A microscopic pathogen, introduced to the United States about 1995, is wiping out a relative of the oak tree called the tanoak in the western states of California and Oregon. Only about ten percent of known tanoaks remain. At a slower rate, the same organism is also attacking 15 other deciduous, or leaf-bearing, trees and plants. And there is alarming evidence that the world's tallest tree, the soaring California redwood, may be next.
The awe-inspiring redwood, which can live 2,000 years and top 100 meters in height, is such a signature symbol of the northern California coast that the area is called "the Redwood Empire." Logging almost wiped out that empire until harvesting was regulated.
Today, more than 250,000 hectares of redwood forest remain in timber production. And safe havens for the trees were established in state and national parks. No wonder scientists and preservationists are worried about the recent discovery that the deadly Phytophthora ramorum organism has migrated from broadleaf trees to the redwoods, which are needle-bearing conifers.
Plant pathologists Matteo Garbelotto at the University of California in Berkeley, and David Rizzo at the university's branch in Davis, California, have found DNA evidence of the pathogen in dead redwood branches.
Dr. Garbelotto says Phytophthora, which is a microscopic cousin of the algae that form ocean kelp, is especially virulent because it is carried by the wind as well as moving through soil and water. So far in bushes like rhododendrons and huckleberries, it invades and kills only selected leaves and small branches. But in oaks and tanoaks, it produces enzymes that disintegrate the tree trunk's bark.
"And then once it's gone through the bark, it colonizes the cambium, which is the live part of the tree," Dr. Garbelotto said. "And by doing so, it basically kills it. It destroys the cambium, and once it's done girdling the whole circumference of the tree, the plant is dead."
Dr. Garbelotto says that while blotching has been found on redwood needles, and Phytophthora DNA has been confirmed in small, dead redwood branches, he and Dr. Rizzo have not yet confirmed that entire redwood trees have been killed by the organism.
"The branch that we're looking at is dead, but the branch could be dead for different reasons - many different reasons," he said. "What it may do - it may take out every single branch, one at a time. But that may take, you know, a long, long time. We could talk tens or even hundreds of years for a redwood."
Ken Bovero is an arborist in Mill Valley, California. He first identified Phytophthora in oaks and coined the term "sudden oak death." He says he's cut into three dead redwoods in the forest, seen other distressed redwoods, and found evidence of Phytophthora deep inside the giant trees.
"I saw dark, vertical staining between the sapwood and the heartwood. I also found a heavy odor of fermentation," he said. "It smells as if you had freshly uncorked a bottle of wine, and if you smell the cork, you smell that fermentation. That's what alarmed me. So I sent samples to a laboratory in Davis, California, and they confirmed that Phytophthora fungus was present in the samples that I sent them."
Scientists can do little to stop a blight in the areas where an outbreak has already occurred. In the late 1800s, a blight introduced to the New York Botanical Garden wiped out the entire East Coast population of chestnut trees. About the same time in Australia, a pathogen similar to Phytophthora killed a thousand native species.
Dr. Garbelotto says the spread of disease can be better controlled today than in the days of the chestnut blight. If it's confirmed that Phytophthora is threatening redwood trees, affected stands can be quarantined. Then other redwoods could be sprayed with copper sulfate, which would kill attacking spores and, hopefully, save the Redwood Empire.
Source:VOA News Service
Author: Ted Landphair
First published: January 15, 2002
Labels: Conifers, Forests, Pest Control, Trees
