Department of Plant Pathology, University of California, Davis 95618, USA

Background and objectives
Pitch canker, a disease of pines caused by Fusarium subglutinans f. sp. pini, was first described in 1946 in the south-eastern USA, where it remains a chronic problem in plantations, seed orchards and nurseries. Pitch canker was discovered in California in 1986 and has since been reported to occur in Mexico, Japan and South Africa. In California, pitch canker has become a serious problem on Monterey pine (Pinus radiata), the most widely planted pine species in the state. Inoculation studies and observations in permanent plots both indicate that approximately 97% of Monterey pines are susceptible to pitch canker. The susceptibility of Monterey pine, coupled with its abundance, has allowed pitch canker to spread throughout much of coastal California [1]. Initially the damage was restricted to urban forests, but by 1992 pitch canker was detected in native stands on the Monterey Peninsula. Subsequently, pitch canker was recognized in the other two native populations of Monterey pine in California, at Ano Nuevo and Cambria. The disease is now well established in all three native populations.

Results and conclusions
The rapidity with which the pitch canker epidemic has developed in California has no doubt been facilitated by the pathogen's association with native insects. Insects from which F. s. pini has been isolated include engraver beetles (lps spp.), twig beetles (Pityophthorus spp.) and cone beetles (Conophthorus radiatae). These insects are natural associates of pines which tend to favour weakened trees or parts thereof and thus, in the absence of pitch canker, are not commonly a primary cause of tree mortality. For example, although gallery formation by twig beetles causes some damage to the branches they colonize, the cost to the tree is minimal. On the other hand, by vectoring pitch canker, twig beetle activities contribute to a general decline of the tree which thereby renders it more prone to attack by engraver beetles, which will further weaken or kill the tree. This in turn provides additional substrate for twig and engraver beetles, allowing their populations to increase and enhancing the prospects for insect and/or disease-induced damage to neighbouring trees. This process is under way in native Monterey pine forests, but the extent to which it will proceed in other western forests remains unknown. From our present perspective, several factors suggest that damage will eventually be extensive. Firstly, greenhouse tests show that most low-elevation pine species native to the western USA are comparable to Monterey pine in their susceptibility to pitch canker. This would include Bishop, Coulter, Digger, Jeffrey, knobcone, lodgepole, ponderosa and shore pines. Also susceptible, though perhaps less so, are pinyon pine and Douglas fir. Secondly, the host range of many known and suspected insect vectors extends to one or more of the susceptible pine species, and their geographic ranges largely coincide with that of their host trees. Thus there are ample opportunities for the pathogen to move between susceptible hosts. Finally, there is as yet no evidence that environmental conditions will limit the movement of the pathogen from the coastal regions to more inland montane habitats. Low temperatures may prevent spread of the disease to higher elevations, as the pathogen shows little or no growth below 10C. However, the moderate temperatures which prevail in the foothills of the Sierra Nevada throughout much of the year will not constrain growth and infection by the pitch canker pathogen. There are differences in susceptibility to pitch canker, both within and between species. Over time, therefore, shifts in the species composition of coniferous forests in California are likely. Moreover, even trees which presently appear resistant may eventually succumb to the disease if the pathogen population changes as a result of new introductions and/or sexual recombination. In extreme cases this may mean a conversion of pine forests to oak woodlands.

1. Storer AJ, Gordon TR, Wood DL, Bonello P, 1997. Journal of Forestry 95, 21-26.