A Clemson University plant disease researcher has disturbing news for Southeastern strawberry growers.
Research data show that the fungus causing gray mold in strawberries has become completely resistant to some important pesticides that producers have been relying on for years.
“If the wrong products are used during weather conditions suitable for gray mold disease we will experience the ‘perfect storm,’” said Guido Schnabel, research and Extension plant pathologist at Clemson University.
Gray mold is a serious disease affecting the plant and fruit of the multibillion-dollar crop.
“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,” said Schnabel, who presented his research Saturday at the Southeast Regional Fruit and Vegetable Conference in Savannah.
“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,” said Schnabel.
Gray mold is one of the most common and serious diseases wherever strawberries are grown. The fungus, botrytis cinerea, causes crown rot, tissue blight and fruit rot. During wet seasons on unsprayed plants, losses of up to 90 percent of flowers and fruit can occur.
Schnabel believes growers are unaware of the problem because control failure has not yet occurred. Recently, there have been a lot of years of dry weather that would not support a widespread outbreak of the pathogen. Still, growers applied fungicides 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,” he said. “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.”
Other options effective
But Schnabel also says growers still have sufficient options for effective disease control. “The good news is that a sufficient number of chemical classes that — if rotated correctly — should allow for reliable disease control,” he said.
The goal for Schnabel, who is part of a regional strawberry research project headed by the University of Florida, is to make growers aware of the problem and to get them on board for 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.”
Schnabel’s research group collected samples taken from four counties in North Carolina and from eight counties in South Carolina.
“We found resistance to certain chemical classes in all of the sampled areas,” he said. “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.”
Schnabel received $850,000 from the four-year, $2.9 million USDA National Institute of Food and Agriculture grant that supports efforts to forecast outbreaks of two fungal diseases threatening the nation’s $2.1 billion strawberry crop.
Managing emerging fungicide resistance in strawberries is just one aspect of the proposed research. With the help of a disease forecast system to predict high risk of infection by analyzing air temperature and leaf wetness, the number of total applications can be drastically reduced, said University of Florida plant pathologist Natália Peres, the regional project leader.
Experiments so far have shown that growers can potentially reduce fungicide use by half without compromising disease control.
The grant will enable the team to test the system in other strawberry-producing states, including South Carolina, with the added component of monitoring fungicide resistance and advise growers of alternative fungicide options.