When disease organisms become resistant to a fungicide, spraying is like hitting them with rainwater—expensive rainwater.
In the last three years, Georgia and South Carolina peach growers have saved money from what would have been wasted sprays and lost fruit caused by fungicide-resistant brown rot—thanks to a Profile kit developed by plant pathologists Dr. Guido Schnabel at South Carolina’s Clemson University and Dr. Phil Brannen at the University of Georgia in Athens.
The test should work equally well for any pathogen that sporulates, grows on artificial medium, and develops early in the season, Schnabel said. Among them would be gray mold of grapes and brown rot infections in cherries and other stone fruit, he says. The technique works best with fungi whose spores are easily collected and easily grown in a petri dish.
Extension agents and specialists have been running the tests for peach growers rather than leaving it to growers themselves. It requires collecting spores from several fruit, transferring spores to assay plates so as not to contaminate them, recording data, and interpreting results.
“A Web application facilitates the system,” Schnabel said. “The user simply enters the observations and receives a customized recommendation for resistance management that quickly can be shared with growers via e-mail.”
The test was refined last year to make it even more useful. In the early version, a piece of agar containing three fungicides was cut from what looked like a tube of lip balm and placed in a culture dish with spores from infected fruit. The new version uses a 24-cell assay plate that can be filled with up to five different fungicide-agar mixtures and a control with no fungicide. The new test not only detects fungicide resistance, but it tells exactly which fungicide class or classes the fungus is resistant to.
The test is done by collecting spores from fruit showing symptoms of brown rot, either by using a cotton swab or by pricking the infected area, then transferring the spores into each of six cells on the assay plate. The top cell contains a medium supporting fungal growth but no fungicide. The second cell contains a medium plus DMI (propiconazole) fungicide; cell 3, MBC (thiophanate methyl) fungicide; cell 4, QoI (azoxystrobin) fungicide; cell 5, carboxamide (boscalid) fungicide; and cell 6, pyramide (fluopyram) fungicide.
“Each fungicide represents a different chemical class and is applied at a dose that will only allow resistant pathogens to grow,” Schnabel said. Each plate has enough cells to test four fruit.
After three days of incubation at room temperature, a visual assessment is made of the mycelial growth on the fungicide-amended potato dextrose agar.
Growers do not run the test unless there was infected fruit in their orchard and there was suspicion of fungicide failure, Schnabel said. And they would want to spray soon after seeing the rot. The three-day test allows them to get back into the orchard quickly—and assures them that what’s in the spray tank will be effective in controlling the brown rot.
Earlier this year, Schnabel obtained a grant of $850,000 from the U.S. Department of Agriculture to extend the technique to strawberries for diagnosis of gray mold and Anthracnose.