M. F. Chuang,
H. F. Ni,
H. R. Yang,
S. L. Shu, and
S. Y. Lai, Department of Plant Protection, Chiayi Agricultural Experiment Branch, Agricultural Research Institute, Chiayi, Taiwan; and
Y. L. Jiang, Department of Horticulture, National Taiwan University, Taipei, Taiwan, R.O.C.
Pitaya (Hylocereus undatus and H. polyrhizus Britt. & Rose), a perennial succulent plant grown in the tropics, is becoming an emerging and important fruit plant in Taiwan. In September of 2009 and 2010, a number of pitaya plants were found to have a distinctive canker on stems. The disease expanded quickly to most commercial planting areas in Taiwan (e.g., Pintung, Chiayi, and Chunghua). Symptoms on the stem were small, circular, sunken, orange spots that developed into cankers. Pycnidia were erumpent from the surface of the cankers and the stems subsequently rotted. After surface disinfestation with 0.1% sodium hypochloride, tissues adjacent to cankers were placed on acidified potato dextrose agar (PDA) and incubated at room temperature for 1 week, after which colonies with dark gray-to-black aerial mycelium grew. Hyphae were branched, septate, and brown and disarticulated into 0- to 1-septate arthrospores. Sporulation was induced by culturing on sterile horsetail tree (Casuarina equisetifolia) leaves. Conidia (12.79 ± 0.72 × 5.14 ± 0.30 μm) from pycnidia were one-celled, hyaline, and ovate. The internal transcribed spacer (ITS) region of ribosomal DNA was PCR amplified with primers ITS1 and ITS4 (2) and sequenced. The sequence (GenBank Accession No. HQ439174) showed 99% identity to Neoscytalidium dimidiatum (Penz.) Crous & Slippers (GenBank Accession No. GQ330903). On the basis of morphology and nucleotide-sequence identity, the isolates were identified as N. dimidiatum (1). Pathogenicity tests were conducted in two replicates by inoculating six surface-sterilized detached stems of pitaya with either mycelium or conidia. Mycelial plugs from 2-day-old cultures (incubated at 25°C under near UV) were inoculated to the detached stems after wounding with a sterile needle. Conidial suspensions (103 conidia/ml in 200 μl) were inoculated to nonwounded stems. Noninoculated controls were treated with sterile medium or water. Stems were then incubated in a plastic box at 100% relative humidity and darkness at 30°C for 2 days. The symptoms described above were observed on inoculated stems at 6 to 14 days postinoculation, whereas control stems did not develop any symptoms. N. dimidiatum was reisolated from symptomatic tissues. To our knowledge, this is the first report of N. dimidiatum causing stem canker of pitaya.
References: (1) P. W. Crous et al. Stud. Mycol. 55:235, 2006. (2) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, New York, 1990.