Major-gene resistance to the rust pathogen Coleosporium ipomoeae is common in natural populations of Ipomoea purpurea.

The genetic basis of resistance to pathogens is well studied in crops, yet our understanding of the evolution of this trait in natural populations will be improved by determining how resistance is inherited in a wide range of plant-pathogen interactions. Here, we examined resistance to Coleosporium ipomoeae, a common fungal rust pathogen of Ipomoea purpurea. Natural populations across North Carolina, South Carolina, and Georgia (USA) were surveyed for the presence of C. ipomoeae and seeds were collected. A combination of crosses and controlled infections was then used to determine the genetic basis of qualitative resistance. In one population studied in detail, complete resistance to natural infection and a bulk collection of C. ipomoeae is conferred by a single locus (Rci1), where resistance is dominant to susceptibility. Allelic, major-gene resistance to this same bulk collection of C. ipomoeae appears to also occur in nine other natural populations. The prevalence of this resistance phenotype in natural populations suggests that the evolution of resistance to C. ipomoeae in I. purpurea may be dominated by genes of large phenotypic effect.