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Apogee at king bloom petal fall reduces fireblight shoot strikes

Apogee (prohexadione calcium) applied at petal fall of king bloom provides excellent control of shoot fireblight, says Dr. George Sundin, the Michigan State University plant pathologist who has studied the disease intensely for several years.
Apogee, and not the antibiotic streptomycin, is effective against shoot blight, and growers should not rely on streptomycin after the bloom period.
Streptomycin is the most effective treatment to prevent infection during the bloom period, a time when the fireblight bacteria (Erwinia amylovora) can get inside trees and cause lasting problems—oozing cankers, shoot and rootstock infection, and even tree death. Strep kills bacteria on plant surfaces.
But, Sundin cautions growers against extended use of streptomycin. Streptomycin resistance has developed in the past when a large number of applications—ten or more per season—were made over several seasons, he said.
He recommends that growers use resistance management techniques to keep streptomycin-resistant bacteria from spreading. “Strep is the best material for blossom blight control,” he said. Other materials, like the antibiotic Mycoshield (oxytetracycline) and biological control agents such as Serenade MAX, do not kill the pathogen and do not control blossom blight as well as strep does.
“Strep resistance occurs widely in the western United States and has developed in some regions in Michigan,” he said. In Michigan, Sundin, working with the Michigan Department of Agriculture, obtained a Section 18 specific exemption enabling the use of Kasumin (kasugamycin), a new agricultural antibiotic for fire blight control.
If conditions are conducive to fireblight, Sundin recommends strep, or Kasumin where strep resistance occurs, during bloom followed by the full rate of Apogee at king bloom petal fall. Under less severe fireblight pressure, Apogee can be applied at lower rates two or three times at two-week intervals.
Succulent tissue wounded by wind or storms is vulnerable to infection. “Trauma events such as wind or hail storms not only wound trees but also introduce E. amylovora cells to internal tissues,” he said. “These internalized populations are not affected by streptomycin unless applications are made optimally, within four to six hours of the trauma event.

Mechanism

Sundin, working at MSU with pathologists Molly McGrath, Jessica Koczan, and Megan Kennelly, investigated the mechanism that makes Apogee effective. In a paper they published last year, they said that Apogee induces structural resistance to fireblight infection by thickening the cell walls in developing shoots. It does not reduce the population of the causative bacteria, they found, but impedes their spread.
It takes about two weeks for Apogee’s effects to appear. King bloom petal fall occurs about two weeks before shoot elongation.
Apogee was labeled for use on fruit by the agrichemical company BASF in 2000, at which time both its ability to reduce shoot growth and its ability to reduce shoot fireblight strikes were known. The work by Sundin and his colleagues established how it does that.
In their trials, the researchers also tested another plant growth regulator, paclobutrazol, which is used to suppress shoot growth in ornamentals and in other applications. It, too, thickened cell walls and suppressed fireblight, but to a lesser degree than Apogee.
Finding how these materials work could lead to other important developments. Might thicker cell walls be a genetic trait that occurs naturally in varieties like Red Delicious, which are less fireblight susceptible?
“We’re looking at that now,” Sundin said of his new research.
In fruit breeding programs, the trait for thicker cell walls might be a marker, an early indicator of whether a new seedling might be more or less resistant to fireblight.