Apple pests stand up to antibiotics.

Antibiotic resistance in bacteria may soon force apple farmers to search for a new prescription to fight a deadly tree disease.

Resistance to the common antibiotic streptomycin is sweeping through apple and pear orchards, impairing farmers' ability to fend off fire blight, says Patricia S. McManus, a plant pathologist at the University of Wisconsin-Madison.

Fire blight, a leading killer of fruit trees, is the reason farmers spray more than 11,000 kilograms of streptomycin on U.S. orchards every spring. Streptomycin and the antibiotic oxytetracycline both can keep fire blight in check, although streptomycin does a better job, says McManus.

That may change as two forms of streptomycin resistance proliferate in fire blight bacteria, McManus said at the American Association for the Advancement of Science annual meeting in Washington, D.C., last month. The most common form--caused by a mutation in one bacterial gene--has appeared in orchards in the northwestern United States, Michigan, and New Zealand.

A second form emerged in a single Michigan orchard in 1991. It has since spread to 20 orchards in three counties, McManus says. This resistance is caused by two genes carried on a circle of DNA called a plasmid. Scientists suspect a related bacterium passed the plasmid to fire blight bacteria, she says. In a fourth Michigan county, fire blight bacteria have incorporated the two resistance genes into their chromosomal DNA.

The new antibiotic resistances probably aren't a danger to people, McMaaus says. However, researchers have "no data, no data whatsoever" to determine the environmental and health effects of antibiotic spraying, she says. Unfortunately, adds Anne K. Vidaver of the University of Nebraska-Lincoln, there is no cost-effective alternative.