Spatial and genetic analysis of a flag shoot subpopulation of Erysiphe necator in italy

Erysiphe necator overwinters both as ascospores in cleistothecia and as mycelium in dormant buds of grapevines. Shoots developing from infected buds early in the growing season are covered with dense mycelium and are known as “flag shoots”. Combining epidemiological and Cortesi et al. Phytopathology, page 2 genetic analyses, the objective of this study was to analyze the spatial and genetic structure of a flag shoot subpopulation of E. necator, as a way to assess the contribution of flag shoots as primary inoculum and to determine if the flag shoot subpopulations are clonal, with only one mating type. One vineyard in Tuscany, Italy, was surveyed intensively for flag shoots for eight years; isolations of E. necator were made from flag shoots for five years. We observed distinct disease foci developing around flag shoots early in epidemics, demonstrating a steep dispersal gradient of conidia and the importance of flag shoots as primary inoculum sources. Flag shoots were spatially aggregated within and between years, most likely as a result of short-distance dispersal of conidia from flags early in the season when dormant buds for the next year’s shoots are formed and are susceptible to infection. The two mating types were found in 1:1 ratios in this flag shoot subpopulations. Genotypic diversity, based on ISSR markers, was high in all years, with only two haplotypes occurring twice, and subpopulations were genetically differentiated between years. Similarities between haplotypes were not spatially autocorrelated. One multilocus analysis of population structure is consistent with the hypothesis of random mating, but another is not. These results are not consistent with expectations for either a strictly clonal or strictly randomly mating flag shoot subpopulation. Instead, the hypothesis that the flag shoot subpopulation of E. necator may reproduce both clonally and sexually needs further testing.