Apple pests stand up to antibiotics

Rcjohnsen rcjohnsen at aol.com
Sat Mar 25 18:16:03 EST 2000


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, McManus
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.	—T.H.

Science News, 3-18-00, vol 157, #12, pg.184




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