April 24 Wheat Pest and Disease Update

From Ned Tisserat, CSU Extension Specialist and Professor, Bioagricultural Sciences and Pest Management

Ned sends out information concerning wheat diseases in the state on a routine basis. To subscribe to this email, please email Ned at Ned.Tisserat@ColoState.EDU.

Freeze Damage.   There are reports that some freeze damage may have occurred in wheat south of I70 as a result of last  and early this week’s low temperatures.  I have inclued an excellent review ([download id=”635″]) of determining freeze damage developed by agronomists at Kansas State University.  It may be a week or so before one can accurately assess the damage caused by freezing.

Rust update.  There are reports of stripe rust in south Texas, although the severity seems to be much less than last year.  Dr. Bob Hunger, Oklahoma State, also reported finding stripe rust in trace amounts in one field in southern Oklahoma.  Therefore, it doesn’t appear that there is a buildup of stripe rust to the south of us yet.   That is good news, but we still have a long way to go.  There are more pressing problems at the moment concerning the possible freeze damage.

 

Last week I sent along a draft of a table listing fungicides for rust and leaf spot control.   Dr. Erick DeWolf, Kansas State University, wrote a nice summary of the residual efficacy of fungicides.  That discussion follows.

Residual life of fungicides

The residual life of the fungicide is influence by many factors, including the rate at which the product is applied, the targeted disease, and the level of disease pressure. Fungicides applied at the full-labeled rate will generally have longer residual life. Fungicides will generally provide longer residual life against rust diseases (often more than 21 days) than leaf spot diseases. Some of the products may provide some additional residual life but this extra residual does not always translate into more grain yield.

The research that I have reviewed indicates that fungicides listed in the publication Foliar Fungicide Efficacy Ratings for Wheat Disease Management 2013 will generally provide 21 days of solid protection against fungal diseases. This includes products with the active ingredient tebuconazole that is listed in the table as the product Folicur but is also marketed in a number of generic formulations.

The chart below is just one example of the type of data that supports the 21-day residual activity for the various fungicide products.

fungicide residual small

Preventive vs. curative activity

There is some confusion at times about the preventive and curative activity various fungicides. All the fungicides listed in the publication mentioned above are best applied when the disease is at low levels. The triazole fungicides are generally considered to provide some limited curative activity. This means that they can stop the development of fungi already inside the plant. The triazole-only fungicides include products such as Prosaro, Carmaba, Tilt, and Folicur. Triazole fungicides are also included in most of the mixed mode-of-action products such as Quilt Xcel, Stratego YLD, and Twinline. Both the triazole and strobilurin fungicides provide excellent protection against new infections that is often considered “preventive” activity.

It would be an error to think that a triazole fungicide does not provide preventive activity simply because it also has curative activity. The curative activity is good thing, especially with a disease such as stripe rust where the fungus grows within the plant to cause additional expansion of the stripes.

Summary

The bottom line is that producers have a lot of excellent fungicide options. In my experience, based on the all the data I have seen in research trials in Kansas and other states, the importance of correctly identifying situations where fungicides are needed or not needed is far greater than the choice of fungicide product.

— Erick De Wolf, Extension Plant Pathology, Kansas State University

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