Hedera helix L. (Araliaceae) is a common ornamental species that is able to grow in shaded areas and is often used in parks and gardens. During the fall of 2006, severe outbreaks of a previously unknown powdery mildew were observed in several gardens in Liguria (northern Italy). Both surfaces of young leaves of affected plants were covered with dense, white mycelia and conidia. As the disease progressed, infected leaves turned yellow and dropped. Mycelia and conidia were also observed on young stems. Conidia were hyaline, cylindrical, borne singly, and measured 38 to 51 × 12 to 18 (average 42 × 16) μm. Single germ tubes, moderately long (average 26 μm), developed at the end of conidia. Appressoria of germ tubes and hyphae were lobed (three to four lobes). Conidiophores, 68 to 82 × 7 to 8 (average75 × 8) μm, showed foot cells measuring 39 to 60 × 7 to 8 (average 52 × 8) μm, followed by one shorter cell measuring 19 to 28 × 8 to 9 (average 23 × 9) μm. Fibrosin bodies were absent. Chasmothecia were numerous, spherical, amber-colored then brown at maturity, with diameters ranging from 97 to 140 (average 120) μm, containing four asci shortly stalked, 57 to 72 × 32 to 51 (average 65 × 41 μm). Ascospores were ellipsoid and measured 24 to 34 × 15 to 20 (average 30 × 17) μm. The internal transcribed spacer (ITS) region of rDNA was amplified using the primers ITS4/ITS6 and sequenced. BLASTn analysis (1) of the 613-bp fragment showed an E-value of 0.0 with Erysiphe heraclei. The nucleotide sequence has been assigned GenBank Accession No. EU 010381. In GenBank, our nucleotide sequence shows an E-value of 0.0 also with E. betae. However, the comparison of appressorium shape and germ tube length observed on our microorganism with those described for E. betae by Braun (2) suggests that the causal agent of the powdery mildew reported on ivy is E. heraclei. Furthermore, symptoms described on our host, appressorium shape and the length of conidiophores, are different from those of Oidium araliacearum described by Braun (2) on Araliaceae. Inoculations were made by gently pressing diseased leaves onto leaves of five healthy H. helix plants. Three noninoculated plants served as controls. Inoculated and noninoculated plants were maintained in a greenhouse at temperatures between 21 and 25°C. After 15 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 H. helix caused by E. heraclei in Italy. A powdery mildew caused by E. cichoracearum was previously reported on H. canariensis var. azorica in Italy (3), while a powdery mildew on H. helix caused by O. araliacearum and Golovinomyces orontii, respectively, were observed in the United States (4) and Germany. 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) U. Braun. A Monograph of the Erysiphaceae (Powdery Mildews). Cramer, Berlin, Germany, 1987. (3) C. Nali. Plant Dis. 83:198, 1999. (4) G. S. Saenz and S. T. Koike. Plant Dis. 82:127, 1998.
