To determine the identity and distribution of Armillaria species in California, we collected 589 isolates from managed areas (urban landscapes, orchards, and vineyards) and 20 natural forest types. From our collection of isolates, we identified A. mellea, A. gallica, A. nabsnona, and North American biological species X (NABS X). State-wide ranges of each species were estimated based on distributions of forest types from which they were collected. Only A. mellea and A. gallica are widely-distributed in California; A. nabsnona and NABS X appear to have restricted ranges. Armillaria mellea is common in most regions of the state dominated by development, such as the Central Valley, the San Francisco Bay area, and Los Angeles. It was the only species, with the exception of a single isolate of NABS X, isolated from symptomatic hosts, including both forest trees and planted hosts. Armillaria mellea does cause mortality in California forests, but large disease centers are rare. The distribution of A. gallica in California includes the greatest variety of forest types of all Armillaria species identified from our collection. Although it shares many forest types in common with A. mellea, A. gallica is extremely rare in developed areas. The range of A. nabsnona in California appears to be restricted to northern redwood forests. NABS X was only identified in high elevation red and white fir forests of the southern Cascades.
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