Lamium galeobdolon L. (Labiatae) is a common ornamental species that grows in shade areas and often used as a ground cover in gardens. During the summer of 2006, severe outbreaks of a previously unknown powdery mildew were observed on all Lamium spp. plants in some gardens near Biella (northern Italy). Both surfaces of the leaves of affected plants were covered with dense, white mycelia and conidia. As the disease progressed, infected leaves turned yellow and died. Mycelia and conidia also were observed on stems and flowers. Conidia were hyaline, ellipsoid, borne in short chains (with a maximum of five conidia per chain), and measured 29 to 37 × 16 to 20 μm (average 33 × 18 μm). Conidiophores, 91 to 104 μm (average 96 μm) long, showed the foot cell measuring 28 to 49 × 9 to 11 μm (average 38 × 10 μm), followed by three shorter cells measuring 14 to 26 × 9 to 15 μm (average 21 × 11 μm). Fibrosin bodies were absent. Chasmothecia were not observed in the collected samples. The internal transcribed spacer region (ITS) of rDNA was amplified using the primers ITS4/ITS6 (4) and sequenced. BLASTn analysis (1) of the 436 bp obtained showed an E-value of 0.0 with Golovinomyces orontii (Erysiphe orontii.) (3). The nucleotide sequence has been assigned GenBank Accession No. EF 121871. Inoculations were made by gently pressing diseased leaves onto leaves of five healthy L. galeobdolon plants. Five noninoculated plants served as controls. Inoculated and noninoculated plants were maintained in a greenhouse at temperatures between 15 and 28°C. After 10 days, typical powdery mildew colonies developed on inoculated plants. Noninoculated plants did not show symptoms. The pathogenicity test was carried out twice. To our knowledge, this is the first report of the presence of powdery mildew on L. galeobdolon caused by G. orontii in Italy. Blumer (2) was able to reproduce powdery mildew symptoms on L. galeobdolon using populations from cucumber, while Braun (3) reported L. galeobdolon as a possible host of E. orontii. Herbarium specimens of this disease are available at AGROINNOVA Collection, University of Torino, Italy.
References: (1) S. F. Altschul et al. Nucleic Acids Res. 25:3389, 1997. (2) S. Blumer. Ber. Schweiz. Bot. Ges. 62:384, 1952. (3) U. Braun. A Monograph of the Erysiphaceae (Powdery Mildews). Cramer, Berlin, GDR, 1987. (4) D. E. L. Cooke and J. M. Duncan. Mycol. Res. 101:667, 1997.
