Plant resistance to infection by soil fungal pathogens can be influenced by a wide range of genetic and environmental factors. We examined the role that intraspecific neighbors may play in the relative resistance of Allium vineale plants to Sclerotium cepivorum, a white rot fungus specific to the Allium genus that can travel laterally in the soil from plant to plant. We used asexual propagules from plant genotypes known to have different levels of resistance to infection by the fungus, and our planting design incorporated them both as focal individuals and as neighbors. Our results demonstrate that the relative resistance of an individual plant is strongly affect by the relative resistance or susceptibility of its neighbors, with genotypes that are resistant to the pathogen becoming susceptible when planted with susceptible neighbors and vice versa. In addition, the most susceptible genotype also had the highest performance in the absence of the pathogen. As competitive interactions increased over the course of the experiment, the relative importance of neighbor genotype on resistance to the pathogen declined, although the overall patterns did not change. This work indicates that the presence of intraspecific neighbors may substantially alter the susceptibility of individual plants to soil pathogens. Previous work has demonstrated that the effect of mycorrhizal fungi varies with the genotype of neighboring A. vineale plants, and that mycorrhizae can influence plant-pathogen interactions, emphasizing the overall importance of soil fungi in structuring A. vineale populations.
