She clipped some samples and took them back to her lab for analysis. “The second time I cursed was when I looked at those petri plates,” she remembers. The culture tested positive: It was sudden oak death.The disease primarily affects tan oaks, which grow along the coast of California and southern Oregon. It’s caused by a pathogen called Phytophthora ramorum. It’s a peculiar organism: It appears similar to a fungus but is actually a water mold, more closely related to kelp than fungi. A related species, Phytophthora infestans, was responsible for the 19th-century Irish potato famine, during which about a million people died and millions more fled the country. Phytophthora ramorum is causing a natural disaster on a similar scale in western forests: It has killed over 30 million trees in California and Oregon in just 20 years.
The spores start in the canopy, reproducing in the leaves and fine twigs. They move around—from tree to tree, or from upper canopy to bark—thanks to wind, rain, and fog. Once they reach the bark, the infection forms cankers: dead spots and breaches. The foliage of a sick tree can turn brown in weeks, but, contrary to the disease’s name, actual death usually comes after years of infection. Spores in the soil form long-resting structures that don’t spread as easily, but they can survive and spread from soil or leaf litter.Many kinds of vegetation can be infected, but tan oaks are the most susceptible to illness and death. They play a critical ecosystem role; for animals, they are often the most important nut-producing tree in forests dominated by pines (whose nuts are smaller and harder to access) and redwoods. An infestation would have massive economic implications for Oregon, where forestry and wood products are the state’s third-largest industry, worth over $8 billion.