The killer that is laying waste to the oak trees of California is almost invisible. Sudden oak death got its name before anyone knew why the trees were dying; eventually the cause turned out to be Phytophthera ramorum. The only outward sign of infection is a dark, bleeding ooze that leaks down the bark. But a second fungus, hypoxylon, is almost always associated with the disease–this one fruits into a distinctive black growth that emerges from the bark and looks like a mound of dark, hardened foam. But inside the tree the infection is running rampant.
While tanoaks and several species of oaks are the most likely to die from the disease, several other species are carriers–including redwoods, rhododendrons, and pepperwood. The spores of Phytophthera can’t travel far; they rely on the splash and flow of rainwater to carry them. But hikers, bikers, and ATVs do a much more effective job of moving the infection from place to place. It can also spread on firewood and the equipment used to do tree work. The sad truth is that even if it moves slowly, certain trees–like tanoaks–have no resistance to this disease. Once it reaches them, they will die, and someday they are almost certainly going to be locally (if not totally) extinct.