First Report of a Leaf Spot on Basella alba Caused by a Bipolaris sp. in Florida

Malabar spinach (Basella alba L.) is a fast-growing, perennial vegetable crop grown largely in the tropics of Asia and Africa. This crop is widely used in the cuisine of different regions for its thick, semisucculent leaves, mild flavor, and mucilaginous texture. Leaf spots were observed on both surfaces of symptomatic leaf samples received from a home garden in Homestead, FL in November 2009. The necrotic lesions (up to 2 mm in diameter) were round, semicircular, or irregular-shaped with grayish centers surrounded by dark brown borders. A fungus was consistently isolated from symptomatic tissues on clarified V8 (CV8) agar. Fungal colonies on CV8 agar were black and velvet-like with minimal mycelial growth and conidiophores were dark brown, simple, borne singly or in groups upon the substrate. Conidia were straight, pale to medium golden brown, rounded at the ends with three to six septa, and on average measured 75 × 15 μm (48 to 97 × 9 to 18 μm). Cultural and conidial characteristics of the isolates were closely similar to those of a Bipolaris sp. (1). The internal transcribed spacer (ITS) region (~570 bp) of rDNA was amplified using the primers ITS1/ITS4 and sequenced bidirectionally (GenBank Accession No. JF506092). Subsequent database searches by the BLASTN program indicated that the resulting sequence had a 95% identity over 531 bp with the corresponding gene sequence of Bipolaris portulacae (GenBank Accession No. AY004778.1), a fungal pathogen reported to cause leaf spot on purslane (Portulaca oleracea) (2,3). However, our isolate has consistently smaller conidia and does not match descriptions of B. portulacae (BPI 871173, U.S. National Fungus Collections). The pathogenicity was confirmed through inoculation of healthy Malabar spinach plants with conidia of the isolate reproduced on CV8. Six Malabar spinach plants were inoculated with a suspension containing 1 × 106 conidia per ml and sprayed until runoff (approximately 15 ml per plant) with a handheld pressurized canister. Another six noninoculated plants served as a control. Immediately after inoculation, plants were covered with plastic bags for 24 h to maintain high relative humidity and maintained in a greenhouse under ambient conditions. Ten days after inoculation, the symptoms described above were observed on leaves of all inoculated plants, whereas symptoms did not develop on the control plants. A Bipolaris sp. was reisolated and identified by the above methods, fulfilling Koch's postulates. This pathogenicity test was carried out three times. To our knowledge, this is the first report of a Bipolaris sp. affecting Malabar spinach in Florida. Further work should be conducted to confirm identity of these isolates. Because of limited plantings of Malabar spinach, the economic importance of this disease in Florida is currently not known. Nevertheless, this pathogen poses a threat to the growing market of continuously produced oriental vegetables in Florida. References: (1) J. L. Alcorn. Mycotaxon 39:361, 1990. (2) S. A. Alfieri, Jr. et al. Bull. 14. Index of Plant Diseases in Florida (Revised). Florida Dep. Agric. Consumer Serv., Div. Plant Ind., 1984. (3) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory. ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/ , 25 January 2010.