The Pitch Canker Epidemic in California

Native Monterey pine (Pinus radiata D. Don) forests are currently found at three disjunct locations in coastal California and on two islands off the coast of Mexico. The mainland forests comprise, in total, approximately 4,500 ha (11), whereas the two island populations are limited to 150 ha on Cedros Island and fewer than 400 trees on Guadalupe Island (28). Collectively, these populations are significant ecological and recreational resources, but they are also a valuable repository of useful genetic traits for improved varieties of Monterey pine, which are widely used by the timber industry. In California, Monterey pine is important as a landscape tree, with an estimated 50 million standing trees as of 1985. Monterey pines were especially popular for plantings on freeway rights-of-way, where they served as visual and sound barriers for the adjacent properties. It was among such trees that a dieback problem became apparent in Santa Cruz County (Fig. 1) during the mid-1980s. In 1986, A. H. McCain, extension plant pathologist at University of California Berkeley, established that the affected trees were suffering from pitch canker (30), caused by Fusarium circinatum Nirenberg & O’Donnell (=Fusarium subglutinans (Wollenweb. & Reinking) Nelson, Toussoun, & Marasas f. sp. pini). Subsequent surveys showed the disease to be widespread in coastal Santa Cruz County, with a clearly disjunct infestation being found farther inland, in Alameda County (Fig. 1). Curiously, the disease was also found in Christmas tree farms in Los Angeles and San Diego counties, more than 450 km to the southeast (8).