Occurrence of Root and Stem Rot Caused by Rhizoctonia solani AG-4 HGI on Torenia fournieri in China.

Torenia fournieri (wishbone flower) is widely cultivated for ornamental and medicinal use in southern China. In July 2021, severe root and stem rot of T. fournieri were observed in approximately 70% of the 3,000 plants grown in an ornamental plant nursery under natural conditions located in Guangzhou, Guangdong Province, China. Lesions on the roots and basal stems were water-soaked and dark brown. In late infection, the whole plant wilted and died. Eventually, infected plants lose their ornamental and economic value. Fifteen diseased samples were collected, and diseased tissues (4 × 4 mm) were surface sterilized in 1% NaOCl for 45 s, rinsed with sterile water, dried, and incubated for 3 d on potato dextrose agar (PDA) medium at 25°C in the dark. Pure cultures were obtained by transferring hyphal tips to new plates of PDA and incubated at 25°C for 5 d. A fungus was consistently isolated from symptomatic root and stem samples (100% isolation rate). The colonies on PDA medium were initially white, turning light brown and with age, no sclerotia was present after 15 days. Width of hyphae was 3.78 to 11.10 μm , which with a constriction at the base of hyphal branches, septa near the branch, and right-angled branching. Multinucleate hyphal cells (3 to 12 nuclei per cell) were determined by 4'-6-diamidino-2-phenylindole (DAPI) staining (Yang et al. 2013). These characteristics matched those described for Rhizoctonia solani (Sneh et al. 1991). A representative isolate LZ1 was chosen for molecular identification, the internal transcribed spacer (ITS) region was amplified using the primers ITS1/ ITS4, and the resulting sequence (633 bp) were deposited in GenBank (accession no. MZ798228). A BLASTn search of the ITS sequence exhibited 99% identity with that of R. solani AG-4 HGI (MK430998). The phylogenetic analysis (neighbor-joining method and 1,000 bootstrap values) showed LZ1 was clearly assigned to group of R. solani AG-4 HGI. For pathogenicity testing, ten healthy T. fournieri plants (approximately 18 cm high) were planted into plastic pot filled with sterilized soil. Inoculum of R. solani was produced by adding a PDA agar plug (5 mm in diameter) from a 6-day-old colony to a flask with 150 ml of potato dextrose broth (PDB), shaken at 25°C and 150 rpm for 7 days, and poured evenly into the sterilized soil (50ml per pot ). Another 10 plants were poured with sterile PDB as controls. All the inoculated plants placed in a greenhouse at 25°C and 70-90% humidity conditions. After 14 days, rot symptoms of the roots and basal part of stems similar to those in the nursery were observed on all inoculated plants, while no visible symptoms were observed on the controls. We reisolated the fungal pathogen from the inoculated plants and identified to be R. solani AG-4 HGI by microscopy and molecular characteristics mentioned above. R. solani AG-4 HGI was previously reported causing Impatiens walleriana Stem Rot and muskmelon fruit rot in Liaoning, China (Sun et al. 2015). To the best of our knowledge, this is the first report of Rhizoctonia solani AG-4 HGI causing root and stem rot on Torenia fournieri in China as well as worldwide. This finding contributes to expand the host range known for R. solani AG-4 HGI and will assist in developing effective control strategies of this disease to minimize the losses.