Spatial population structure in an obligate plant pathogen colonizing oak Quercus robur

1. The spatial population structure of parasites may have profound effects on both ecological and evolutionary dynamics. Strong gene flow among local populations has been hypothesized to prevent differential performance on local and foreign hosts. 2. We studied the spatial population structure of an obligate pathogen, the oak mildew (Microsphaera alphitoides) on the pedunculate oak (Quercus robur). Patterns of distribution and colonization-extinction dynamics were recorded in a set of 113 trees over 3 years, and in an additional set of 77 trees over 2 years. Colonization of uninfected hosts was examined in a set of 100 experimentally transplanted hosts, and differential performance on local and foreign hosts tested by reciprocal inoculations conducted in the laboratory. 3. Across years, the mildew occupied a high proportion of host trees ( >= 2/3). Patterns of mildew infection showed either no spatial aggregation (years 2003, 2005), or aggregation at a large spatial scale (up to 400 m; year 2004). High and even incidence of infection reflects efficient dispersal: during a single year, transplanted uninfected host plants were rapidly colonized in all parts of the landscape. 4. Local mildew populations still performed best on their original hosts: in reciprocal inoculations conducted on mature foliage, local mildew populations infected a significantly higher proportion of leaves from their original host tree than from other trees. Yet, even on the local host less than half of the leaves were infected. 5. To account for the observed patterns, two selection processes are invoked. The colonization experiment suggests that mildew spores are spread widely, with selection filtering out the genotypes that cannot infect the focal host. Local mildew populations are subject to further selection events later in the summer, favouring the strains best adapted to phenological changes in the local environment. 6. In conclusion, the patterns observed and processes inferred in M. alphitoides suggest that complex selection pressures may affect local parasite populations, blurring any clear-cut relation between gene flow and performance on local and foreign hosts.