Fungicide Resistance in Strawberries

Posted: June 2, 2014

Botrytis or gray mold is a major disease for strawberry growers, and there is some new information on fungicide resistance that growers should have.

I'd like to thank Dr. Guido Schnabel, Clemson University, for his input and work, as what he and his colleagues are doing is very helpful to growers everywhere. This information is summarized from work being conducted in the southeastern U.S. - at the Univ. of FL and Clemson Univ. – where researchers have been testing botrytis isolates from strawberry fields for resistance to commonly-used fungicides. They tested over 1800 samples from 183 farms in 2012 and 2013, and found the following:

  1. More than 75% of the isolates tested were resistant to thiophanate-methyl (Topsin M). Resistance to this material is not a big surprise, as this is an older material that was known to be at high risk for resistance development.
  2. Roughly half of the isolates tested in each year were resistant to pyraclostrobin (the active ingredient in Cabrio, and one of the active ingredients in Pristine), though a smaller percentage (29% and 5% in 2012 and 2013, respectively) were resistant to boscalid, the other active ingredient in Pristine.
  3. Resistance to both thiophanate-methyl and pyraclostrobin were found in essentially every location in both years, though not in all samples, meaning that resistant isolates existed on nearly every farm.
  4. In 2012 and 2013, respectively, 29 and 17% of isolates were resistant to cyprodinil, which is one of the active ingredients in Switch, with very low levels of resistance to fludioxinil, the other active ingredient in Switch.
  5. About 1/4 of botrytis isolates were resistant to fenhexamid (the active ingredient in Elevate).

    And, of extreme interest…

  6. More than half of the isolates were resistant to fungicides in more than one chemical class. 33% of the isolates were resistant to fungicides in either three or four different chemical classes.

This cannot be dismissed as purely a problem in other states – Dr. Schnabel has done limited testing in MD and PA, with the help of Bob Rouse, and found significant resistance in those samples, too.

The first question some folks might have is whether you could buy plants infected with resistant isolates. Nurseries are very aware of potential resistance issues, and are generally very careful about fungicide rotations – after all, they have a lot at stake if they can't control diseases. Also, some materials that are at high risk of resistance development are prohibited from nursery use. So I'm more concerned about use on individual farms. Every now and then, I talk to someone who thinks they are rotating fungicides, but then when asked which ones they use, lists the names of 2 or 3 products with ingredients in the same fungicide class. I also know that when you have small acreages, it's tempting to buy one or two products at a time, and use those until they are gone, rather than accumulate products in your pesticide shed. So, those practices are a concern. Please read on for what you can do to help.

First, be sure to use any cultural controls that you can to avoid disease issues, cut down on botrytis inoculum on your farm, and minimize the need for sprays. Every spray avoided is avoidance of an opportunity for resistance development. Cultural controls consist of removing dead leaves from plasticulture fields in the spring (that's where a lot of inoculum overwinters), and basically, anything that helps the field to stay dry, because diseases need moisture to sporulate. So, keep weeds controlled, rows narrowed back, and possibly consider a wider row spacing in matted-row production, or slightly wider plant spacing on plasticulture beds. Keep fields picked to the greatest extent that you can, and encourage harvesters to remove rotten fruit from the field. Cultural controls generally have other benefits like improving plant growth and fruit quality.

Second, don't just spray on a schedule – spray only when you have a reason. Even if you don't see botrytis, inoculum is out there, and every spray exposes what inoculum is there to the material you are using. This applies to any crop – not just strawberries. If the weather is dry and no rain is forecast, there is probably no reason to spray, at least not for diseases.

Third, consider a break from any at-risk fungicides that you have used more commonly in the past. Dr. Schnabel mentioned that resistance development has not progressed on farms where growers moved to other products and then were careful about how they used them.

Fourth, either rotate at-risk products with a broad-spectrum fungicide like captan (0-day PHI but 24-hr REI) or thiram (3-day PHIand 24-hr REI), or include them in a tank mix. You may have seen last month's article about concern with captan use during bloom and bees, but by now, you should be past bloom, and so for any disease issues for the remainder of the year, you should be able to use broad-spectrum materials, IF you need to spray.

Finally, be sure to consult FRAC codes on the fungicide package, or Table 6.14 in the Mid-Atlantic Berry guide for information on the fungicides that fall under different chemical classes.


  • Fernandez-Ortuno, D., A. Grabke, P. K. Bryson, A. Amiri, N. A. Peres, and G. Schnabel. 2014.
  • Fungicide Resistance Profiles in Botrytis cinerea from Strawberries Fields of Seven Southern U.S. States. Plant Disease 98(6):825-833. Online at with access limited. A summary of the work can be found here:

Contact Information

Kathy Demchak
  • Senior Extension Associate
Phone: 814-863-2303