Archivio Journal of Plant Pathology

Abstract:

During summer and the following autumn 2015, about a hundred plants of Phlox paniculata growing in a garden near Biella (northern Italy) showed symptoms and signs of an unknown powdery mildew. Leaves, stems and inflorescences were covered by a white mycelium that produced hyaline, elliptical conidia measuring 28-35×16-21 (mean: 31×18) μm. Conidia germinated apically with short, rather clavate germ tubes. Fibrosin bodies were absent. Many chasmothecia, 100-162 (mean: 130) μm in size, formed dark patches on all the affected tissues, particularly on the upper leaf surface. Chasmothecia contained 8-15 shortly stalked, 2- spored asci measuring 44-85×24-40 (mean: 62×29) μm. Spores were ellipsoid to subglobose and measured 21-30×14-21 (mean: 25×18) μm. The ITS region of rDNA extracted from fruiting bodies was amplified using the primers ITS1/ITS4 (Altschul et al., 1997) and sequenced (GenBank accession No. KT953357). The 510 bp amplicon had 99% homology with the sequence of Golovinomyces magnicellulatus (AB769441.1), confirming the relationship between G. magnicellulatus and P. paniculata as recently reported in the phylogenetic analysis of the genus Golovinomyces (Takamatsu et al., 2013). In pathogenicity tests, leaves affected by powdery mildew were gently pressed onto three healthy plants of P. paniculata, which were then were maintained at temperatures ranging from 20 to 26°C. Three non-inoculated plants were used as controls. Fifteen days post inoculation, the first symptoms appeared only on inoculated plants. G. magnicellulatus has been reported on P. paniculata in Great Britain (Jones and Baker, 2007). This is the first report of G. magnicellulatus on P. paniculata in Italy.