In June 2009, dieback of distal branches and resin exudation associated with bark lesions were observed in an adult Cupressus macrocarpa tree in Sonoma County, California (Glenn Ellen; 38°21′N, 122°31′W, elevation 233 m). The fungal pathogen, Seiridium unicorne (Cooke and Ellis) Sutton, was obtained by plating fragments of necrotic bark from the margins of branch cankers on potato dextrose agar (PDA). Identification was based on cultural, morphological, and molecular traits (2,3). Colonies on PDA were dense, cottony, off-white at first and then turning pale gray-green, and 2.3 and 4.3 cm in diameter after 1 and 2 weeks of growth at 20°C, respectively. Colonies of the fungus showed a faster radial growth at 20°C than at 25°C. Acervuli were abundantly produced on water agar amended with autoclaved cypress seeds after 2 to 3 weeks at 18°C under a mixture of fluorescent and near UV light. Conidia were six celled (five euseptate), fusiform, 20.9 to 35.2 × 7.11 to 10.57 μm, straight or slightly curved, with four, brown median cells, and with end cells bearing unbranched appendages 2 to 5 μm long. The DNA sequence of a portion of the β-tubulin locus (GenBank Accession No. HQ678171) revealed a 100% homology with sequences of S. unicorne isolates from Portugal and South Africa, while being clearly distinct from sequences of S. cupressi and S. cardinale isolates (2). Greenhouse stem inoculations were performed by underbark placement of a 3-mm plug taken from the margins of a colony of the fungus grown on PDA. Inoculations were repeated twice in the spring and fall of 2010 on 10 C. macrocarpa saplings grown in pots for 3 years. Three months postinoculation, the pathogen could be successfully reisolated from the edges of 15 to 30 mm long elliptical lesions, present on each one of the inoculated saplings. The observed S. unicorne isolate is atypical because of its shorter appendages compared with the form reported in the literature (2,3). Because of its shorter conidial appendages and in vitro temperature optimum of 18 to 20°C, the fungus described here is similar to an unnamed Coryneum sp. observed by Wagener on C. macrocarpa (4). S. unicorne is a pathogen of many Cupressaceae in Africa, New Zealand, Japan, and some U.S. states (Georgia, South Carolina, Kansas, and Texas) (3), and although it was mentioned in a USDA Plant Quarantine Division report from 1963 as found on cypress in San Francisco (1), it has never been officially reported from California. Since similar disease symptoms were observed on many Cupressaceae in the course of an extensive survey performed in 2009 in California, it may be important to evaluate the relative incidence of S. unicorne compared with that of S. cardinale, a pathogen more commonly reported in association with the disease (4). References: (1) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/fungushost/fungushost.cfm , 1/19/2011. (2) P. Krokene et al. Mycologia 96:1352, 2004. (3) N. A. Tisserat. Plant Dis. 75:138, 1991. (4) W. W. Wagener. J. Agric. Res. 58:1, 1939.