First Report of Fruit Rot on Hylocereus undatus Caused by Bipolaris cactivora in South Florida

Pitahaya (Hylocereus undatus (Haw.) Britton & Rose), a cactus grown for its edible fruit, is gaining popularity in South Florida as part of the specialty tropical fruit market. In July 2009, flowers and fruit were discovered with an uncharacterized rot. Small, circular, light brown, depressed lesions expanded to form large areas of rot on flowers and fruit in 7 to 10 days. The lesions produced large amounts of dark fungal spores. Single-spore isolates were identified morphologically and by aligning internal transcribed spacer (ITS) and glyceraldehyde-3-phosphate dehydrogenase (gpd) DNA sequences from the isolates with previously published sequences of Bipolaris, Drechslera, and Cochliobolus species. Conidia from the dark, blackish brown colonies were formed at the tips of pale golden brown, straight to flexuous conidiophores, 99 (184) 313 × 3 (6) 8 μm and slightly swollen at the apex and base. Conidia were pale-to-medium golden brown, smooth and clavate with a protuberant hilum, 24 (40) 51 × 9 (10) 13 μm, and two to four distoseptate. The isolates closely match descriptions of Bipolaris cactivora (= Drechslera cactivora) (3,4), although isolates from pitahaya had smaller conidia (30 to 65 μm) than previously reported. Conidial characteristics from a B. cactivora herbarium specimen BPI 431621 (U.S. National Fungus Collections) closely matched (29 (36) 50 × 8 (9) 11 μm, two to four distoseptate) our isolates. ITS (GenBank Accession Nox. HM598677-79) sequences aligned most closely (99.7% homology) with another B. cactivora isolate from China (GU390882), and both ITS and gpd (GenBank Accession Nos. HM598680-82) sequences indicate a close relationship to Bipolaris indica. Wounded or nonwounded mature pitahaya fruit and mature stems were inoculated with either a mycelia plug or a 15-μl 0.3% agar drop containing 105 conidia ml-1. Lesion diameters were measured after 7 days at 25°C, the fungus was reisolated on potato dextrose agar (PDA) and its identity was confirmed. Mean lesion diameters on mature fruit were 6.0 to 10.8 mm, depending on the inoculation method, and sporulation began 6 days after inoculation. On mature plant stems, wound-inoculated treatments formed 1.8 to 3.4 mm lesions, but nonwounded inoculations and controls were negative. Lesions were light tan, circular, and did not sporulate. To our knowledge, this is the first report of fruit rot caused by B. cactivora on pitahaya in Florida. The same pathogen causes stem rot of the Cactaceae in Europe and the United States (2) and a fruit rot on pitahaya in Japan (4). In Florida, it has been reported as causing a leaf spot on Portulaca oleracea (1). Our results indicate that B. cactivora causes flower and fruit rot on pitahaya, but does not seriously affect mature plant stems. The flower rot does not appear to significantly increase incidence but may provide inoculum for the fruit rot. The high incidence of fruit rot affecting commercial operations in Miami-Dade County over the past 2 years requires an effective disease management strategy. References: (1) S. A. Alfieri, Jr. et al. Bull. 14. Index of Plant Diseases in Florida (Revised). Florida Dep. Agric. Consumer Serv., Div. Plant Ind., 1984. (2) R. D. Durbin et al. Phytopathology 45:509, 1955. (3) M. B. Ellis. Page 432 in: Dematiaceous Hyphomycetes. Commonwealth Mycological Institute, Kew, England. 1971. (4) S. Taba et al. J. Gen. Plant Pathol. 73:374, 2007.