Dead terminal shoots on apple trees caused by infection by fireblight bacteria.
COURTESY OF KERIK COX
Apple growers in New York State were not granted the Section 18 emergency use permit they requested from the U.S. Environmental Protection Agency. That would have allowed them to use the antibiotic kasugamycin (Kasumin) to prevent fireblight. Instead, they need to follow a different action plan if they are in an area where the bacteria may be resistant to streptomycin.
“We won’t be able to use kasugamycin,” said Dr. Kerik Cox, the Cornell University plant pathologist who sought the emergency use permit from EPA. “Unlike in Michigan, where it was granted, New York growers have not seen economic damage from fireblight at the same level. Michigan growers lose acres of trees and millions of dollars annually to fireblight.”
Streptomycin-resistant fireblight strains were discovered last fall on four apple farms in western New York, Cox said, and it is not yet known how widespread resistance may be. “It’s a fairly scary situation for growers in that area,” he said. “The areas of these outbreaks are regarded as high risk for streptomycin-resistant fireblight in 2012.”
The Cornell pathologists, Cox and Dr. Herb Aldwinckle, have developed a program they advise all New York growers use every year against fireblight, and they have added an “action plan” for the high-risk areas.
The general recommendations are:
—Remove all fireblight cankers during winter pruning.
—Apply copper sprays at green tip and on all new plantings of young trees.
—Heed warnings of fireblight infection periods, and apply recommended materials promptly.
—Seriously consider sprays of prohexadione calcium (Apogee).
—Prune out and destroy fireblight strikes promptly.
—If severe blossom blight occurs after streptomycin was applied in accordance with predicted fireblight infection periods, contact Cornell Cooperative Extension for testing to determine whether resistance has occurred.
In the high-risk areas, the pathologists recommend:
—Add oxytetracycline to the tank mix with streptomycin, both at full rates, when blossom infection periods are forecast.
—Use prohexadione calcium sprays at the high rate, applied at two to three inches of shoot growth.
Not first outbreak
This is not New York’s first encounter with strep-resistant fireblight, Cox said. In 2002, there was an outbreak after two New York growers planted trees from a Michigan nursery. That year, Michigan had a devastating outbreak of fireblight that killed millions of trees, and some of the bacteria were strep-resistant, including those from the nursery. Those trees planted in New York were destroyed.
This new infection was first spotted in Wolcott, New York, and additional ones were found in Wayne and Ontario counties. All infected trees there are thought to have been destroyed.
However, growers are being urged to examine new trees carefully, paying close attention to trees from New York and Michigan nurseries in areas where strep-resistant fireblight strains are known to occur.
Cornell pathologists have these recommendations for all new tree plantings:
—If possible, plant varieties grafted on fireblight-resistant rootstocks.
—Examine trees carefully for fireblight infections before planting. Infected trees should be discarded, and samples submitted for strep‐resistance testing.
—Apply a copper spray immediately after planting.
—Scout new plantings at seven-day intervals for fireblight strikes until June 30. Remove infected trees. Scout seven to ten days after hail or severe summer storms and at the end of the season (mid‐September). Disease forecasting models (http://newa.nrcc.cornell.edu/newaModel/ apple_disease) can provide an estimate of symptom emergence following a storm or other trauma event.
—If possible, remove flowers before they open. Since most new plantings have many blossoms the first year, and many orchards are high density, blossom removal may not be feasible.
—Apply copper and a tank mix of streptomycin and oxytetracycline at the full label rate for each during any remaining bloom based on blossom blight predictions. The disease forecasting models run nearly until August and have an adjustable bloom date to account for asynchronous or late bloom in new plantings.
—Trees should receive a second copper spray at a stage equivalent to bloom.
—Samples of any infections seen after planting should be submitted for strep-resistance testing.