First Report of Leaf Spot Caused by a Phoma sp. on Clematis × jackmanii in Italy

The genus Clematis, belonging to the Ranunculaceae family, is widely used in gardens and is very much appreciated for its climbing attitude as well as rich flower production. In the fall of 2006, in a private garden located near Biella (northern Italy), a severe foliar disease was observed on 2-year-old plants of Clematis × jackmanii. Small necrotic spots were observed on the upper and lower sides of infected leaves. At temperatures of 15 to 25°C, spots enlarged to form round areas that were 2 to 7 cm in diameter and well defined by a brown margin. Severely infected leaves wilted without abscising. The disease occurred on 100% of the plants of the C. × jackmanii hybrid in one garden. Stems and flowers were not affected by the disease. From infected leaves, a fungus was consistently isolated on potato dextrose agar (PDA) amended with 25 mg/liter of streptomycin. The fungus was grown on PDA and maintained at 22°C (12-h light, 12-h dark). After 10 days, black pycnidia 132 to 340 μm in diameter developed, releasing abundant hyaline, elliptical, nonseptate, conidia measuring 5.1 to 8.3 (6.8) × 1.6 to 3.4 (2.7) μm. On the basis of its morphological characteristics, the fungus was identified as a Phoma sp. (2). The internal transcribed spacer region of rDNA was amplified using primers ITS4/ITS6 (1,3), sequenced (GenBank Accession No. EF566917), and identified as a Phoma sp. Pathogenicity tests were performed by spraying leaves of healthy 1-year-old potted C. × jackmanii (cvs. Superba, Mrs N. Thomson, and Vagebond) plants with a spore and mycelial suspension (4 × 10⁵ spores or mycelial fragments per ml). Noninoculated plants served as controls. Five plants per cultivar were used for each treatment. Plants were covered with plastic bags for 3 days after inoculation and kept in a growth chamber at 18 to 20°C. Symptoms previously described developed on leaves of all tested cultivars 10 days after inoculation, while control plants remained healthy. On the infected leaves, pycnidia and conidia with the same dimensions and characteristics as previously described were observed. The fungus was consistently reisolated from the lesions of the inoculated plants. The pathogenicity test was carried out twice. The presence of Ascochyta clematidina, then renamed as Phoma clematidina, on Clematis species has been reported in the United States (4) and subsequently in the Netherlands, Britain, and New Zealand. References: (1) S. F. Altschud et al. Nucleic Acids Res. 25:3389, 1997. (2) G. H. Boerema and G. J. Bollen. Persoonia 8:111, 1975. (3) D. E. L. Cooke and J. M. Duncan. Mycol. Res. 101:667, 1997. (4) W. O. Gloyer. J. Agric. Res. 4:331, 1915.