WSU professor warns of fire blight antibiotic resistance

MOSES LAKE — Frank Zhao, a professor of plant pathology at Washington State University, spoke at the April 4 Fire Blight Webinar hosted by Michigan State University, providing an update on current research and results of fire blight antibiotics and resistances at WSU.

According to WSU’s Tree Fruit website, fire blight is a disease affecting crops of pears and apples.

“Infections commonly occur during bloom or on late blooms during the three weeks following petal fall. Increased acreage of highly susceptible apple varieties on highly susceptible rootstocks has increased the danger that infected blocks will suffer significant damage. In Washington there have been minor outbreaks annually since 1991 and serious damage in about 5 to 10% of orchards in 1993, 1997, 1998, 2005, 2009, 2012, 2015, 2016, 2017 and 2018.”

Zhao discussed three common fire blight antibiotics during the webinar, including streptomycin, oxytetracycline and kasugamycin.

“So far, streptomycin remains the best tool we have to manage blossom blight,” Zhao said. “Unlike human pathogens or diseases, using antibodies to control fire blight is mostly for prevention, and not for a cure, because once it is infected, the delivered antibiotics will not kill the bacteria inside the plants.”

Bacteria can develop multiple mechanisms to become resistant to antibiotics, Zhao said.

“Most likely, in a lot of cases in our control fire blight, it is mainly due to bacteria producing some kind of enzyme that can degrade those antibodies, or (the bacteria) becomes mutated so that the antibiotics will not kill the bacteria, because the antibodies cannot actually bind into the gene,” he said.

Streptomycin resistance dates back at least 50 years.

“Streptomycin resistance was first reported in California in 1971, and also in Oregon and Washington in 1972,” Zhao said, “and later was found in the Eastern United States, including Michigan, as well as New York, and also found in other western states like Idaho, Utah, and in the Midwest, Missouri, and the other states as well.”

Zhao said that streptomycin resistance is much higher in the Western United States

“The mechanism for this resistance is a mutation of the target gene rpsL, and this gene is located in the chromosome, and this kind of mutation is mainly due to selective pressure, because we probably apply (streptomycin) several times and because we are using streptomycin for a long, long time.”

Zhao turned to new developments, outlining recent data of a pathogen in pear orchards in California

“(What) we recently found in California is alarming,” Zhao said. “This fire blight pathogen contained both streptomycin-resistant genes as well as oxytetracycline-resistant genes. And when we tested this resistant gene … in pears, we were then using oxytetracycline and streptomycin for control and you can see the efficacy of those antibiotics has been greatly reduced.”

The data from the last few years in Washington is much less clear regarding streptomycin, oxytetracycline and kasugamycin.

“In the past two years my lab … screened for those three antibiotics, and surprisingly, we didn’t find any streptomycin or oxytetracycline resistant genes in both years in Washington.”

Zhao explained that since the lab can’t locate streptomycin-resistant genes in specific samples, the results don’t help discover why streptomycin resistance persists in Washington.

Zhao said that the lab did discover resistance to kasugamycin in 2023.

“We found 30-some isolates that show some kind of resistance or tolerance to kasugamycin,” Zhao said. “Among those isolates, 68% were from pears and 32% were isolated from apples.”

Zhao gave WSU’s recommendation for which antibiotic mixtures to use on crops in Washington.

“Now in Washington, we kind of recommend these two mixtures of antibiotics, including kasugamycin with oxytetracycline, both at full rates, or streptomycin with oxytetracycline, both at full rates,” Zhao said, “but for this mixture, in Washington, we only recommend them once per season.”

For more information on fire blight, visit bit.ly/WSUFireBlight.