Fusarium subglutinans, a recent introduction into California, is the causal agent of pitch canker disease in Monterey pine. It is phoretic on many scolytid species. The engraver beetles Ips paraconfusus and I. mexicanus and the cone beetle Conophthorus radiatae are known to vector the pathogen, and twig beetles, Pityophthorus spp., are implicated as vectors. In the absence of F. s. pini, I. paraconfusus outbreaks are brief and sporadic, and I. mexicanus is usually a secondary species. Pityophthorus spp. usually infest shade weakened branches in the lower crown. These species function largely as primary decomposers of dead and dying trees. Infestation rates of C. radiatae vary from area to area, and its economic importance is restricted to sites where seed is harvested.
The impact of these scolytids is greater in the presence of the pitch canker pathogen. They may vector the pathogen to trees, without infesting those trees, creating weakened trees or branch tips which are subsequently used for breeding. Branch tip dieback followed by bole cankers frequently results in tree mortality that is usually associated with bark beetle infestation. The aesthetic value of these trees is reduced, costs associated with tree removal and disposal are high, the genetic diversity of a tree species with a limited native distribution but high global economic value is threatened, and quarantine measures may restrict the movement of infected material. Estimated dollar values of these economic impacts are not currently available.
An early indication of the presence of F. s. pini in an area may be the establishment of associations with insects. Insects which were previously unimportant from an economic perspective, and unstudied due to their low densities, now need to be considered as potential vectors. Increased economic impacts of the scolytid species considered here, and other scolytid and cohabiting species, are likely, because F. s. pini and many of the insect vectors have a wide host range.
Key words: pitch canker, Fusarium subglutinans, Monterey pine, Scolytidae, exotic tree diseases.
Correspondence: Andrew J. Storer, Division of Entomology, Plant and Soil Microbiology, 201 Wellman Hall, Berkeley, CA 94720 USA
E-mail: storer@nature.berkeley.edu