Technical Abstract: High densities of planted hosts and frequent irrigation have contributed to severe Armillaria root disease in Golden Gate Park, San Francisco, CA. Our objective was to assess the relative contribution of vegetative growth and basidiospore dispersal to the colonization of the park by Armillaria mellea. We investigated the genetic structure of A. mellea at a fine spatial scale using microsatellite data. Ninety-five unique multilocus genotypes were identified among 166 isolates. Only 28 genotypes (29%) were shared by two or more isolates (clones). The largest two clones, resulting from vegetative growth of one genotype, measured 216 m and 322 m. Spatial autocorrelograms of kinship coefficients, with and without clones, converged at an average distance of 130 m, indicating that this distance constitutes the linear spatial dimension above which clonality does not affect the genetic structure of the population. Up to 100 m, genetic similarity between pairs of isolates decreased linearly with an increase in spatial distance. Beyond 100 m, a random spatial distribution of genotypes was observed, consistent with an establishment from sexual spores from distant sources. The absence of multilocus linkage disequilibrium and the high proportions of genotypes detected only once suggest that most infections in the park resulted from basidiospores. However, 29% of genotypes infected multiple trees as a result of subterranean, vegetative growth.