Three years ago, Russ Shlagel, owner of Shlagel Farms, Waldorf, Md., thought he was doing the right thing by applying certain fungicides to prevent gray mold in his 6-acre U-pick strawberry patch.
He was surprised to find—after sending plant samples to be analyzed through a program at Clemson University—that none of the materials he was applying was working. Gray mold, caused by the fungus Botrytis cinerea, had grown resistant to all of them.
Now Shlagel and up to 90 other growers each year routinely send samples of plants showing gray mold symptoms to be analyzed early in the season for fungicide resistance.
Guido Schnabel, professor of plant pathology at Clemson, started the program in response to increased concerns about gray mold and its mounting fungicide resistance.
Gray mold is the No. 1 problem farmers in the Southeast and Midwest face when growing strawberries, Schnabel says.
“It’s a problem that has to be attended to every year because inoculum is always present,” he says.
When an infection occurs, growers need a fungicide that actually works. That’s where Schnabel comes in.
He leads a group that analyzes samples growers submit and then report back, telling growers whether gray mold has developed resistance to specific fungicides in their area.
“It’s important to know your location-specific resistance profile to avoid spraying something that doesn’t work,” Schnabel says.
Over the years, growers have developed a greater awareness and understanding of resistance issues, he says, and the ability to control gray mold seems to be improving.
“We are making really good progress understanding how gray mold is developing and what the impact is of our chemical control methods on the gray mold population,” Schnabel says.
He is able to offer his service free to growers in the Southeast thanks to grant money, but he has to charge for other areas such as Michigan or Arkansas.
The 2014 price for testing samples from one strawberry field was $100.
Unlike some diseases that appear only sporadically, gray mold is very aggressive. It produces an abundance of spores and causes secondary infections, so controlling an initial infection is a must, he says.
In a typical year, gray mold may not cause any damage with an effective spray program.
If you didn’t spray at all for it,” he says, “you can expect 20 to 30 percent yield loss before harvest and more after harvest.”
In a bad year, a grower who doesn’t spray can lose an entire crop.
The fungus produces a gray spore mass on the berry before harvest, but seemingly healthy fruit may also develop symptoms after harvest, Schnabel says.
Work is a “crop-saver”
Strawberry growers are appreciative of Schnabel’s efforts.
“The work that he does is almost invaluable,” Shlagel says. “It’s a money-saver as well as a crop-saver that he’s providing.”
Since strawberries are an annual crop that doesn’t carry over from year to year and Shlagel only sprays six to eight times per season, he found it hard to believe that there could be resistance issues.
But then he realized that plants are sprayed at nurseries before they even arrive at his ranch.
In February, he sent Schnabel plant material that never had been sprayed in Maryland and that showed symptoms of gray mold and blossoms that had suffered winter kill. The fungus showed signs of resistance.
“If you don’t know this ahead of time, you’re out there spraying and wondering why things aren’t working for you,” Shlagel says.
Results can change each year.
This year, he was able to use fungicides that had resistance issues three years ago.
Botrytis can be a significant issue in South Carolina, especially during wet years, says Eric Hunter, owner of Hunter Farms, a small growing operation in Easley.
“(Schnabel’s program) helps us out a lot,” Hunter says. “We may be spraying a chemical that (it) has grown resistant to, and we wouldn’t know otherwise if it wasn’t for his tests.”
Hunter turns in samples to Schnabel’s lab and finds out within a week if gray mold is resistant to a chemical.
“It’s very farm specific,” he says. A grower down the road might have different results.
In Florida, Schnabel’s efforts are complemented by the work of Natália Peres, associate professor of plant pathology at the University of Florida’s Gulf Coast Research and Education Center in Wimauma.
Through a project called “Precision Decision Management for Sustainable Strawberry Production in the Eastern U.S.,” Peres monitors weather conditions in Florida and lets strawberry growers know when and what they should spray to prevent gray mold.
By visiting a website—agroclimate.org/tools/strawberry—growers can access a map that shows risk levels for gray mold as reported by six weather stations. Peres plans to expand the network to include two weather stations in South Carolina within a year.
Weather is the key to determining the need for spraying fungicides, Peres says.
The stations monitor the duration of leaf wetness on strawberry plants by measuring moisture resulting from rain, overhead irrigation and overnight dew. Temperatures during the period when the plant is wet also are monitored.
If conditions warrant, an alert is sent by text or email to subscribers, and growers are advised of the gray mold risk level and told when and what to spray. A mobile app prototype also is being tested.
Schnabel’s research has determined that the fungicide Switch is the only one that is highly effective in combating gray mold in Florida, so Peres says it’s important to ensure that the chemical is not overused.
If resistance develops to Switch, “Growers would be in big trouble,” she says. “This helps them to only spray when it’s really needed.”
Currently, about half of Florida’s strawberry growers use the AgroClimate system.
In the past, growers typically sprayed once a week, because they were not sure of the risk level.
“Now they can go three or four weeks without spraying,” Peres says.
So far, the program has been funded by the U.S. Department of Agriculture at no cost to growers.
“We’ll keep it free as long as have other sources of funding,” Peres says.
Schnabel says he tests at least 10 samples (fungal isolates) per field and looks at all available registered chemical classes to figure out which active ingredients still work.
The resistance profile of a field is based on several factors, including the spray habits of the grower, whether other small fruit are around and the nursery source.
“The Botrytis population can come in from several different sources,” including nearby weeds or crops, plants purchased from the nursery, he says. Or it may have survived from the strawberry crop from previous years.
All the testing is done at Schnabel’s lab to ensure consistency and to avoid subjective assessment of data.
The program has been in effect four years and covers Florida, South Carolina, North Carolina, Pennsylvania, Virginia and Georgia.