Microsatellite analysis of the population structure in Phytophthora nicotianae

Nine validated SSRs were designed and used to characterize 268 isolates from a broad range of hosts and geographic localities. A total of 129 multilocus genotypes (MLG) were identified with markers showing polymorphisms ranging from 4 (locus P2039) to 24 alleles (locus 1509). Analyses revealed a preferential clonal reproduction in field orchards while sexual reproduction seemed to be more diffused in nurseries. A strong association between genetic groups and host of recovery was revealed for most isolates although this association was less evident for isolates from nurseries. In contrast, a significant geographical structuring was recovered only for isolates from tobacco (sourced in Australia and United States) and from Citrus maxima (sourced in Vietnam), while a typical panmictic distribution characterized the majority of isolates including those from other Citrus species. These difference were ascribed to the different propagation and cultivation systems. Isolates obtained from potted ornamental and citrus (excepted pommelo) were likely to be diffused worldwide with infected plant material (mainly potted plants). Conversely, tobacco is propagated by seeds which do not contribute to the spread of the pathogen and plantlets are very rarely transplanted in areas different from those in which have been produced. As regards to C. maxima, this species is a native plant of Vietnam and plant material was not introduced from other countries suggesting a specific co-evolution of P. nicotianae and C. maxima.