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Question: How much early blight resistance to SDHI fungicides are you seeing in the fields and how is it impacting current control strategies?

{Sponsored} In the recent past, early blight resistance was between 90% to 100% for the major SDHI fungicides on the market. New strategies had to be implemented to bring that resistance back down to a manageable level.

The SDHI fungicides are being threatened again as farmers rely heavily on these FRAC Group 7 fungicides to deal with developing resistance to strobilurin fungicides (FRAC Group 11). In order to keep the efficacy of the SDHI fungicides, it is imperative that farmers not overuse these chemistries.

What’s the bottom line? Only rotation and tight control will help extend the shelf life of the SDHI fungicides.

Here are a few very specific steps farmers need to take in order to reduce the development of early blight resistance and preserve the utility of SDHI fungicides.

1. Rotate different FRAC Groups throughout the season

Rotating effective modes of actions, identified by different FRAC Groups, prevents populations of pathogens from becoming more resistant to a class of chemistry. Potato farmers do not have many tools, so it is imperative that the life of fungicides are stretched as long as possible.

In situations where older products regain utility, it may be possible to add a different FRAC Group into your SDHI rotation. By varying products or giving one product a break, chemistries can become efficacious again. An example of this is Rovral® brand 4 flowable fungicide, a FRAC Group 2 fungicide, which growers used for white mold in the past, but then stopped using as pathogens developed resistance. Now, Rovral brand 4 flowable fungicide is seeing a resurgence in use for a different pathogen, early blight, because fungal populations are susceptible to it. This gives growers another mode of action to add to their rotation.

2. Follow labels and keep meticulous records

Always follow the manufacturer’s guidelines and track very carefully how often and how much each FRAC Group is applied to every acre. When the strobilurin fungicides came out in the early 2000s, they were heavily abused because of their level of control and resistance developed in the Midwest within two years of release.

Follow the manufacturer’s recommendations for SDHI pesticides such as Endura® fungicide. It is very easy to overapply specific FRAC Groups without realizing it — especially since some of the newer products on the market are a mix of chemistries. When a mix is used along with a duplicate mode of action, too many applications of a single FRAC Group may accidentally be made. Always look at the FRAC Groups on product labels, keep meticulous records and double-check all calculations to be sure you’re not exceeding recommendations.

3. Include a protectant fungicide in the mix

Include a protectant fungicide such as chlorothalonil or mancozeb in your program. While these chemistries are older and not as effective as the newer chemistries on the market, they provide broad spectrum control and using them along with the modern SDHI fungicides will help avoid the development of resistance.

4. Utilize disease forecasting tools to minimize sprays

The other piece to this puzzle is to tightly control timing of sprays. Weather-based disease forecasting tools predict when conditions are favorable for fungi to proliferate. By incorporating disease forecasts to time fungicide applications, growers may decrease the selection pressure placed upon the various FRAC Groups.

Timing fungicide application to disease forecasts, rotating effective modes of action, use of protectant fungicides and careful record keeping to prevent the overuse of the same FRAC Group may help prolong the life of key fungicide chemistries. If you mismanage fungicides, you may develop resistance in a single season.

See more Spud Science at spudman.com/fmcspudscience.

Always read and follow all label directions, restrictions and precautions for use. Some products may not be registered for use in all states of the United States. FMC and Rovral are trademarks of FMC Corporation or an affiliate. Endura is a trademark of BASF. © 2018 FMC Corporation. All rights reserved. 18-FMC0494 04/18

Meet the Expert Dr. Phillip Wharton is currently associate professor of potato pathology at University of Idaho. Wharton earned his doctorate in plant disease resistance in 1997. He spent the following two years as a post-doctoral researcher at Purdue Dr. Phillip WhartonUniversity before moving to Michigan State University in 1999. At MSU he studied the biology and epidemiology of diseases of tree and small fruit (cherries, blueberries, strawberries, and grapes) before concentrating his efforts in 2004 on late blight, Rhizoctonia, Fusarium dry rot and other diseases of potato. Wharton currently conducts research on disease forecasting, crop protection, host-pathogen interactions, post-harvest disease management of vegetable crops and fungicide resistance.

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