First report of web blight on toothache plant caused by Rhizoctonia solani AG-4 HG-I in Italy

During the summer of 2017, a web blight occurred on seedlings of Spilanthes oleracea growing in a commercial farm located near Albenga (Northern Italy). Affected plants showed a collar rot followed by leaf wilting and stem collapse. Rhizoctonia solani was consistently isolated from affected tissues. On potato dextrose agar (PDA), colonies were light-brown, not aerial, with coarse hyphae and produced few, small, flat, brown sclerotia with a crusty surface, up to 1 mm in diameter. The characterization was determined by using R. solani tester strains belonging to the anastomosis groups AG-1, AG-2, AG-4, AG-7, and AG-11. R. solani from S. oleracea anastomosed only with R. solani AG-4 (Fusion Frequency < 30%) (Sneh et al. 1991). Primers ITS1/ITS4 were used to amplify the Internal Transcribed Spacer (ITS) region of rDNA. BLASTn analysis (Altschul et al. 1997) of the 700 bp sequence (GenBank accession number MG366819) showed 99% similarity with the sequence AY154307 of R. solani AG-4 HG-I. One of the isolates was grown on sterilized wheat kernels that were distributed (3 g/l) close to the collar of 15 60-day-old healthy plants of S. oleracea grown in pots (12 cm in diameter). Controls were treated with non-infected kernels. All plants were maintained in a moistened plastic bag, at the temperature of 23 ± 1 °C. About three days after the inoculation, a collar rot developed on all the inoculated plants and R. solani was reisolated from symptomatic tissues. Controls remained healthy. This is the first report of R. solani on S. oleracea in Italy as well as worldwide.