—by Ross Courtney
When Frank Zhao, the Washington State University endowed chair for plant pathology, was hired three years ago, growers asked him to look for resistance in the fire blight pathogen to antibiotics.
He found resistance to kasugamycin.
“The industry is not happy,” Zhao said.
True, but no one is ready to panic just yet.
“Just because there is resistance to one product doesn’t mean the sky is falling,” said Mike Doerr, an account manager for Wilbur-Ellis.
Zhao’s revelation that Erwinia amylovora, the bacterium that causes fire blight, is showing resistance to one of the industry’s mainstay antibiotics just reinforces the usual advice for growers: Limit use of the product and rotate it with other materials, Doerr said.
The good news is that Zhao caught it early, said Greg Pickel, business development manager for G.S. Long Co. “It’s not prevalent, but it is present,” Pickel said.
After 18 years spent researching fire blight at the University of Illinois, Zhao began at WSU in 2021. One of his first goals was to collect samples he could screen for antibiotic resistance.
In 2022, he detected something going on in a few samples, but at such low levels he called it evidence of “tolerance,” not resistance.
But it got worse.
In 2023, 38 isolates from 10 pear locations did not die when treated with 100 parts per million of kasugamycin, the full rate recommended by WSU. In 2024, five isolates from two locations withstood treatment at 150 parts per million.
Also in 2024, he documented the mutation in the gene that kasugamycin targets.
Kasugamycin was first registered in 2014 by the U.S. Environmental Protection Agency. Ten years is a short time for resistance to develop, Zhao said. “That’s quick,” he said.
The industry has weathered resistance in fire blight products before.
In the early 1990s, growers and researchers noticed resistance to streptomycin, Doerr said. They stopped using it for three years. Subsequent testing told them the resistant strain of fire blight was diminished in the absence of the selection pressure of regular streptomycin sprays.
So, crop advisors reintroduced streptomycin — now as part of a rotation. Meanwhile, predictive tools for monitoring weather conditions and moisture levels have improved, and WSU wrote Cougar Blight, the fire blight degree-day model on the university’s Decision Aid System website. Together, these advances help growers use all their fire blight products more efficiently.
“This is a good resistance management story that we still refer to today,” Doerr said. As part of his recent research, Zhao did not find resistance to streptomycin.
Zhao’s news about kasugamycin resistance was not a shock, Doerr said. Growers and advisors were already suspicious.
Pickel hadn’t seen any field failures and is glad Zhao confirmed resistance early.
Already, the industry limits use of kasugamycin to once per year and tries to save it for high needs, such as on Pink Lady, which is susceptible to fire blight, he said. Growers also use copper, plant growth regulators and plant defense elicitors such as Actigard (acibenzolar-S-methyl).
His advisors also rotate the material.
Kasugamycin and streptomycin have different modes of action but are both bactericidal, which means they kill bacteria. Oxytetracyline is a bacteriostatic, a preventative treatment applied to flowers that limits Erwinia growth. It is less prone to resistance.
“The key is, just be very judicious with your use of the product,” Pickel said. •
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