Other new fungicides being tested may eventually bring some relief. Two old materials, Captan and Thiram, can control the disease, but much higher rates are necessary, compared to the newer fungicides.

Resistance to multiple fungicides got growers’ attention fast.

“If we’d had resistance only to one product, they would probably not even notice because most of them use more than one fungicide,” Peres says. “The problem is that some of the fungicides they’ve been using have the same mode of action.”

She thinks the possibility of resistance increased because growers tended to use fungicides on a set schedule rather than applying them when they were truly needed.

“In most cases, the label states the limited number of applications you can use. If you use more than that, and sometimes growers do, you are more likely to get resistant Botrytis,” she says.

“The entire Botrytis population does not have resistance to all fungicides. Some have multiple resistance to two fungicides, some to three, some to four. But what we do know is that if you spray more, you select for more resistant strains.”

University of Florida scientists have been evaluating Botrytis isolates collected since 2001. That research shows that 15 percent to 20 percent of Botrytis isolates may have resistance to all fungicides except Switch and fludioxonil, while some may be resistant to only one or two.

Still, growers should not let this push them to overuse Switch, Peres says, because, in that case, the fungus will very likely develop resistance to it, too.

“It is overuse that causedthe problem,” she says. Changing cultural practices also might have had something to do with it, too.

“Some growers have been leaving the plastic row cover on the ground to re-use the following season. They kill the plants and just leave them on the cover. That saves the cost of the plastic.

“But our hypothesis is that it keeps the inoculum and the resistant strains there from one year to another. In the past, when they removed the old plants and the plastic cover, then worked the soil and planted again, that may not have been the case,” she says.

Botrytis can be a particularly virulent foe. It has a number of other hosts including tomatoes, some ornamental plants and some weeds. The fungus quickly evolves and can change from one year to the next.

It produces a lot of spores, which get blown up into the air. If even a few Botrytis fungi in a field are resistant to fungicides, those few can still spread material a great distance.

Fungicide resistance should motivate growers to use the Strawberry Advisory System Peres devised to alert growers when conditions are right for Botrytis and anthracnose, two major diseases affecting the crop.