—by Matt Milkovich
Where there is stone fruit, there will be pathogens — especially in Midwest and East Coast growing regions.
Two tree fruit pathologists discussed pathogen management in stone fruit during the Great Lakes Fruit, Vegetable and Farm Market EXPO in December. Penn State University’s Kari Peter and Michigan State University’s George Sundin offered the latest advice on control options.
Brown rot represents the most consistent, and impactful, fungal disease of stone fruit, Peter said. It thrives in hot, wet conditions, and growers face limited management options. She recommended cultural controls such as removing fruit mummies left from the previous season — which can turn into spore factories — as well as pruning out cankers. She also recommended pruning during the season to create more air movement in the canopy and keep conditions a bit drier and cooler.
Unlike brown rot, powdery mildew typically thrives in dry conditions and does not pose a consistent problem from year to year. When it does hit, it can cause rusty spot, rendering fruit unmarketable, Peter said.
During dry years, growers should try to control powdery mildew early in the season, roughly late April to mid-June in the mid-Atlantic region. Rutgers University researchers have developed a four-spray management schedule: petal fall, shuck split, first cover and second cover. If the chemicals are applied at the right times, they will “nail” powdery mildew, she said.
Regardless of the disease, management requires a multipronged approach: Use fungicides with a rotation of different modes of action; make sure trees have adequate nutrient and water levels; cut out diseased wood and remove shoots during the season to create air movement in the canopy; and plant resistant varieties, Peter said.
Sundin said bacterial canker flares up periodically in Michigan sweet cherries, especially if a bad frost hits during bloom. Bactericide management options are limited. Growers previously applied copper products, but sweet cherry trees are susceptible to phytotoxicity from copper, and the bacterial canker pathogen has developed resistance to copper, which must be applied judiciously.
The antibiotic Kasumin (kasugamycin) was recently registered to treat bacterial canker on cherries at bloom, though that option may be too expensive for processing sweet cherries. Kasumin treatments might offer a better option in more profitable fresh cherries, which are typically grown in higher-density systems where canker can spread more easily between trees, he said.
Pruning out infected branches is an effective cultural control for bacterial canker, Sundin said.
Bacterial spot can be difficult to manage in peaches, especially in susceptible cultivars. There are cultivars with greater resistance to bacterial spot, but unfortunately many popular varieties are highly susceptible to the disease, Sundin said.
Control options for bacterial spot are limited. Peach trees are sensitive to phytotoxicity from copper applications, so growers must reduce the amount of copper as the season progresses, lessening its effectiveness, he said. •
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