Begonia semperflorens Link et Otto (family Begoniaceae) is an herbaceous, flowering, ornamental plant, extensively cultivated in northeastern China. In July to August 2013, during a period of hot, humid, and rainy weather, a widespread aerial (web) blight was observed on the campus of Jilin Agricultural University and in many landscape plantings in Changchun, China. More than 80% of begonia plants were damaged by this disease, and they were replaced with other ornamental plants. Symptoms began as small, water-soaked spots on the lower stem, later spreading to the upper stem and leaves. In humid weather, the disease developed severely in less than a week, and a web-like brown mycelium covered the affected tissues. Then leaves became necrotic and dropped, and the plants frequently collapsed. Small pieces from the margin of symptomatic stem and leaf tissues collected from the campus of Jilin Agricultural University were surface sterilized in 70% alcohol for 30 s and 0.5% sodium hypochlorite for 1 min, plated on potato dextrose agar (PDA), and five isolates with morphological characteristics of Rhizoctonia solani Kühn were obtained. Hyphal tips were transferred to a fresh plate of PDA and the cultures were examined for morphological characters microscopically. Mycelia of all isolates were branched at right angles with a septum near the branch and a slight constriction at the branch base. Fungal colonies were initially white, turned brown with age, and produced irregularly shaped, brown sclerotia (0.6 to 6.2 × 0.5 to 20 µm) after 5 days on PDA. Hyphal cells were determined to be multinucleate when stained with 1% safranin O and 3% KOH solution (Bandoni 1979). The internal transcribed spacer (ITS) region of the nuclear rDNA was amplified by using the primers ITS4/ITS5 (Cooke and Duncan 1997). The ITS sequences of isolates SJHT1 (GenBank Accession No. HG934425), SJHT2 (HG934426), and SJHT3 (HG934427) had 100% sequence identity with R. solani AG1-IB (GU585667 and JN254787). Pathogenicity tests were performed on healthy, potted B. semperflorens plants using five isolates. Ten potted plants were inoculated near the base of the stem and on the leaf with a 6-mm-diameter mycelial plug from 3-day-old PDA cultures for each isolate, and the inoculation sites were covered with moistened, sterile, absorbent cotton. Ten plants inoculated only with PDA plugs served as controls. The plants were placed in a greenhouse and covered with plastic domes to maintain high humidity and temperature (ranging from 25 to 33°C). Three to five days after the artificial inoculation, all inoculated plants showed the same symptoms previously described, whereas control plants remained healthy. Rhizoctonia solani was reisolated from inoculated plants and its cultural characteristics were similar to those of the original isolates. There was no fungus isolated from control plants. To our knowledge, this is the first report of R. solani AG1-IB causing aerial (web) blight on B. semperflorens in China. Rhizoctonia solani has been reported on Begonia in the United States, Australia, and Bulgaria (Farr and Rossman 2015). This new finding on begonia in China represents a threat both to begonia production and to the use of begonias in the landscape. © The American Phytopathological Society.
