FIRST REPORT OF MYROTHECIUM RORIDUM CAUSING ROUND SPOT ON DENDROBIUM CANDIDUM IN CHINA

Abstract

Dendrobium candidum (family Orchidaceae) is grown as a medicinal plants in China. In July 2013, in some plastic greenhouses of Puer (Southwest Yunnan) round spots were observed on the leaves of D. candidum plants with 20-30% incidence. Initial symptoms included black to brown circular or subcircular lesions 0.5-2.0 cm in diameter, surrounded by water-soaked tissue and later by concentric rings, grey brown and dark green. Infected leaves turned yellow, wilted, and decayed. Symptomatic leaf tissues (4x5 mm fragments) were surface-sterilized and cultured onto potato dextrose agar (PDA) at 25°C in darkness. Developing colonies were white, floccose at the beginning, then produced sporodo- chia in dark greyish-green concentric rings bearing slimy spore masses. Conidia were smooth, hyaline, cylindrical with rounded ends, 4.9-8.9x1.4-2.8 μm in size. DNA was extracted from a fungal isolate denoted YNPE-SH9 using a DNA extraction kit (OMEGA, USA). The internal tran- scribed spacer (ITS) region and 5.8S rDNA gene were am- plified using primer pair of ITS1 and ITS4 (White et al., 1990). BLAST search of the obtained sequence (GenBank accession No. KM986033) revealed a 100% identity with two Myrothecium roridum strains (AJ301995, AJ302001). Leaves of 10 healthy greenhouse-grown D. candidum plants at the seedling stage were inoculated by injecting a conidial sus- pension of M. roridum (1.1×105 conidia per ml) and kept at 28°C, 80% relative humidity and 14-h photoperiod. Seed- lings inoculated with sterile distilled water served as con- trols. Symptoms like those seen in naturally infected plants were observed in all inoculated D. candidum seedlings seven days post inoculation while control plants remained symp- tomless. To our knowledge, this is the first report of natural infection by M. roridum on Dendrobium spp. in China.