Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/11/2001
Publication Date: 9/22/2001
Interpretive Summary: Armillaria root disease (causal fungal agent, Armillaria mellea) affects native forest trees, orchard trees, landscape trees, and grapevines in California. We determined the impact of Armillaria root disease on native trees in coastal mixed-hardwood forests. Four 3000 m2 plots, consisting of black oak, coast live oak, tanoak, madrone, laurel, and Douglas-fir (a total of 404 trees), were intensively sampled for signs of Armillaria species. We identified two Armillaria species, Armillaria mellea and Armillaria gallica, from tree roots. The most abundant tree species in each plot was the tree species most frequently infected by Armillaria mellea or Armillaria gallica. Both species were equally common. Based on the pattern of infection in the roots of sampled trees, Armillaria mellea appears to be a more aggressive pathogen than Armillaria gallica. Even though Armillaria mellea appears to be an aggressive pathogen on forest trees, only one plot, ,where past logging likely exacerbated the disease, was severely impacted b this root pathogen.
Technical Abstract: Incidence of Armillaria root disease and the population structure of associated Armillaria species was studied in California mixed-hardwood forests. Four 3000 m2 plots, consisting of Quercus kelloggii, Q. agrifolia, Lithocarpus densiflorus, Arbutus menziesii, Umbellularia californica, and Pseudotsuga menziesii, were intensively sampled. Root collars were examined for mycelial fans, decayed wood, and rhizomorphs. From 404 trees (333 living, 71 dead), A. 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. mellea isolates were recovered from mycelial fans on living trees. Recent mortality due to A. mellea was only found in one plot. Armillaria gallica was mainly identified from epiphytic rhizomorphs. Only one to three somatic incompatibility groups (SIGs) for each species were found in each plot. Estimated sizes of SIGs varied from the extent of a single root system to the entire width of the plot. Based on our results, both A. mellea and A. gallica are common in mixed-hardwood forests, yet A. mellea appears to be a more aggressive pathogen. Armillaria gallica is capable of attacking live hosts, but occurs on living roots more frequently as epiphytic rhizomorphs.