Good Fruit Grower

February 15

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16 FEBRUARY 15, 2015 GOOD FRUIT GROWER A fter years of quietly accepting Armillaria as a disease that not only kills trees but passes a death sentence on stone fruit sites that become infected, there is new hope that such sites may one day be replanted with resistant trees. That springs from the discovery that the Amur choke- cherry, Prunus maackii, a wild cherry species from the region along the Siberian and Manchurian border, shows a high level of resistance to Armillaria. Its resistance comes from plant-produced compounds with fungicidal activity that prevent the disease from developing. The resistance was discovered by plant pathologist Dr. Ray Hammerschmidt at Michigan State University. His special area of research is in induced resistance; he studies plants that secrete compounds that fend off pests that attack them. He began working with Armillaria more than 20 years ago and has been screening Prunus species looking for resistance. He developed a relatively rapid assay based on the observation that Armillaria initially colonizes tissues between the periderm and the wood. Hammerschmidt concluded that P. maackii carries a gene or genes that produce fungicidal compounds. "The outer bark of P. maackii was determined to contain compounds that are antifungal," he said. It's not like typical single-gene resistance, as in scab resistance in apples, but is more of an entire pathway. "The compounds may also serve as good biochemical markers for resistance screening of other species and hybrids from crosses," he said. Diseases Amur chokecherry, used as a rootstock, may protect stone fruits from Armillaria. by Richard Lehnert ARMILLARIA resistance explored Above: Amy Iezzoni is incorporating the chokecherry Prunus maackii in her breeding work and search for disease-resistant varieties and rootstocks. Right: Prunus maackii growing in an orchard heavily infected with Armillaria. The tree was planted in October 2013, and the photo was taken August 2014. RICHARD LEHNERT/GOOD FRUIT GROWER COURTESY OF AMY IEZZONI

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