First Report of Binucleate Rhizoctonia (Ceratobasidium) AG F Causing Dry Rot Canker of Sugar Beet in Idaho

In August 2018, three separate samples of sugar beet from southwest Idaho were received by the University of Idaho Parma Diagnostic laboratory for diagnosis. Above-ground symptoms of wilting and yellowing of leaves were present. However, below-ground symptoms showed several dry sunken lesions over affected brown tissue penetrating into the tap root. In addition, these lesions consisted of a series of concentric circles, and black sclerotia were also present. To determine the causal agent, pieces of affected material (approximately 3 to 5 mm³) were surface disinfected in 2% (v/v) sodium hypochlorite for 60 s, followed by two rinses of sterile distilled water. The pieces were placed onto water agar amended with penicillin G (0.2 g/liter) and streptomycin sulfate (0.8 g/liter). Rhizoctonia-like colonies were consistently observed (approximately 70%) after 3 days at 21°C, and hyphal tips were transferred to a fresh plate of potato dextrose agar (PDA). After 2 weeks, colonies were tan to light brown with several sclerotia present. Septate hyphae, ranging from 3 to 8 μm in width, were observed. A single representative colony from each sample was selected for sequencing of the rDNA ITS region. DNA was extracted using a Wizard DNA purification kit for food in conjunction with a Kingfisher ML magnetic particle processor (ThermoFisher). PCR and sequencing were done as previously described (Woodhall et al. 2013). The resulting DNA sequence (GenBank MT275688) was 100% identical to a binucleate Rhizoctonia (BNR) AG F isolate previously associated with dry rot of sugar beet in Nebraska (GenBank KC842197). To determine pathogenicity, two PDA plugs (10 mm²) from a 2-week-old culture of the BNR AG F isolate were used to inoculate 15-day-old roots of sugar beet variety SV 043N. The roots were grown in pots containing 0.5 liters of peat moss, vermiculite, and sand mix (2:1:1). Control plants were inoculated with a sterile PDA plug. Each treatment was replicated 10 times. After 48 days, brown sunken lesions were observed on 50% of the AG F inoculated roots, whereas no lesions were observed on plants inoculated with sterile PDA. Isolations were attempted from five replicates of the symptomatic material, and BNR was recovered each time, thereby fulfilling Koch’s postulates. This is the first report of AG F in Idaho and the first report of dry rot canker on sugar beet in Idaho. Dry rot canker was first identified from Utah in 1921 (Richards 1921). More than 75 years later, the disease was reported in Nebraska and the causal agent identified as BNR AG F (Harveson and Bolton 2013). Presently Rhizoctonia AGs A, E, K, 2-2, and 4 are associated with sugar beets in Idaho (Strausbaugh et al. 2011); growers should consider the presence of AG F in fields prior to planting. Appropriate crop rotation and fungicidal treatments to minimize the impact of this pathogen in sugar beet production should be considered.