Plant species are attacked by a multitude of insect herbivores and pathogens, and develop resistance against these attackers. If resistance improves plant fitness in the presence of the attackers, then plant populations should evolve to maximal levels of resistance, and in the process lose genetic variation for those traits. Nevertheless, numerous studies have shown significant genetic variation in resistance in natural plant populations. We predict that spatial structure of plant genotypes can influence attackers' colonization on plants, which may feed back to the maintenance of genotypic variation in a plant population.

To examine the prediction, we conducted common garden experiments, manipulating genotypic diversity of the tall goldenrod Solidago altissima. Then we estimated additive and non-additive effects of genotypic diversity on abundance of Uroleucon aphids and infestation of Coleosporium fungi, respectively.

The synergistic effects of genotypic diversity on both aphid abundance and fungal infestation were detected. Our results suggest dispersal of aphids and fungi would contribute to the synergistic effects, and spatial structure of plant genotypes contribute to the maintenance of genotypic variation in resistance.