First report of Phytophthora nicotianae causing blight of Dendrobium candidum candidum in Zhejiang province, China

Dendrobium candidum is a traditional Chinese medicinal herb, used raw or processed for health-care products in recent years. Due to the high economic value, there is huge demand and over-collecting of this plant, and hence the resources of wild D. candidum are endangered. In order to meet the increasing demands, tissue cultured seedlings are grown on large scale fields by local enterprises. Since 2001, a root and basal stem rot and die-back disease has been observed on D. candidum in Yiwu city, Zhejiang province, China. In the field, 1-year-old infected transplanted seedlings demonstrate typical symptoms of Phytophthora root and basal stem rot. Symptom progression is acropetal and plants die within a few days. Tap roots are completely rotted with black-brown to black sunken lesions frequently extending up the stem. In contrast, 2- to 3-year-old infected plants only have die-back symptoms with almost no root and basal stem rot. Typical black-brown lesions are observed on the tender top parts of a plant, causing top wilt and defoliation. Dendrobium candidum blight has not been observed in the wild.

Five isolates of a Phytophthora species from diseased material were initially identified as Phytophthora nicotianae using morphological features (Stamps et al., 1990; Erwin & Ribeiro, 1996). No oospores were observed in the tissue. Mating types were determined by pairing known P. nicotianae testers strains. Oospores were produced following pairing testers strains and all isolates proved to be of the A2 mating type. PCR amplification of genomic DNA using the primers ITS6 and ITS4 generated sequences of approximately 890 bp. The sequences were submitted to the GenBank database (Accession No. EF140984-88). Sequence analysis showed that the ITS sequences obtained were identical with others obtained from P. nicotianae (Estevez de Jensen et al., 2006), thus confirming the identity of the pathogen.

Mycelial plugs of the five Phytophthora isolates taken from agar medium, were inoculated into the basal stem of 2-month-old transplanted seedlings grown in pasteurized potting substance (mainly chipped pine bark) and to the top parts (new shoots) of 2-year-old plants. Plants were grown in the greenhouse at 25ºC, where they developed typical disease symptoms after 7 days. Non-inoculated controls did not develop disease symptoms. The pathogenic isolate was re-isolated from all diseased plants. Isolates have been deposited at the Biotechnology Institute, Zhejiang University. To the best of our knowledge, this is first record of Phytophthora blight of D. candidum in China.