Roundup

Dousing fire blight

Give fire blight plenty of rain and warmth during apple bloom, or wind-driven rains or hail before mid-summer, and the blossom- and shoot-blackening disease will explode across Idaho apple orchards.

Not only do today's high-density plantings, intensive fertilization, and overhead irrigation fuel the spread and severity of fire blight, but several of Idaho's most marketable new varietiesÑincluding Fuji, Braeburn, and GalaÑare unusually susceptible.

For the past three springs at Parma, UI extension plant pathologist Krishna Mohan has deliberately infected the tips of young shoots in 22 apple varieties with the disease-causing bacterium Erwinia amylovora. Then he has measured the progress of the infection through those shoots. Consistently, fire blight has roared through the full length of Fuji shoots, and nearly as far in Braeburns and Galas.

Plant pathologist Krishna Mohan tests experimental apple varieties.

Currently, orchardists battle fire blight with aggressive pruning and with the antibiotic streptomycin. "Under weather conditions that favor the disease, fire blight can put the grower out of business," says Mohan. The situation is poised to worsen as streptomycin-resistant strains of fire blight, now widespread in Washington, build in Idaho.

To ensure that future new varieties have acceptable disease resistance, Mohan and UI fruit physiologist Essie Fallahi are participating in a nationwide variety development project. At Parma, one of six sites in the U.S., they are evaluating the growth, production, quality, and disease resistance of 20 experimental varieties.

They are also cooperating with four other western universities to fine-tune environmental forecasting models that will let growers know whenÑand whereÑto spray most effectively.

In addition, Fallahi is investigating a new growth regulator that restrains shoot development. The control strategy might work, says Fallahi, because fire blight flourishes in tender new growth and is rebuffed by mature tissue.

"The battle continues," he says, "because we want the Fuji." --Marlene Fritz