In 1995 a microscopic fungus called Phytophthora ramorum, or P. ramorum, was first detected in the forests of the western United States. P. ramorum infects trees and causes particularly serious damage in oak trees: in many infected oaks, leaves wither rapidly, large cracks appear in the bark, and the trees die. A spread of P. ramorum represents a serious threat to the forests in the western states. Several methods of protecting the forests have been proposed.

First, stopping P. ramorum spores from spreading would surely be an effective method. Spores are small particles through which all fungi, including P. ramorum, reproduce. Researchers have discovered that many P. ramorum spores can be found along hiking or biking trails, suggesting human-assisted spread by way of shoes and bicycle tires. A few measures to prevent such human-assisted spread—like encouraging hikers to wash their shoes and installing new bike scrubbers on bicycle trails—would be an effective and low-cost way to stop the spread of P. ramorum.

Second, there are a few fungicidal (fungus-fighting) chemicals that can be used to protect the oak trees. Some of these chemicals stimulate the oak trees’ natural defenses against the P. ramorum fungus and have been found in small-scale tryouts to significantly reduce the likelihood that the oaks will be infected.

A third way to fight P. ramorum is a practice called clear-cutting. This approach starts with cutting and burning the diseased oaks, but it also involves cutting and burning the seemingly healthy vegetation (bushes and other kinds of trees) surrounding the oaks. This is done because some of the surrounding plants and trees may be infected even though they do not show any symptoms of the disease. Clearing large areas of vegetation in places where diseased trees are found is often an efficient measure to stop the spread of infections.