White Dendrobium (Dendrobium candidum Wall. ex Lindl.) is a traditional Chinese medicinal herb that is used raw or processed for health care products in China (4). It is propagated by tissue-cultured seedlings largely because of its dramatic decrease in the wild from heavy market demand. In July 2011, a wilt disease was observed on 1-year-old seedlings in a cultivated field in Yueqing City, Zhejiang Province, China. Initial symptoms were one to several chlorotic leaves in a cluster of plants. As the disease progressed, leaves wilted before dropping. Vascular discoloration was not observed in infected stems. Individual plants wilted, bent, and rotted starting from the base and eventually all plants in a cluster wilted. Three strains isolated from stem tissues collected at different sites were grown on potato dextrose agar (PDA) and carnation leaf agar (CLA) under a 12-h alternating cycle of light and dark at a temperature of 25°C. Macroconidia were sparse, three-septate, slightly curved, and ranged from (21.1) 23.9 to 32.7 (36.9) × (2.8) 3.3 to 4.2 (4.8) μm. Microconidia were abundant, unicellular, oval to reniform, and ranged from (5.3) 6.0 to 8.2 (9.4) × (2.2) 2.5 to 3.3 (3.6) μm on PDA. Abundant chlamydospores formed after 3 weeks on CLA. Chlamydospores were single and sometimes in pairs, terminal and intercalary, rough walled, and measured (6.7) 7.6 to 10.2 (11.9) × (5.9) 6.8 to 8.8 (10.4) μm. Strains were identified as Fusarium oxysporum on the basis of morphological features (2). PCR amplification of genomic DNA using the primers ITS6 and ITS4 as well as ef1 and ef2 (translation elongation factor 1-α gene) generated sequences of approximately 500 bp and 670 bp, respectively. ITS sequences were submitted to the NCBI database (Accession Nos. JQ809654, JQ809655, and JQ809656), and were identical to the sequence for F. oxysporum, Accession No. HQ379654 (3). Three sequences of a portion of TEF-1α (Accession Nos. JQ809657, JQ809658, and JQ809659) had 99% similarity with an isolate of F. oxysporum isolate from chickpea (Accession No. HQ731057) (1), thus confirming the identity of the pathogen. The roots of five to seven transplanted seedlings of a cluster grown for 2 months in pasteurized potting media (primarily chipped pine bark) were inoculated with 15 ml of a 107 conidia ml–1 suspension of each isolate. There were 10 replicates for each isolate treatment. Sterile water was used as a negative control. Plants were grown in the greenhouse at 28 ± 2°C. Lateral roots turned brown 15 days after inoculation. Leaves started to turn yellow 1 month after inoculation and the plants began to wilt 2 months after inoculation. Noninoculated controls did not develop disease symptoms. The pathogenic isolates were reisolated from all diseased plants. Isolates have been deposited at the Biotechnology Institute, Zhejiang University (F_01, F_02 and F_03). To the best of our knowledge, this is first record of Fusarium wilt of D. candidum in China.
References: (1) D. Jiménez-Fernández et al. Plant Dis. 95:860, 2011. (2) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Ames, IA, 2006. (3) N. Rajmohan et al. Plant Pathol. 131:631, 2011. (4) J. Z. Zhang et al. Plant Pathol. 57:370, 2008.