First Report of Rhizoctonia solani, AG4 HG-II Causing Rhizoctonia Root Rot in Beta vulgaris in Minnesota, USA

Sugar beet (Beta vulgaris) taproots are often infected with soilborne pathogens included Rhizoctonia, Fusarium, Aphanomyces, Pythium and Geotrichum. These evidenced to cause substantial qualitative and quantitative losses [1]. Till today, 13 anastomosis groups (AG1-AG13) of R. solani have been reported in a wide range of crops, among them AG 2-2 mainly cause root and crown rot in sugar beet [2]. In August/2019-sugar-beet growing season, dark brown to black irregular water-soaked lesions were observed on the sugar beet taproot in the production areas of Foxhome (46.2774° N, 96.3100° W), Minnesota. Approximately 2-5% of disease incidence was recorded. Small pieces (10 mm²) of symptomatic root tissues were cut from the margins between infected and healthy tissue, and disinfected with 10% sodium hypochlorite for 1 min, rinsed three times with autoclaved water, and then placed on potato dextrose agar (50% PDA) for incubation at 35°C in the dark for 2 days. Subsequently, single hyphal tips were transferred to potato dextrose ager (PDA). In total, 8 isolates were developed from diseased beet collected from Foxhome. Macroscopically, isolates were developed light brown fungal biomass on PDA in 14 days. Microscopically, hyphae branch at a 90° angle, and septa near the branches and slight constrictions at the bases of the branches were present [3]. Based on macroscopic and microscopic characteristics, the fungus was speculated to be Rhizoctonia solani. Determining the subgroups, genomic DNAs of 8-isolates were used for polymerase chain reaction (PCR) using ITS-1 and ITS-2 primers [4,5]. PCR amplicons were cleaned with E.Z.N.A ®Cycle Pure Kit, OMEGA, and 8 samples were sent for Sanger sequencing to GenScript (Piscataway, NJ). The sequences were identical, and Blast analysis showed 100% sequence homology to R. solani AG4HG-II (Genbank accessions: MN106353.1). The nucleotide sequence was deposited in Genbank (MT108198.1). Pathogenicity assay was performed with 3-week old sugar beet susceptible cultivar which was inoculated with a 2-week old culture -mycelial plug (5 mm²) was placed at a 1/2-inch depth. The study was done twice with four replicates and 10 plants per replicate. Six weeks of post-inoculation, all inoculated plants developed similar symptoms of taproot rot as discussed above, whereas mock-inoculated plants were healthy. Pathogen re-isolated from the infected root tissue and was further confirmed morphologically and molecularly, fulfilled Koch’s postulates. AG4 HG-II previously reported to cause disease on sugar beet in Idaho [6]. To our best knowledge, this is the first report of R. solani AG4 HG-II on sugar beet in Minnesota, USA.