Genetic differentiation between two subpopulations of Erysiphe necator in Italy

Erysiphe necator (syn. Uncinula necator) overwinters both as ascospores in cleistothecia and as mycelium in dormant buds of grapevines. Shoots developing from infected buds are known as ‘flag shoots’. We showed in a previous study that a flag shoot subpopulation of E. necator deviates from a strictly clonal or strictly randomly mating mode of reproduction. In this study we expand the investigation to a subpopulation overwintering strictly as ascospores with the objectives of: i) analyse its multilocus genetic structure, and ii) determine whether there is genetic differentiation between the two subpopulations. Finally, the two subpopulations, as defined by biological traits, were also compared using genetic markers developed to distinguish flag shoot and ascospore fungal biotypes, as defined in early studies. Two vineyards of Tuscany (central Italy), were intensively sampled for two years for flag shoots and for ascospore infections at the same time early in the epidemic. Isolates of the two mating types were found in 1:1 ratios in both subpopulations. Genotypic diversity, based on ISSR markers, was high in both years. Multilocus analysis of the population structure was not consistent with the hypothesis of random mating in either subpopulation. The two subpopulations were genetically differentiated within years based on ISSR markers. In the vineyard where the fungus overwinters as mycelium in dormant buds the subpopulation comprised both flag shoot and ascospore biotypes as shown by both sets of markers. By contrast, the subpopulation overwintering as ascospores comprised almost exclusively isolates of the ascospore biotype. These results indicate that the E. necator population in Tuscany is genetically divided in subpopulations in the two geographic areas considered. Moreover, E. necator may reproduce both clonally and sexually in both subpopulations, with restricted gene flow be-tween them. However, genetic differentiation did not correlate strictly with the overwintering strategies of the fungus because both biotypes were found within flag shoot isolates.