Diseases of Conifers in California

California has 52 native coniferous tree species and collectively, these populate approximately 15 % of all land in the state (6,170,500 ha), over a wide range of climatic conditions. It is not surprising that a number of pathogens have evolved to take advantage of this extensive resource. In this chapter, three groups of conifer pathogens are given special attention. Group 1 is the “heavy hitters”. This group includes three genera of pathogens that have exacted a particularly heavy toll on conifers in California, including: (1) the Arceuthobium genus of dwarf mistletoes (15 species), (2) two species of Heterobasidion, which cause root rot problems for almost all of California’s conifers, and (3) three varieties of the blackstain root disease fungus (Leptographium wageneri) which kill yellow pines, Pinyon pine and Douglas-fir. Group 2 includes 25 additional pathogens of conifers which are common in California but do not cause great levels of damage in this state. A few of these, however, have caused serious damage when they have found their way to other countries with Mediterranean climates, and therefore merit attention in this report. Finally, Group 3 includes five pathogens that have been introduced into California. Phytophthora lateralis, which causes Port-Orford-cedar root disease, is described in depth in this chapter. The remaining four (Cronatium ribicola, causal agent of white pine blister rust, Fusarium subglutanans, causal agent of pitch canker, Phytophthora ramorum, causal agent of sudden oak death, and Bursaphelenchus xylophylus, the pine wood nematode) cause so much damage elsewhere around the world that entire chapters of this book are dedicated to them and they are only briefly covered in this chapter. While Cronartium ribicola, Phytophthora lateralis, Fusarium subglutanans are highly damaging to California conifers, the other two pathogens are not (P. ramorum primarily damages California hardwoods, while damage by B. xylophylus to California pines is minor). A great deal of information already exists on the diseases of California conifers. However, the potential for export of these pathogens to other countries with Mediterranean climates and the extent of damage that could occur are seldom discussed. This report includes a discussion of this risk for each of 54 pathogens that are found in California. Six were determined to present a high risk of causing severe damage if exported to other suitable environments. These include: Cytospora spp., Diplodia spp., Dothistroma pini, Cronartium ribicola, Phytophthora ramorum, and Fusarium subglutanans. Four present a medium-high risk of causing severe damage if exported including Heterobasidion irregulare, H. occidentale, Phomopsis lokoyae and Endocronartium occidentale. Another 13 present a medium risk, while the remaining 13 present a low degree of risk. Ultimately, land managers who are charged with protecting the forests of these receiving countries will need to adapt this information to their own particular situations in order to prevent potentially dangerous pathogen introductions. Factors such as the extent and distribution of suitable conifer hosts, the characteristics of various existing avenues of introduction, and the effectiveness of existing quarantine regulations and infrastructure will need to be considered for each of the pathogens listed in this chapter.