Rhododendron species cultivation has a long history in northern Italy, where a wide selection of cultivars and hybrids is grown. In spring 2001, a previously unknown bud blast was observed on several rhododendron cultivars growing in gardens and parks in the Province of Biella, Italy. Flower petioles and twigs, but not leaves, showed extensive necrosis, and flower production was reduced. The first symptoms on infected flower buds are brown necrotic areas near the base, which increase in size until the whole bud appears brown and water-soaked. Infected buds shrink, but do not rot or disintegrate, and remain on the bush. Coremia (2.0 mm high and 0.5 mm wide), which appeared as black specks on outer bud scales, produced large numbers of spherical spores apically that measured 3.6 to 5.5 μm in diameter. The fungus Pycnostysanus azaleae (1) was consistently isolated after disinfested infected buds (1 min in 5.15% commercial NaOCl solution) were cultured on potato dextrose agar amended with streptomycin sulfate at 100 mg/liter. Pathogenicity of three isolates of P. azaleae was confirmed by inoculating with a spore suspension 30 buds of 2 rhododendron plants (cv. Rosso scuro grande tardiva) grown in containers (14 cm diameter). Buds were punctured with a sterile needle before inoculation. Noninoculated plants served as controls. Wounded buds were covered with plastic bags to maintain high relative humidity, and all plants were maintained in growth chambers at 20°C for 9 days, and then transferred outdoors where temperatures were 20 to 25°C. Inoculated plants developed typical bud symptoms 20 days after inoculation, with 80% of inoculated buds showing symptoms. Control plants remained symptomless. The pathogen was consistently reisolated from artificially inoculated plants. In garden observations, different cultivars showed a wide range of susceptibility to the pathogen, mainly related to their flowering period. To our knowledge, this is the first report of the presence of P. azaleae in Italy. Outbreaks of P. azaleae were previously reported in the United States (1), United Kingdom (2), and Germany (3).
References: (1) W. H. Davis. Phytopathology 29:517, 1939. (2) P. J. Howell and R. K. S. Wood. Ann. Appl. Biol. 50:723, 1962. (3) W. Schmalscheidt. Immergrune Blater 26:35, 1985.