Powdery mildew of Michaelmas daisy in Italy.

Abstract

Michaelmas daisy (Aster novi-belgii), family Asteraceae, is a herbaceous plant used in low maintenance gardens and for cut flower production. During summer 2011, three-year-old plants grown in a garden near Biella (northern Italy) showed symptoms of an unknown powdery mildew. Both leaf surfaces were covered with white mycelium that was also observed on stems and flower calyxes. Conidiophores were erect, with a cylindrical foot cell measuring 45-85×10-12 μm (average: 64×11 μm) followed by 1-2 shorter cells, measuring 11-36×6-16 μm (average: 22×11 μm). Conidia were hyaline, elliptical, borne in short chains and meas- ured 31-43×16-23 μm (average: 37×19 μm). Fibrosin bodies and chasmothecia were not observed. The internal transcribed spacer (ITS) region of rDNA was amplified using the primers ITS1/ ITS4 and sequenced (Altschul et al., 1997) (GenBank accession No. JQ015394). The 558-bp amplicon had 95% homology with the sequence of Golovinomyces cichoracearum (AB077627). Path- ogenicity was confirmed by gently pressing diseased leaves onto leaves of five healthy A. novi-belgii plants. Five non-inoculated plants served as control. Plants were maintained at temperatures from 15 to 25°C. Ten days post inoculation, symptoms of pow- dery mildew developed only on inoculated plants. G. ci- choracearum (syn. E. cichoracearum) has been reported on A. novi-belgii from several countries, among which the USA (Farr et al., 1989) and Denmark (Mork et al., 2011). To the best of our knowledge, this is the first report of G. cichoracearum affecting A. novi-belgii in Italy.