By Heather Marie Kelly, Extension Plant Pathologist

More observations of stripe rust in wheat have been reported in West Tennessee. The answer to the question, “Do I spray now or wait until bloom?” depends on amount of disease present, variety, growth stage and weather.

If you have a susceptible stripe rust variety, can easily find stripe rust, and the field is more than a week away from bloom, then it would be recommended to apply a fungicide. In the same situation where the field is very close to blooming, holding the fungicide application until bloom can manage the stripe rust and protect from Fusarium head blight (scab). 

Be sure to avoid products with strobilurins (QoI, Fungicide code 11 on the label) when applying near or at bloom due to the risk of increasing mycotoxin (DON) levels if there is a scab epidemic. Be sure to check the Fusarium head blight prediction site to guide fungicide application decisions at bloom. With warmer temperatures and rain during bloom, scab risk will increase.

Alternatively, if little to no stripe rust or other diseases are present and/or you have a resistant stripe rust variety, a fungicide application can wait, but scouting the field should be continued on a weekly basis. Another management tool to consult is the Wheat Fungicide Table to check fungicide efficacy against the diseases you are trying to manage. A couple of notes on choosing fungicides:

• Combination products (fungicides with mixed modes of action) usually provide the best management of stripe rust infections that are already established.
• A maximum of 4 fluid ounces of a tebuconazole product may be applied per acre per crop season. Meaning if you apply 4 fluid ounces of a tebuconazole fungicide earlier in the season, you cannot apply a product containing tebuconazole (such as Prosaro) at a later application.

While only observations on stripe rust in Tennessee have been reported, leaf rust has been reported in Mississippi and Arkansas. It is more important to manage stripe rust timely than leaf rust as stripe rust can be more devastating to yields; hence it is important to make sure the rust is correctly identified.

Stripe rust develops best within 50-64 F with intermittent rain or dew and rapidly loses it’s ability to cause new infections above 70 F. While daytime temperatures are predicted to be above 70 F in Tennessee, nighttime temperatures and temperatures within wheat canopies can still support rust infection and development. Stripe rust is best identified by tiny, yellow to bright orange pustules that form distinct stripes on the leaves. 

Although, on some varieties and on the lower leaves there might not be such striking stripes and symptoms vary depending on a variety’s susceptibility. The other rust that could be seen is leaf rust which produces small round or oblong raised pustules that are orange red in color. Leaf rust pustules are more scattered and larger in size compared to stripe rust. Leaf rust develops within a warmer temperature range (64-77 F). If uncertain about rust identification, you can consult your county agent and additional information at UTcrops.com.