Strategies for controlling bacterial spot
Growers in the eastern United States should select peach varieties that are resistant to the disease.
The O’Henry peach variety is a poster child, highly susceptible to bacterial spot. Symptoms include fruit spots, leaf spots, and twig cankers.
Bacterial 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 developed 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 oxytetracycline (FireLine, Mycoshield).
“When establishing an orchard in areas prone to bacterial 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.”
The strategy for using copper is to place a bacteria-killing barrier on the surface of the tree before the bacteria 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 terminal or lateral buds. These cankers are normally first visible at bloom and are a major source of the season’s first bacteria that can infect emerging leaves.”
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 wetness 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 temperature; 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 conditions 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 heavily 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 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 equivalent (MCE) and application rate is 100 gallons per acre):
—Dormant bud to bud swell: copper 1.0 - 2.5 lb per acre
—Bud burst (top of buds have opened) to ¼-inch green: 0.75 to 1.25 lb per acre
—Pink to 25 percent bloom: 0.5 – 1.0 lb per acre
—First petal fall to 50 percent petal fall: 0.25 – 0.75 lb per acre
When risk of potential injury from copper becomes high, he recommends lowering the rate and adding antibiotics.
—First shuck-split to 50 percent shuck-split: FireLine 17WP or Mycoshield 0.75 – 1.0 lb may be tank-mixed with 0.10 – 0.20 lb copper per acre.
—Shucks off: FireLine 17WP or Mycoshield at 0.75 lb per acre may be tank-mixed with 0.10 – 0.20 lb copper per acre.
—Additional sprays as needed based on weather and disease conditions.
Time of applications is critical; sprays should be applied prior to rainfall but with sufficient time for the chemicals to dry, he said.
“Discontinue copper if unacceptable injury occurs,” he added. “Antibacterial activity of oxytetracycline has a short period of a few days and is readily washed off with rainfall.”
The earliest lesions on fruit that develops bacterial spot are seen about three weeks after petal fall.
Developing fruit lesions have a water-soaked appearance, sometimes gummy, with a small necrotic area in the center, he said. As the lesions mature, they turn brown to black and enlarge. Infections during petal fall develop into large, open lesions that extend deep into the fruit flesh, sometimes almost to the pit by harvest. Infections after initiation of pit hardening usually remain near the fruit surface. These shallow lesions may coalesce, resulting in skin cracking.