First Report of Bipolaris sacchari Causing Leaf Spot on Lygodium japonicum and L. microphyllum in Florida

Lygodium japonicum (Thunb.) Sw. (Japanese climbing fern) and L. microphyllum (Cav.) R.Br. (Old World climbing fern) are invasive, noxious weeds in Florida. During 2001, L. japonicum sporelings were collected from natural sites in Hamilton, Highland, and Madison counties and transported to Broward County (Fort Lauderdale) for research use. During February 2002, leaf spots were observed on pinnules (leaflets) of these containerized plants growing in a shadehouse. A fungus with Bipolaris-like spores was isolated from affected pinnules and purified and stored for future evaluation. In early 2005, the fungus was grown on 1.5% water agar, with sterile, wheat straw pieces embedded in the agar surface, at 26°C and a 12-h photoperiod using cool-white fluorescent and Verilum full-spectrum bulbs. Conidia were 80.5 ± 14.5 μm (range 53.2 to 123.4 μm) × 15.1 ± 1.6 μm (range 12.1 to 19.4 μm), pale brown, slightly curved, narrowly ellipsoid, without a protuberant hilum, distoseptate (8 ± 1, range 6 to 10), and germinated from both polar cells. Conidiophores were septate and smooth. Conidiogenous nodes were smooth. On the basis of these characteristics, the fungus was identified as Bipolaris sacchari (E. Butler) Shoem. (1,2). Pathogenicity toward L. japonicum and L. microphyllum was determined using conidia produced on potato dextrose agar at 26°C and a 12-h photoperiod. A 1 × 10⁶ conidia ml^-1 suspension was sprayed until runoff on healthy plants grown in 450-ml containers. Control plants were sprayed with sterile water. There were four replicate plants per treatment. Plants were covered with plastic bags to maintain high humidity and placed in a growth chamber with a 12-h photoperiod at 28°C (light cycle) and 22°C (dark cycle). Bags were removed after 72 h, and small (1 to 2 mm), water-soaked spots were evident throughout the plant canopy on both Lygodium spp. Plants were incubated for three more weeks under the same photoperiod and temperatures with ˜70% relative humidity (light cycle) and ˜45% relative humidity (dark cycle), and then evaluated for disease. At least 50% of L. microphyllum pinnules and 25% of L. japonicum pinnules on each inoculated plant had small (1 to 2 mm), brown leaf spots or larger (approximately 5 mm) necrotic spots. B. sacchari was reisolated from both types of spots from both Lygodium spp.; there was no evidence of fungal sporulation on the plants. No symptoms were apparent on control plants. To our knowledge, this is the first report of B. sacchari on a non-Poaceae host in Florida. References: (1) J. L. Alcorn. Ann. Rev. Phytopathol. 26:37, 1988. (2) A. Sivanesan. Graminicolous Species of Bipolaris, Curvularia, Dreschslera, Exserohilum and Their Teleomorphs. CAB International, Wallingford, Oxon, UK, 1987.