First Report of Blackleg of Ajuga multiflora Caused by Phoma multirostrata in China.

Abstract

Ajuga multiflora Bunge (Labiatae) is an ornamental that is widely planted in gardens in southern China. Blackleg symptoms were observed on A. multiflora during the autumn of 2005 in the Hangzhou area, Zhejiang Province. A. multiflora had to be replaced by other landscape plants during the past 2 years because of a serious outbreak of the disease. When the plant is in continuous cultivation for more than 2 years, disease incidence is more than 80%. Brown water-soaked spots appeared initially on the base of the petioles. Blackleg lesions from multiple infections may coalesce. Lesions often expand rapidly along the petioles followed by wilting and drying of individual leaves, ultimately resulting in the death of plants. The disease can occur at any time of year, but is most severe during the flowering stage. A fungus was consistently isolated from symptomatic tissues on potato dextrose agar. After 14 days on SNA (synthetic low nutrient agar; 1.0 g of KH2PO4, 1.0 g of KNO3, 0.5 g of MgSO4·7H2O, 0.5 g of KCl, 0.2 g of glucose, 0.2 g of sucrose, and 20 g of agar per liter) medium, the fungus produced light olive-to-gray mycelium, and later, generated brown, globose to subglobose pycnidia with hyaline, unicellular, and ellipsoidal conidia. Diameter of pycnidia ranged from 163 to 260 μm (average 203 μm). Conidiophores measured 3.5 to 6.2 × 2.0 to 3.2 μm (average 5.4 × 2.3 μm). The fungus was identified as Phoma multirostrata (P.N. Mathur, S.K. Menon & Thirum) Dorenb. & Boerema based on its morphological characteristics (1). The fungus grew between 4 and 36°C and optimum growth was at 23°C. To fulfill Koch's postulates, 15 healthy 20-day-old A. multiflora seedlings were planted in sterile soil artificially infested with P. multirostrata, which was grown on a mixture (1:10 w/w) of corn meal and sand. Soil was amended with 20% (w/w) of inoculum mixture. Another 15 plants grown in noninfested soil served as controls. All plants were grown at 22 to 25°C in the greenhouse. After 5 to 7 days, inoculated plants showed symptoms closely resembling those seen on plants from the garden. P. multirostrata was isolated from the margins of necrotic tissues on diseased plants, but not from the control plants. To our knowledge, this is the first report of P. multirostrata causing blackleg of A. multiflora in China. Reference: (1) B. C. Sutton. The Coelomycetes. CMI, Kew, Surrey, UK, 1980.