Genetic Structure of a Plasmopara viticola Population in an Isolated Italian Mountain Vineyard

In this paper we investigated the genetic population structure of Plasmopara viticola, the causal organism of downy mildew on grapevines (Vitis spp.). We selected an isolated and newly established mountain vineyard, where the disease was observed for the first time. As soon as disease symptoms appeared, they were collected (three samplings) and they were genetically analysed by means of four microsatellite markers. Our study revealed that a human-mediated allochthon oosporic import, rather than naturally immigrating secondary propagules, presumably provided the starting inoculum that initiated the very mild epidemic. Fifteen genotypes initiated the disease during three of the at least seven infection periods. Genotypes participating in the epidemic showed a very variable and strain-specific aptitude in generating secondary lesions. Only two genotypes multiplied asexually and generated some tens of secondary lesions, probably because of the unfavourable climatic conditions of the mountain region. The mountain P.viti cola population appeared with a poor gene pool, genetically separated and isolated- by-distance from the valley natural stocks as well as recently established by a founder effect event.