First report of multinucleate Rhizoctonia solani AG4 HG-I causing crown and root rot on strawberry in Italy

Strawberry (Fragaria × ananassa Duch.) is a crop of great economic importance in Italy, where it is grown in soil and under soilless conditions. In March 2019, about 30 to 35% of plants (cv. Portola) grown in a peat substrate under soilless conditions in a farm located in Cuneo Province died. The examination of 10 plants showed crown and root rot over 100% of the root/crown. Affected plants showed brown necrotic tissues in basal leaves and petiole necrosis. Crown and root tissues were cleaned thoroughly from soil residues under tap water. Portions (about 3 to 5 mm) from crowns and roots were cut and surface disinfected with a water solution of NaClO at 0.5% for 2 min and rinsed in sterile water. The tissue fragments were plated on potato dextrose agar (PDA) amended with 100 mg/liter of streptomycin sulfate and incubated at 25°C. After 3 days, fungal colonies with septate hyphae and right-angled branching similar to Rhizoctonia solani were observed with high frequency (90%) (Sneh et al. 1991). To confirm the species identity, hyphal tips were transferred from the obtained colonies to PDA and grown for 10 days at 22 ± 1°C. Mycelium was light brown, compact, with radial growth. The hyphal width varied from 8.5 to 10 μm. Sclerotia were not present. DNA was then extracted from a single representative isolate (RH230), and rDNA ITS sequencing was conducted as described by Aiello et al. (2017). The rDNA ITS sequence of RH230 (GenBank accession no. MZ373271) was 100% identical (603/603 bp) to part of another sequence previously identified as R. solani AG4 HG-I (MK583647, Claerbout et al. 2019). Twenty-day-old healthy plants of cultivar Portola were planted in a steam-disinfested peat soil (12-liter pots) infested with 1 g/liter of wheat kernels colonized for 10 days with the isolate RH230 to evaluate the pathogenicity. Control plants were planted in a steam-disinfested peat substrate amended with noninoculated sterilized wheat kernels. Six plants per treatments were used and kept in a greenhouse at 25 ± 3°C. Crown and root rot similar to that observed in the farm developed 40 to 55 days after inoculation and resulted in 50 to 66% dead plants during two repeated trials. Fungal colonies morphologically similar to R. solani were consistently reisolated from affected crowns, and the resequencing of the rDNA ITS region fulfilled Koch’s postulates. Control plants remained healthy. Rhizoctonia isolates of AG-A and AG-G anastomosis groups were found as pathogens of strawberry in Italy (Manici and Bonora 2007), while the AG4 HG-I was reported in Israel (Sharon et al. 2007). R. solani AG4 HG-I was found on other hosts (Aiello et al. 2017); however, to our knowledge, this is the first report on strawberry in Italy. The disease could become a significant problem for soilless culture strawberry in Italy, causing severe yield losses.