Be on the Lookout for Wheat Rust Diseases

Carl A. Bradley, Extension Plant Pathologist, University of Kentucky

Stripe rust (Fig. 1) was diagnosed by the Plant Disease Diagnostic Laboratory at the UKREC this week in a wheat sample that came from Lyon County.  In addition, leaf rust (Fig. 2) of wheat has been observed in states south of Kentucky (Arkansas and Mississippi), and appears to be moving northward.  Some wheat varieties have high levels of resistance to these diseases; therefore, it is important to know the susceptibility of the varieties planted. Resistant varieties likely will not require any additional management for rust disease control; however, a foliar fungicide application may need to be considered for susceptible varieties. The 2016 multi-state university foliar fungicide efficacy table for wheat diseases can be found here:

Leaf Rust May be Brewing on Your Farm

Leaf rust is brewing in some fields planted to rust-susceptible varieties in spite of the recent dry weather. I get the sense that disease severity may increase sharply in fields that have not been treated with a fungicide, and that experience at least one significant rain event over the next 30 days.

Fusarium head blight risk has been uniformly low across the state this spring because of the abnormally dry weather. As a result, many fields that may have otherwise been treated with a fungicide, were not. Our Extension grains crop specialist, Jim Herbek, thinks most fields will be harvested about a week to 10 days earlier than normal this spring. If the 7-10 day-period before harvest is a time when the crop dries to harvest moisture, and with it now being only April 30, that means many fields will still have green tissue for at least another 30 days. That is plenty of time for leaf rust to do considerable damage to vulnerable fields of wheat.

Modern fungicides are all extremely effective against leaf rust.  Some, like tebuconazole (e.g., Folicur® and generics), are very cheap and can be applied up to 30 days to harvest. My main purpose in writing this article is to prompt you to look at wheat fields planted to leaf rust-susceptible varieties that have not been sprayed with a fungicide this spring, and evaluate the rust risk; if necessary, act while you still have time.

Stripe rust is also visible in some fields, but at generally low levels. Hot and dry weather is very unfavorable for stripe rust, and we have had plenty of both during the last month. In addition, some fields are showing late infections of barley yellow dwarf which may result in a shorter grain fill period and limit the leaf rust risk. The main goal when it comes to leaf rust is to keep the flag leaf as clean as possible.  Infections in the lower canopy infections have little impact in terms of reducing yield.