Good Fruit Grower

February 15th

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Diseases & Disorders Strategies for controlling BACTERIAL SPOT Growers in the eastern United States should select peach varieties that are resistant to the disease. by Richard Lehnert B acterial spot is a serious problem for peach growers in the eastern United States, but no big worry for western growers. This has effectively divided the country on which peach varieties they grow. Western peach breeders rarely screen for bacterial spot resistance, while for eastern growers, resistance is a primary tool for control. Dr. David Ritchie, a plant pathologist at North Carolina State University in Raleigh and a leading authority on the disease, spoke about it during the Great Lakes Fruit, Vegetable, and Farm Market Expo in Grand Rapids, Michigan, in December. "There is a wide range of susceptibility within peach varieties, from highly resistant varieties such as Sentinel and Clayton to highly susceptible varieties like O'Henry and Ryan Sun," he said. "Most, if not all, varieties devel- oped west of the Rocky Mountains are highly susceptible because bacterial spot is so uncommon in the west that breeding programs cannot screen new selections for susceptibility." Ritchie has developed a management approach that is widely used by eastern growers. Management of bacterial spot on peaches should start with host resistance when available for cultivars adapted to the growing area, he said. "Bacterial spot is very difficult to control on highly susceptible varieties. Spray options are very limited to copper-containing materials and the antibiotic oxytetra- cycline (FireLine, Mycoshield). "When establishing an orchard in areas prone to bac- terial spot, varieties having at least moderate resistance should be considered. In years when disease pressure is low, such varieties will sustain little to no fruit loss; in years when disease pressure is high, chemical sprays are much more effective on less susceptible varieties." NEW HOLLAND MODIFIED TN4040V w/cab Narrowest & Lowest Profile Tractor on the Market Call for Details. The O'Henry peach variety is a poster child, highly susceptible to bacterial spot. Symptoms include fruit spots, leaf spots, and twig cankers. The strategy for using copper is to place a bacteria- killing barrier on the surface of the tree before the bacte- ria emerge from overwintering sites. The bacterial pathogen overwinters in the tree in leaf scars, buds, and other protected sites on the tree surface. These areas can be killed and form necrotic areas (spring cankers) at ter- minal or lateral buds. These cankers are normally first vis- ible at bloom and are a major source of the season's first bacteria that can infect emerging leaves." Epidemics From his studies of epidemics over the last three decades, Ritchie concluded that there is a period when fruit is very susceptible and that the environment at that time is critical for fruit infection. This period is from petal fall through shucks-off—and sometimes extends through pit hardening. Frequent and extended rain events during that period resulted in more fruit infection. "In addition to frequency and time of precipitation relative to seasonal tree development, the duration of wetness is important," he said. Wetness in the morning is more easily tolerated than wetness that lasts a day or more. This sensitivity to wet- ness is evident in the symptoms. Tips of leaves, where moisture evaporates more slowly, show the spot lesions and chlorosis more quickly than the base of leaves. Bacterial infection seems to be independent of tem- perature; however, temperature can influence the rate that bacteria multiply and thus bacterial inoculum levels and the rate of symptom development, he added. It is difficult to present a spray program for bacterial spot that fits every orchard, he said, because so much depends on the variety's susceptibility and local condi- tions within an orchard. Places that do not dry quickly might have more disease, but places that dry quickly because of wind can be susceptible because blowing, sandy soil can cause abrasions that foster the disease. Orchards having bacterial spot can be further infected by airblasting sprays through wet foliage. Ritchie recommends a spray program that relies heav- ily on use of copper, but warned that copper is hard to manage and that some injury to leaves—reddish spots and shot holes—can be expected. "Peaches and other stone fruits are very sensitive to "Serving "Serving Area Farmers Since 1939" "Your Ag Area FarmersEquipment Since 1939" Specialist" 1308 East Mead YAKIMA,WA 509-457-8105 YAKIMA,WAburrowstractor.com 509-457-8105 1308 East Mead burrowstractor.com QUALITY SERVICE QUALITY PARTS 30 FEBRUARY 15, 2012 GOOD FRUIT GROWER 800-827-5138 800-827-5138 QUALITY SERVICE • QUALITY PARTS 927 Wenatchee Ave WENATCHEE,WA 509-888-0355 927 Wenatchee Ave WENATCHEE,WA 509-888-0355 "Your Ag Equipment Specialist" copper, thus extreme care must be taken when copper is used during the growing season," he said. "Sprayers should be properly calibrated, the correct rate of copper used in the appropriate volume of spray water, and foliage monitored before subsequent sprays are applied, to determine if unacceptable injury is occurring or likely to occur with another spray. Also, copper can accumulate on the trees if adequate rainfall fails to occur between applications." The program Richie recommends to growers is as follows (copper rates are in pounds of metallic copper www.goodfruit.com I saved big $$ at Burrows Tractor. You will too! david ritchie

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