Ecological genetic interactions between a clonal host plant (Spartina pectinata) and associated rust fungi (Puccinia seymouriana and Puccinia sparganioides)

The spatial scale of genetic diversity among patches of a host plant could affect the likelihood of pathogen adaptation to the host. If host patches are genetically distinct, pathogen adaptation to local host genotypes may occur. To study this issue, we focused on the ecological and genetic interactions between two rust fungi, Puccinia seymouriana and P. sparganioides, and the clonal prairie grass, Spartina pectinata. In a field transplant experiment, disease levels differed among plants from different patches, suggesting variation in resistance. Over a 4.5-km scale, disease levels were not higher on plants transplanted back into their source patch as opposed to other locations, providing no evidence for local adaptation in the pathogen. However, on the spatial scales examined (ranging from 0.2 km to 120 km), there was no relationship between the physical distance separating host patches and their similarity in isozyme banding patterns, implying that plants from more distant patches are not necessarily more genetically distinct than plants from nearby patches. Plants derived from the most distant location had, on average, the lowest mean number of pustules at the end of the summer, suggesting the need for reciprocal transplant studies to be performed on a larger spatial scale.