Thymus × citriodorus is well known for the citrus aroma released by its leaves and is grown as a potted plant in northern Italy. This species is widely used in gardens and landscapes and for culinary purposes. In the Liguria Region alone, 1.5 million plants are grown. In the winter of 2002, extensive chlorosis was observed on potted plants of Thymus × citriodorus cv. Silver Queen grown outdoors on commercial farms near Albenga. Initial symptoms included stem necrosis at the soil level and darkening of leaves. As stem necrosis progressed, infected plants wilted and died. Wilt, characterized by the presence of soft and watery tissues, occurred within a few days on young plants. Necrotic tissues became covered with whitish mycelium that produced dark sclerotia. Sclerotinia sclerotiorum (Lib.) de Bary (1) was consistently recovered from infected stem pieces of Thymus × citriodorus. The diseased stem tissue was disinfested for 1 min in 1% NaOCl and plated on potato dextrose agar (PDA) amended with 100 ppm of streptomycin sulfate. Sclerotia produced on PDA were ellipsoid and measured 5.2 to 4.4 × 2.1 to 1.5 mm (average 3.5 × 3.0 mm). Pathogenicity of three isolates obtained from infected plants was confirmed by inoculating 30-day-old plants grown in 14-cm-diameter pots in a screenhouse. Inoculum that consisted of wheat kernels infested with mycelium and sclerotia of each isolate was placed on the soil surface around the base of each of 10 plants. Noninoculated plants served as controls. The inoculation trial was repeated once. All plants were kept at temperatures ranging between 5 and 26°C and watered as needed. Inoculated plants developed symptoms of leaf yellowing within 13 days, soon followed by the appearance of white mycelium, and eventually wilted. Control plants remained symptomless. White mycelium and sclerotia developed on infected tissues and S. sclerotiorum was reisolated from inoculated plants. To our knowledge, this is the first report of white mold of Thymus × citriodorus caused by S. sclerotiorum. The economic importance of this disease for the crop can be considered low.
Reference: (1) N. F. Buchwald. Den. Kgl. Veterin.er-og Landbohojskoles Aarsskrift, 1949.
