Plant species can have ecological impacts on co-occurring species by altering their resistance to natural enemies. Associational resistance occurs when one species reduces enemy damage to neighboring species, whereas associational susceptibility increases enemy damage to neighboring species. In a previous study, Ipomoea tricolor 'Pearly Gates' plants, endosymbiotic with alkaloid-producing Periglandula fungi developed fewer nematode galls and produced less biomass than non-endosymbiotic plants. To explore whether endosymbiont-mediated resistance could extend to neighboring species, we grew endosymbiotic or non-endosymbiotic I. tricolor with corn (Zea mays) in soil inoculated with Southern root-knot nematodes (Meloidogyne incognita) or no inoculation controls. Both nematode and endosymbiont treatments reduced total plant biomass per pot, but corn produced significantly more biomass in the nematode addition treatment when morning glory was endosymbiotic, consistent with associational resistance. These results suggest that the Periglandula endosymbiont of I. tricolor can enhance the growth of co-occurring plants in the presence of natural enemies.
