Clemson University scientists have found the fungus that causes gray mold in strawberries has become completely resistant to many key pesticides that are in common use in the U.S.

Plant disease researcher Guido Schnabel says the fungus Botrytis cinerea causes crown rot, tissue blight and fruit rot, and during wet seasons can result in up to 90% flower and fruit losses.

"If the wrong products are used during weather conditions suitable for gray mold disease we will experience the ‘perfect storm'," he says.

"I will have to let growers know that some products we have been using are no longer effective and resistance to other products is already emerging.

"There are several chemical classes available to control Botrytis blight, crown rot and gray mold disease. The ones that have been used frequently are now quickly selecting for resistance in the pathogen."

He says many southeastern U.S. growers are still unaware of the problem as a control failure has not yet happened, with dry weather over recent years that would not have supported a widespread outbreak of the pathogen. He adds that many growers that apply fungicides are unknowingly selecting for resistance.

"Growers may think they are controlling the disease with weekly sprays, but in reality it is the relatively dry weather that prevented the pathogen from taking off.

"Resistance has built up to such levels that the use of some chemicals is no longer justifiable. It’s just going to take one wet year and the wrong chemicals and we are going to have a big problem. I really am glad we caught this early to be able to counteract."

Schnabel is also part of a regional strawberry research project headed by University of Florida to raise awareness for growers and enlist them in a regional resistance-monitoring program.

"We are in the process of offering to growers in South Carolina, Georgia, North Carolina and Florida a service that will give them the information they need to make science-based decision about spraying and keeping it effective."

He says researchers took 12 samples in South and North Carolina to see how resistant the plants were.

"We found resistance to certain chemical classes in all of the sampled areas - resistance is based on point mutations in the fungicide target genes, which really is the worst possible kind of resistance. That means even an increase of the dose will not matter."

Photo: UFL

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