Nobody likes rotten strawberries. That’s why growers tend to apply fungicides to their fields on regular schedules The problem with spraying antifungal agents when the calendar — or your gut feeling — tells you to is that you can end up applying more fungicide than you need to.
A related problem is that you might unknowingly apply fungicides to which diseases have become resistant. By the time you figure that out, you may have wasted hundreds of dollars on fungicides, and Botrytis gray mold or anthracnose fruit rot may have gained ground.
The good news is that researchers have developed what seem to be promising solutions to each of these dilemmas.
With the help of a $2.9 million U.S. Department of Agriculture specialty crop initiative grant, Natalia Peres, associate plant pathology professor at the University of Florida’s Institute of Food and Agricultural Sciences in Balm, has developed a model that alerts you when it’s time to spray.
The computerized system analyzes leaf wetness in conjunction with the temperature to come up with a probability of Botrytis gray mold or anthracnose fruit rot developing during a certain timeframe.
And at Clemson University in South Carolina, Guido Schnabel, professor and Extension fruit pathologist, has developed a kit that helps you quickly determine whether a particular kind of rot is resistant to the fungicide you plan to spray.
Reducing fungicide applications can be a boon to growers. Peres estimates that fungicide alone costs about $50 per acre, not counting labor, time and equipment costs.
She looks for two main factors when determining whether to issue a spray alert—leaf wetness and the temperature during that wetness period. In general, if the temperature tops 65 degrees and the leaves are damp, it’s time to spray.
“If it reaches the threshold we set up for each disease, we send an alert,” she says. The alert can be in the form of an e-mail or text message for which growers can sign up or a Web announcement.
Of course, Peres’ calculations are much more scientific and also take into account other factors, such as the particular host, pathogen and environment as well as fungicide performance during past seasons.
Peres is pleased with the results so far. Over the past two years, those who followed the program and sprayed only when conditions were optimal for disease have been able to reduce pesticide use by up to 50 percent, she says.
The project also is being evaluated in North Carolina, South Carolina, Ohio and Iowa. If it is successful there, systems will be developed for those states.
Meantime, Peres says she hopes more growers will familiarize themselves with the program.
Disease predictor passes the test
Grower Carl Grooms, owner of Fancy Farms in Plant City, Fla., has been testing Peres’ system and is impressed with what he’s seen so far.
In the past, Grooms often would spray automatically once a week. Now, if Peres indicates there’s a two-week window with no sign of disease on the horizon, he won’t spray. “We’re learning that maybe we don’t need to spray as much as we do,” he says. “This pinpoints the date of application more precisely.”
Grooms has set aside a portion of his 220 acres of strawberries as a test section where he follows Peres’ instructions precisely. For the rest of his acreage, he also factors in his 38 years of experience as a grower.
Grooms estimates that he sprayed about one-third fewer times when he followed Peres’ system. He says he would have wasted 10 fungicide applications this season if he had done it the old way.
So far, he says results have been the same in both areas, and the test section has used less fungicide and saved time and wear and tear on equipment.
Testing for resistance
At Clemson, Schnabel also has been testing Peres’ system with good preliminary results.
“The warning system looks just as good as the regular grower standard, but we’re using much fewer sprays,” he says.
But Schnabel’s main focus is on his own project.
“I’m interested in figuring out how we can incorporate more of those reduced-risk fungicides into our spray program without promoting resistance development in the gray mold fungus,” he says.
The fungus that causes molds is the “world champion in producing resistance,” he says. “It can really generate resistance to all chemical classes that we have available to us.”
By next season, he will have developed an assay that will tell in about two to three days if a chemical class is still effective against a fungus.
Researchers simply collect some strawberries with symptoms of gray mold, transfer the spores into test plates that contain a medium that will indicate if it is sensitive or resistant to a particular fungicide.
“If the fungus grows enough on the fungicide- amended medium after two days, then we know the fungus is resistant, and we shouldn’t continue to use that chemical class any longer,” Schnabel says. “We have to switch to something else.
“With this system, I think we can really make some good, science-based decisions in regard to what we should spray or what we shouldn’t spray. It’s a much more thorough and science-based way to control diseases.”
Schnabel, along with colleagues at the University of Georgia, developed the test for brown rot on peaches, but he says it seems to work even better with Botrytis on strawberries.
With a little training, growers should be able to conduct the test themselves, he says.
The test plates have special markings so growers can measure the fungal growth. They then enter that information into a Web application, which generates a response for each chemical class about what steps the grower should take.
“It’s a very timely way of receiving information,” Schnabel says.
An $850,000 USDA specialty crop initiative grant funded development and the first few years of testing. Eventually, if the test is marketed commercially, he expects it to be reasonably priced.
“The information you’re getting is so valuable that I think that it will definitely be worthwhile for growers to do this,” he says.
The test can be used with any disease caused by a fungus that sporulates readily, including gray mold on grapes and citrus or brown rot on cherries and other stone fruits.
The strawberry disease forecast tool can be accessed at http://agroclimate.org/tools/ strawberry.