Past forest management promoted root disease in Yosemite Valley

Root disease is one of the most important vegetation-management considerations in Yosemite Valley. Large trees with root decay have fallen in the valley causing human fatalities and property damage. Many of the problems associated with root disease in Yosemite Valley can be traced back to the area's history of vegetation management. Wildfire suppression and meadow draining were implemented after the arrival of Euroamericans in the mid-19th century. These practices created conditions that encouraged the development of a dense conifer forest within the valley. Tree removals for vista clearance, campground and lodging construction, and bark beetle control projects created thousands of stumps. Many of these stumps have been infected with spores of Heter-obasidion annosum, a fungal pathogen that causes root decay in conifers. The fungus has since spread from initial infection sites into the surrounding forest, creating hundreds of enlarging tree mortality gaps. Park resource managers have established a program of hazardous-tree removal, but efforts to restore natural ecosystem processes must be continuously reconciled with public safety

Genetic Structure of Heterobasidion annosum in White Fir Mortality Centers in California

The structure of Heterobasidion annosum populations was studied in 1.5 mixed-conifer sites in central and northern California. Study sites displayed mortality of white fir trees in enlarging discrete patches (mortality centers). At each site, fungal genotypes were defined by somatic compatibility tests. In two sites, further genetic and molecular analyses were performed on field genotypes and on homokaryons obtained by dedikaryotization of field heterokaryons. Isolates were found to be colonizing mostly the roots and the bole sapwood of white fir trees, and no significant infections of other tree species were observed. Each mortality center was characterized by the presence of several fungal genotypes, all belonging to the S intersterility group. Both homokaryotic and heterokaryotic strains were present in all sites. Multiple genotypes were retrieved in individual trees or stumps. Out of 228 fungal genotypes, 86 % were found only within a single tree or stump, while 14 % had spread to adjacent trees. The two largest genotypes had diameters of 9 and 10 m, and had colonized fiv