First report of southern blight of Dendrobium huoshanense caused by Sclerotium rolfsii in Lu'an, China.

Abstract

Dendrobium, the second largest genus of Orchidacea, are mainly distributed in Asia, Oceania and Europe (Ding et al., 2008). An emerging disease on D. huoshanense with symptoms resembling southern blight Sclerotium rolfsii Sacc. occurred during June to September from 2017 to 2020 in Huoshan, Lu'an, China. The yield and quality of D. huoshanense were seriously affected especially under greenhouse conditions in July, with a disease incidence of 42%. Early symptoms consisted of water-soaked lesions on the lower stem tissue. Then leaves turned pale green and withered, a dense white mycelial mat were formed on the lower stem along with 1- to 2-mm-diameter, white round sclerotia. Subsequently, infected stems collapsed with the whole stem yellow and withered, and with many dark brown sclerotia on withered stems. Sclerotia from 5 diseased plants were disinfected in 1% mercury bichloride for 10 s, rinsed in 75% ethanol for 30 s and then rinsed 4 times with sterile water, dried, and transferred to potato dextrose agar (PDA) with the cross section and incubated at 27 °C in darkness. Mycelia were selected from the edge of the colony and transferred to new PDA plates for purification of cultures after 3 days. Fungal colonies with white mycelia and abundant sclerotia developed after 7 days of incubation. Isolate morphological characteristics confirmed the identity as S. rolfsii (Mordue, 1974). To further confirm the identity of the causal fungus, the complete internal transcribed spacer (ITS) rDNA region, the elongation factor-1a gene (EF1a) and the large subunit (LSU) region of one representative isolate, MB-1, was amplified using the primers ITS1/ITS4 (White et al. 1990), EF1-983F/EF1-2218R (Rehner and Buckley 2005), and NL1/NL4 (O' Donnell et al. 1997), respectively. Sequences have been deposited in GenBank (Accession No. OM946593 for ITS, OL416131 for EF1a and OL373917 for LSU). There was 100% sequence identity to Athelia rolfsii (S. rolfsii) with the sequences MN610008.1 (ITS) and MW322687.1 (EF1a), and 99.84% sequence identity to MT225781.1 (LSU). Pathogenicity tests were performed by wound inoculation with culture discs of mycelia ( = 5 mm) on the stem base of 10-12 cm height plants (four stems per pot, one stem inoculated with one culture disc, four replicates). Four healthy stems of the plants served as a control. The test was conducted in an artificial climate incubator at 26±1 °C with a 16h light/8h dark photoperiod and plants were enclosed for 72 h in polyethylene bags (relative humidity = 90%). Two days post-inoculation, white mycelia developed in the wound and the leaves at the upper and lower parts of the wound began to turn yellow. Then the stem softened and turned yellow. White sclerotia were formed by the sixth day. By 12 days post-inoculation, stems were atrophied and collapsed, sclerotia were yellow brown, and most leaves showed yellow discolouration and fell off. S. rolfsii was re-isolated from infected plants. Control plants remained healthy. To our knowledge, this is the first report of S. rolfsii causing southern blight on D. huoshanense in Lu'an, China. Confirmation of the causal agent provides a basis for management of S. rolfsii in cultivated D. huoshanense.