The Gall of Yukon's Poplars
Peter Lesniak, editor of the Yukon News had a question he thought would interest his readers and other northerners. What was known about the "big, gnarled galls" that appeared on poplars near Whitehorse and elsewhere in the Yukon?
"These aberrations," he wrote, "seem to occur in groves of trees. Northern poplars seem to be more susceptible than southern ones." He noted also that the galls were harvested to become "works of art, fence ornaments and pieces of furniture.
Doggone. He had me there. Galled and lumpy spruce are common enough in Alaska, and one can see them turned into everything from porch rails to oversized mosquito sculptures by creative folk from Homer to Coldfoot. But poplar? Aspen? Cottonwood? By any name, poplar galls were something I'd never noticed.
However, what Lesniak described sounded like some manner of disease. Not only were the plants altered from their normal configuration, a probable indicator of ill health right there, but the galls showed up in northern trees and in groves, which indicates either contagion by proximity or the presence of a disease-susceptible clone. That was likely, since poplars can send up new trunks from old roots; a grove may be actually a single individual, rather than a collection of seed-grown and unrelated members of the species. That galls showed up in northern trees hinted that the trees might be stressed by growing near the limits of their range, where they would have little energy to spare for warding off invading organisms.
But that was all guesswork. Expert advice was needed. Naturally, though Lesniak's letter arrived in winter, I put off following up on his query until late spring, which meant that the annual Black Hole into which scientists vanish was open: field season had begun.
Nevertheless, one of the busy professionals took time to return my phone message. Tony Gasbarro is both an associate professor in the forest sciences division at the University of Alaska Fairbanks and a forester for the Cooperative Extension Service. The man knows his trees.
That meant that he spoke only with scientific caution. "Hmm," he said. "Without actually examining the galls, I'd hate to say what caused them." Then he went on to recommend other experts whom he considered more expert than himself.
Sometimes you have to be firm with these modest-type scientists. Sometimes it does you very little good. After admitting that the other experts were either in the field or off presenting papers before they went into the field, Gasbarro offered to send some basic information on poplar ailments. "It's probably caused by a canker disease," he said, "or something like it. But remember, you really can't tell without actually examining the trees."
The information soon arrived. It was a portion of the U.S. Department of Agriculture's Handbook 677, "A Guide to Insect, Disease, and Animal Pests of Poplars." It's enough to make one pity the poor trees, filled as it is with descriptions of vile ailments that can afflict poplars and photographs showing their damaging effects. An illustration accompanying the first disease description did indeed show a large, lumpy swelling on a poplar trunk. The caption said, "Old canker."
This particular gall-raiser is a fungus, Septoria musiva. The organism overwinters in part in fallen infected leaves and young cankers. In spring, the fungal spores are blown by wind or splashed by rain onto new poplar victims. The severity of the disease it causes, septoria canker, depends in part on the natural resistance of the poplar clone. Very susceptible ones are killed outright. Many show severe damage to younger shoots. Tough, resistant ones battle and live with the canker, walling it off with woody tissue to form rough galls on trunk or branches.
Forester Gasbarro is unquestionably right, that one can't be sure what the galls are without examining them. Yet the canker theory sound plausible. One would expect Yukon trees to be tough---and have a fair amount of gall.
