First Report of Powdery Mildew Caused by Golovinomyces orontii on Campanula glomerata in Italy.

Abstract

Clustered bellflower (Campanula glomerata), Campanulaceae family, is an herbaceous perennial plant belonging to the native flora in Italy, and it is appreciated for the production of blue inflorescences. During the winter of 2018, about 20% of 4-month-old plants of this spontaneous species growing in a greenhouse of the Centre of Competence Agroinnova, University of Torino, located in Grugliasco (Torino province, northern Italy) showed symptoms and signs of a powdery mildew. A thin, white mycelium covered leaves, in particular the adaxial surface and petioles, causing necrosis on the affected areas within a month. As the disease progressed, the mycelium density became thicker along the primary veining and resulted in leaf curved distortion. Hyphae produced erect conidiophores with cylindrical, sometimes curved at the base, foot cells measuring 44 to 106 × 9 to 12 µm (average: 81 × 10 µm). Foot cells were followed by one to three shorter cells, measuring 11 to 28 × 8 to 13 µm (average: 17 × 10 µm). Hyaline, elliptical conidia were in chains (up to five conidia per chain) and measured 26 to 41 × 13 to 22 µm (average: 31 × 17 µm) (n = 50). They were lacking fibrosin bodies and germinated from an end. The perfect stage of the microorganism was not observed. The DNA of the pathogen was extracted from mycelium, conidiophores, and conidia collected from affected leaves using the E.Z.N.A. Fungal DNA Mini Kit (Omega Bio-Tek, Darmstadt, Germany). A polymerase chain reaction was performed using either primers ITS1 and ITS4 (White et al. 1990) or ITS1 and PM6 (Takamatsu and Kano 2001), to amplify the internal transcribed spacer region of rDNA. The NCBI blast analysis of 513- and 446-bp sequences (GenBank accession nos. MH079551 and MH079552, respectively) showed a 100% identity with Golovinomyces orontii (AB769464 and AB769465 from Cymbalaria muralis and Campanula rapunculoides, respectively). In the pathogenicity test, some leaves affected by the pathogen were gently pressed onto leaves of three C. glomerata 2-month-old healthy plants. Inoculated plants were maintained in a greenhouse, at temperatures ranging from 20 to 26°C. Three healthy noninoculated plants were grown in the same greenhouse and served as controls. Symptoms and signs of powdery mildew appeared on inoculated plants 11 days after the artificial inoculation, whereas controls remained healthy. The pathogenicity test was carried out twice with the same result. G. orontii has been reported on C. glomerata in Finland, Norway, and the USSR, and on Campanula spp. in several European countries (Braun 1995). In Italy, G. orontii has been identified on C. rapunculoides (Garibaldi et al. 2012), whereas this is the first report of this pathogen on C. glomerata. The economic importance of this disease is at present limited in Italy, but it could spread to several Campanula spp., because the cultivation of these species is increasing among the ornamentals.