research done at K-State suggests that the average yield response to a foliar fungicide on a susceptible variety in a high disease pressure situation is about 10%. The yield response for stripe rust can be more than 20% when conditions favor disease development on susceptible varieties, and stripe rust has been detected on the flag leaves. Using this figure along with estimates of a field’s yield potential and the value of wheat grain, we can quickly estimate the breakeven point for a fungicide application (taking into consideration the cost of the product and application per acre, expected bu/acre return, and the price of grain).
In general, fields with more than a 40 bu/acre yield potential are good candidates for a fungicide application when conditions are favorable for disease. Fields that have been heavily damaged by virus infection or freeze damage may not meet the economic yield threshold for an application.
Kelsey Andersen Onofre, Extension Plant Pathology
Erick DeWolf, Extension Plant Pathology