Incidence, host relations and population structure of Armillaria ostoyae in Colorado campgrounds

Armillaria root disease is common and widely distributed in campgrounds of southwestern Colorado. Armillaria ostoyae spreads clonally underground and kills and decays tree roots, causing mortality or predisposing the trees to windthrow. We intensively surveyed and mapped genets (clones) of the pathogen in two campgrounds on the San Juan National Forest and one on Grand Mesa National Forest (GMNF). Three additional campgrounds on the GMNF were also surveyed. Infection (based on mycelium under the bark on or near the root collar) of all sampled live trees was 10.5% (range 7.5–15.0) inside campgrounds and 12.7% (3.3–25.9) immediately outside campgrounds, suggesting that campground construction and management practices have not exacerbated the disease. Dominant trees had significantly greater incidence of infection than trees in other crown classes. Isolates of A. ostoyae were obtained from 379 trees, and genets were identified by somatic incompatibility tests and variation in DNA microsatellite markers. The pathogen occurred as one large genet in the spruce-fir campground and several large genets in each of the mixed conifer campgrounds. Based on the size and distribution of the genets, the campgrounds appeared to be almost completely colonized by mosaics of centuries old genets, and little expansion of genet territories (or disease centers) appeared possible without loss of territory by another genet. The diseased trees in the campgrounds were randomly distributed rather than being organized into discrete disease centers. Live subalpine fir had significantly lower incidence of infection (7.0%) than Engelmann spruce, blue spruce and Douglas-fir (12.0, 12.3 and 15.7%, respectively), but evidence suggests a higher rate of mortality in subalpine fir. Crown thinning and dieback were useful symptoms for detection, but basal resinosis was the most efficient symptom indicating infection. Use of a combination of aboveground symptoms to select trees for more intensive examination is the most efficient approach to detection of infected trees. The disease poses difficult obstacles to long-term management of safe vegetation in developed sites.