First report of Phytophthora siskiyouensis causing disease on Italian alder in Foster City, California

Phytophthora species cause cankers on the stems of many forest and landscape trees. In November of 2006, Italian alder trees, Alnus cordata, were reported to be dying with symptoms of bleeding cankers located at the base of the stem. The trees were located in a business development outside of a library in Foster City, California. Several of the trees had already been removed as hazardous. Successful isolations were made at the leading edge of the canker from the wood cambium interface onto PARP selective medium. A homothallic Phytophthora with primarily paragynous antheridia grew out in the media. The sporangia, produced easily on carrot agar plugs in soil water were ovoid to ellipsoid in shape. Oospores were mostly globose and aplerotic. The intergenic transcribed spacer region of rDNA of the oomycete matched with 100% identity to Phytophthora siskiyouensis, a pathogen associated with tan oak and also found in the soil and water in coastal Oregon. Pathogenicity experiments were conducted on Italian, red and white alder. This Phytophthora may be endemic to California. Foster City shares a marine- influenced climate with coastal Oregon.