Existence of different physiological forms within genetically diverse strains of Ampelomyces quisqualis

Powdery mildew fungi are parasitized by strains of the genetically distinct Ampelomyces quisqualis. To investigate whether differences in the phylogeny and other cultural, morphological and physiological characteristics of these different strains are related to differences in their geographic origins or the host species from which they were isolated, several A. quisqualis strains isolated from different species of Erysiphaceae collected in different countries and possessing different ITS rDNA sequences were selected and characterized. The results revealed some significant variation among the selected strains, which provides evidence for the existence of different physiological forms within the A. quisqualis species. Two groups that display differential growth on artificial media were identified. These groups also differ in the morphology of their mycelium, but not in the morphology of their pycnidia and conidia. Temperature greatly affected the in vitro growth of the A. quisqualis strains and growth rate was closely correlated to colony color. Differences in the conidial germination of distinct strains were observed during the recognition phase of the parasitic relationship. The germination of each of the investigated strains was greatly stimulated by all of the examined powdery mildew species and not only by the conidia of their original hosts. An Italian strain isolated from grapevine in the Trentino Alto-Adige region was identified as the strain that germinates the most quickly in the presence of powdery mildew conidia. Phylogenetic analysis revealed that these A. quisqualis strains can be classified into five different genetic groups, which generally correlate with the fungal host of origin and morphological and growth characteristics.