| Oaks are dominant trees in certain environments, including many urban landscapes. Their effect as landscape trees with great shade-providing canopies is highly valued. Some spectacular southern live oak-enhanced landscapes include St. Charles Avenue in New Orleans, the oak-lined avenues of southern plantations, and Louisiana State University's campus. In addition to their beauty, oak trees are important in supplying acorns for food for a variety of animals such as squirrels, birds, and deer. Oaks provide high quality hardwood timbers, and they are a minor source of drugs, tannins, and dyes. Ceratocystis fagacearum, an ascomycete fungus, apparently introduced into the United States, attacks and kills oak trees, particularly red oaks and live oaks. The disease was first described from Wisconsin localities in 1942, and since then it has spread south to Texas and east into the Appalachian Mountains. Although it is prevalent in twenty-two states, the geographical origin of the fungus is still not known. In 2004 the disease had been detected as far southeast as Harris County, Texas, near Houston. It can be spread several ways. Oak bark beetles and sap beetles (nitidulids) are potential vectors of the disease that are attracted to the sweet odor emitted by the sticky matrix in which the fungal spores are produced. Spore-contaminated insects spread the fungus, but only to nearby trees. In Texas spread from root-to-root transmission is more effective between oaks that grow in close proximity. Insects and root grafts are not the only ways to extend the areas infected with oak wilt, people and vehicles also perform the deed as well. Help stop the spread of the disease: The management program developed at Texas A&M University advocates 1) removal of infected oaks to prevent fruiting and further transmission; 2) obliteration of neighboring oak trees if root spread is expected; 3) cutting the roots connecting an infected and uninfected tree; 4) never pruning in the spring or mid-summer, the season that the spore mats are produced and attract the vectors; and, finally, 5) quarantine laws to aid in the control of this disease spreading from state to state and even into Canada. Unfortunately, in 2004 another exotic oak pathogen, Phytophthora ramorum, appeared via nursery stock to threaten doubly the lives of southern live oaks. --D. R. Bagaley |
