African daisy (Osteospermum sp.) is an ornamental plant grown in the winter as a potted plant in Liguria (northern Italy) and is generally marketed in early-to-late spring in Italy and central and northern Europe. In the winter of 2006, stem lesions, general chlorosis, wilt, and plant death occurred in a greenhouse nursery. Affected plants were characterized by the presence of soft, watery tissues. Necrotic tissues were covered with a white, cottony mycelium. During periods of high humidity, black sclerotia differentiated within the mycelium. To recover the pathogen, diseased stem tissue was surface sterilized for 1 min in 1% NaOCl and plated on potato dextrose agar (PDA) amended with 100 mg/l of streptomycin sulfate. Sclerotinia sclerotiorum (Lib.) de Bary (2) was consistently isolated from affected tissue. Sclerotia produced on PDA measured 1.3 to 3.1 × 1.5 to 3.5 mm in diameter. The internal transcribed spacer (ITS) region of rDNA was amplified using primers ITS4/ITS6 and then sequenced. BLASTn analysis (1) of the 633 bp showed a 100% identity with S. sclerotiorum. The nucleotide sequence was assigned GenBank Accession No. EU 556701. Pathogenicity of two isolates obtained from infected plants was confirmed by inoculating 10 80-day-old plants grown in 14-cm-diameter pots. Inoculum consisted of 1 cm2 of mycelium excised from a 10-day-old PDA culture of each isolate and placed on the soil surface around the base of each plant. Ten plants were inoculated with each isolate and 10 noninoculated plants served as controls. Plants were maintained in a greenhouse under shade at temperatures of 10 to 22°C (average 19°C), in high relative humidity (>90%), and were watered as needed. The trial was conducted twice. All inoculated plants developed leaf yellowing within 12 days of inoculation. White, cottony mycelium and black sclerotia developed on stems and at the base of all inoculated plants, which eventually wilted. Control plants remained symptomless. S. sclerotiorum was reisolated from the stems of inoculated plants. This disease has been reported on an Osteospermum sp. in the United States (3) and Argentina (4). To our knowledge, this is the first report of white mold on an Osteospermum sp. in Italy as well as in Europe. Currently, the economic importance of this disease is limited.
References: (1) S. F. Altschul et al. Nucleic Acids Res. 25:3389, 1997. (2) N. F. Buchwald. Den. Kgl. Veterin.er-og Landbohojskoles Aarsskrift. 75, 1949. (3) H. S. Gill. Plant Dis. Rep. 59:82, 1975. (4) E. R. Wright et al. Plant Dis. 89:1014, 2005.
