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Pathogen Resistance Issues

Wednesday, December 1, 2010
filed under: Optimizing Plant Development/Yields

Is there an issue with developing disease resistance to products like Headline if used repeatedly? Headline and Quadris are both strobilurin fungicides. It is well known that strobilurins are considered high risk for fungal pathogens developing resistance to them. This high-risk status has been determined by the Fungicide Resistance Action Committee (FRAC), an international committee that evaluates the likelihood of resistance to fungicides.

“Plant pathogenic fungi developing resistance to strobilurin fungicides is not new,” says Dr. Carl Bradley of the University of Illinois. “This has already occurred in potatoes and other crop and disease systems where multiple fungicide applications occur during the growing season.” It was recently reported that the disease ‘soybean frogeye leaf spot’ was found to be resistant to strobilurin fungicides. Bradley was surprised to find this resistance so soon. “Every time you apply a fungicide, you increase the selection pressure and the opportunity to select out individuals in the pathogen population that have resistance or reduced sensitivity to the fungicide,” Bradley has written.

But there is very good news for sunflower producers who use strobilurins such as Headline and Quadris to control rust and for yield enhancement. No rust pathogen has ever been found to be resistant to strobilurins. Dr. Sam Markell NDSU extension plant pathologist, an expert in rust, reports that rust resistance to strobilurins has not been found anywhere in the world. This is the case in situations where multiple applications are made annually. Examples are European producers controlling strip rust in wheat, and Brazilian producers applying strobilurins to control Asian soybean rust.

Resistance in Alternaria would be a concern to Markell if that disease were prevalent in the U.S. production region. One disease of possible concern for resistance could be Phomopsis. However, at this point there are no recommendations for fungicide control. Phomopsis did show up in commercial fields this year, and research on fungicide use to manage the disease will be accelerated.

Most university plant pathologists do not recommend a fungicide for yield enhancement. Yield data have been inconsistent from one location to another on crops from corn to sunflower. Markell found a 600-lb. yield increase with Headline compared to Folicur in one of his sunflower rust nurseries in 2009. But there was no statistical difference in nurseries at other locations that year.

Plant health is an issue in sunflower — a crop that is vulnerable to a number of pathogens. Markell notes that he wouldn’t hesitate to add some very inexpensive Folicur to his sunflower fields when spraying for head insects. “It really is a matter of perception. If the grower is finding that a fungicide works for him (in the absence of visible disease) and is cost effective, then he should use it as long as pathogen resistance issues don’t show up,” Markell says.

Crop protection companies have been emphasizing research and introduction of new fungicides. This emphasis is partly due to the dominance of glyphosate in the herbicide market. Growers can expect a number of new fungicides over the next several years. Some of these are new classes of chemistry with various modes of action.

There are new fungicides in the registration pipeline, some of which may be available by the 2012 season. Some may be more specific to one or more pathogens. These are new classes of fungicides with different modes of action, and they should offer new opportunities for sunflower producers. — Larry Kleingartner
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