Ecology of Armillaria spp. in mixed-hardwood forests of California

Incidence of Armillaria root disease and the population structure of associ ated Armillaria spp. was studied in California mixed-hardwood forests. Four 3,000-m(2) plots, consisting of Quercus kelloggii, Q. agrifolia, Lithocarp us densiflorus, Arbutus menziesii, Umbellularia californica, and Pseudotsug a menziesii, were intensively sampled. Root collars were examined for mycel ial fans, decayed wood, and rhizomorphs. From 404 trees (333 living, 71 dea d), Armillaria mellea and A. gallica were recovered and identified from 148 isolates. The most abundant tree species in each plot was the tree species most frequently infected by A. mellea or A. gallica. The majority of A. me llea isolates were recovered from mycelial fans on living trees. Recent mor tality due to A. mellea was found only in one plot. A. gallica was mainly i dentified from epiphytic rhizomorphs. Only one to three somatic incompatibi lity groups (SIGs) for each species were found in each plot. Estimated size s of SIGs varied from the extent of a single root system to the entire widt h of the plot. Based on our results, both A. mellea and A. gallica are comm on in mixed-hardwood forests, yet A. mellea appears to be a more aggressive pathogen. A. gallica is capable of attacking live hosts, but occurs on liv ing roots more frequently as epiphytic rhizomorphs.