Flowering dogwood (Cornus florida L., Cornaceae), is a small deciduous tree whose showy inflorescences, clusters of bright red fruits and red and purple leaves in autumn, make it a much appreciated ornamental. In June of 2008, severe outbreaks of a previously unknown blight were observed in several private gardens near Biella (northern Italy) after a rainy spring with temperatures that ranged from 7 to 25°C. Dogwoods in the gardens were 10 to 15 years old, and the disease was observed on 20 to 30% of 30 trees. First symptoms consisted of blighted leaves and then shoot dieback. As the disease progressed, entire leaves became necrotic and were covered by an abundant, soft, gray, sporulating mycelium. Tissue fragments of 1 mm2 were excised from the margins of the lesions, immersed in a solution containing 1% sodium hypochlorite, plated on potato dextrose agar (PDA) medium, and incubated under constant fluorescent light at 22 ± 1°C for 10 days. Conidiophores were slender and branched with enlarged apical cells bearing smooth, ash-colored conidia 6 to 10 × 6 to 8 (average 9 × 7) μm on short sterigmata. A few, black, irregularly shaped sclerotia (3 to 5 × 1 to 2 mm) were produced on PDA plates incubated for 20 days at 8 ± 1°C. These morphological characteristics identified the fungus as Botrytis cinerea (2). The internal transcribed spacer (ITS) region of rDNA was amplified using primers ITS4/ITS6 and sequenced. BLAST analysis (1) of the 491-bp segment showed a 100% homology with the sequence of Botryotinia fuckeliana (perfect stage of B. cinerea). The nucleotide sequence has been assigned GenBank Accession No. FJ 572049. Pathogenicity tests were performed twice by placing mycelium fragments (1 cm2) of PDA cultures on 30 leaves of 6 healthy 3-year-old potted C. florida plants. Six plants inoculated with PDA alone served as controls. Plants were maintained outdoors at temperatures ranging between 15 and 22°C, spraying leaves with water three times a day. The first foliar lesions similar to those observed in the gardens developed 10 days after inoculation on 23 inoculated leaves, whereas control plants remained healthy. B. cinerea was consistently reisolated from these lesions. To our knowledge, this is the first report of the presence of B. cinerea on C. florida in Italy. The disease has been reported in the United States (4) as well as in Japan (3). At this time, the economic importance of Botrytis blight to flowering dogwoods in Italy is undetermined.
References: (1) S. F. Altschul et al. Nucleic Acids Res. 25:3389, 1997. (2) H. L. Barnett and B. B. Hunter. Illustrated Genera of Imperfect Fungi. Burgess Publishing Company, Minneapolis, MN, 1972. (3) T. Kobayashi. Ann. Phytopathol. Soc. Jpn. 50:528, 1984. (4) C. Westcott. Plants Gard. 7:136, 1951.
