The fungal organism that causes gray mold in strawberries has become completely resistant to some fungicides that Southeastern growers have been relying upon for years.
Should spring weather be cool and wet, strawberry producers may experience the "perfect storm."
With that in mind, Guido Schnabel, a research and Extension plant pathologist at Clemson University, says growers should carefully choose and rotate fungicides.
His research group collected samples from four counties in North Carolina and from eight in South Carolina.
They found resistance to certain chemical classes in all of the sample areas.
The fungicide classes with the greatest levels of resistance were QoIs, SDHIs and benzimidazoles, he says.
QoI fungicides include strobilurins, fenamidone and famoxadone.
SDHIs include fluopyram and boscalid.
Benzimidazole include benomyl and thiabendazole.
But the resistance wasn't uniform, Schnabel says.
"In some areas, benzimidazoles are still active, while in other areas SDHIs are still active," he says.
What has him more concerned is how widespread the resistance is.
"We didn't expect to see that much resistance so widespread," he says.
The resistance is caused by mutation at the fungicidal target site in the fungi.
"The fitness of the resistant isolates compares to the sensitive isolates," Schnabel says. "Even though you back off with one chemical, there's a good chance that the resistant ones will outcompete the sensitive ones."
As a result, it may take several years of not using a fungicide before the fungal organism regains sensitivity to the product.
The fungus, Botrytis cinerea, causes crown rot, tissue blight and fruit rot.
During wet, cool weather, untreated plants can lose up to 90 percent of flowers and fruit.
Growers may be unaware of the resistance problem because they have not experienced total product failure, he says.
In addition, recent dry weather has minimized fungal pressures.
Growers may be spraying regularly, but the dry weather—and not the fungicide—is keeping gray mold in check, Schnbabel says.
To help growers determine whether they have resistant gray mold isolates in their fields and to which chemical or chemicals, Schnabel is part of a team developing field test kits.
These are similar to test kits he helped develop for Southeastern peach growers who had resistant fruit rot isolates in their orchards.
The idea is growers test the gray mold organisms, then avoid using any product to which the organism is resistant.