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CORVALLIS - A mix of "good" bacteria and chemicals looks like the best bet to control fire blight, a disease that kills pear and apple trees in a single season.

Oregon State University plant pathologist Ken Johnson said experiments with two kinds of commercially available bacterial strains showed each is about 60 percent effective as a biological control of fire blight.

Johnson wants to do better. So he is studying mixes of beneficial bacterial strains and genetically modified bacteria under a competitive grant he and colleagues Virginia Stockwell and Joyce Loper won from the U.S. Department of Agriculture. He is also experimenting with combinations of biological and chemical control.

Johnson said he had about "60 percent effective" control in tests with the commercially available biological control agents Blight Ban A506 and Blight Ban C9-1, both of which contain beneficial bacteria.

"Neither treatment is as good as (the antibiotic) streptomycin used to be, but the fire blight pathogen has become resistant to streptomycin," Johnson said.

The bacterial treatments work by crowding out the bad bacteria (the pathogens) on the fruit tree flowers.

"If we crowd out the pathogen, honeybees won't be able to spread the disease throughout the tree or to other trees," Johnson said. "By using biological controls, we can reduce the amount of chemicals needed and we can improve control."

In orchard tests, combinations of biological control agents and the antibiotics streptomycin or ocytetracykcline gave nearly complete fire blight control.

Fire blight disease has been particularly common this year near Hood River and in southern Washington state.