Growing strawberriesin Florida just got tougher. This spring, growers discovered that most of the usual fungicides had little effect on an outbreak of Botrytis cinerea.

It is solid evidence of the fungus developing widespread resistance to the chemical controls, says Natalia Peres, University of Florida plant pathologist at the Gulf Coast Research and Education Center at Wimauma.

Botrytis, commonly called gray mold, causes fruit rot and sometimes results in considerable yield loss. The fungus can also remain dormant on strawberries, then attack fruit being stored, causing rejections at the retail level. Both situations make this fungicide resistance particularly distressing.

“Most fungicides growers have been using have been highly effective but they only had one target on the fungal cell. In that case, it only takes one single cell-based mutation to develop resistance,” says Peres, whose research had indicated that this situation could occur, given the right circumstances.

“Diseases are different from a weed problem, where you can see the herbicide-resistant weeds developing in the field. You can’t see this disease until you already have lesions that produce millions of spores. You only see the disease resistant problem when there is a major outbreak.

“Botrytis-resistant strains have been building up in fields over the past two or three years, but there has not been a big outbreak. This year, conditions for it were right. It needs to have all the stars aligned.”

That alignment unfortunately occurred in a major way this season. Fungicides affected by resistance included Topsin M, Abound, Cabrio, Flint, Elevate, Scala, Captevate, and Pristine. The only material not affected was Switch.

However, the fungus shows resistance to one of the two compounds in Switch, cyprodinil, and is only being controlled by its other half, fludioxonil.

“Another factor with Switch was that it is the most expensive fungicide, so growers have not been using it as much during this season when prices were generally low as they did the others,” Peres says.

Cyprodinil is also a component of Inspire Super, a fungicide recently cleared for use on strawberries. Peres says the fungus will most likely have what she terms cross-resistance to the new material.