Powdery Mildew Caused by Golovinomyces biocellatus on Spearmint (Mentha spicata) in Italy.

Spearmint (Mentha spicata) is grown for its aromatic and carminative oil and as an ornamental. During the fall of 2009, 4-month-old plants grown on a commercial farm located near Albenga (northern Italy) showed signs and symptoms of an unknown powdery mildew. The adaxial leaf surfaces were covered with white mycelia and conidia, while the abaxial surfaces were less infected. As the disease progressed, infected leaves turned yellow and wilted. Mycelia were also observed on stems. Conidia were hyaline, elliptical to doliform, borne in short chains (two to three conidia per chain), and measured 35 × 21 μm (30 to 43 × 18 to 26 μm). Conidiophores measured 86 × 11 μm (76 to 97 × 9 to 13 μm) followed by one to three shorter cells, measuring, respectively, 22 × 11 μm (13 to 28 × 9 to 15 μm). Fibrosin bodies were absent. Chasmothecia were not observed in the collected samples. The internal transcribed spacer (ITS) region of rDNA was amplified using the primers ITS4/ITS6 and sequenced (1) (GenBank Accession No. HM053470). The 567-bp amplicon had 100% homology with the sequence of Golovinomyces biocellatus (GenBank Accession No. AB307675). Pathogenicity was confirmed through inoculation by gently pressing diseased leaves onto leaves of healthy M. spicata plants. Three plants were inoculated, while the same number of noninoculated plants served as a control. Plants were maintained at temperatures from 18 to 25°C. Twelve days after inoculation, typical symptoms of powdery mildew developed on inoculated plants. The fungus observed on inoculated plants was morphologically identical to that originally observed. Noninoculated plants did not show symptoms. The pathogenicity test was carried out twice. G. biocellatus has been reported on Mentha spp. in Australia (3) and the United States (4) and the pathogen is present on other plant genera in Italy. A similar powdery mildew of M. spicata was attributed to Erysiphe orontii in the United States (2). The economic importance of this disease is currently limited in Italy because of the limited planting of this host, but potted aromatic plants represent a steadily increasing crop in Italy. Voucher specimens are available at the Agroinnova Collection, University of Torino. References: (1) S. F. Altschul et al. Nucleic Acids Res. 25:3389, 1997. (2) S. T. Koike and G. S. Saenz. Plant Dis. 83:399, 1999. (3) J. R. Liberato and J. H. Cunnington. Australas. Plant Dis. Notes 2:83, 2007. (4) D. B. Marcum et al. Plant Dis. 94:276, 2010.