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Sugar beet (Beta vulgarisL.) is one of the major sugar sources in the world. Rhizoctonia solanicauses damping-off and crown and root rot that can result in significant yield and economic losses. R. solaniAG 2–2 IIIB is the most damaging anastomosis group in sugar beet production. In this study, we evaluated three different types of inoculums, namely barley grains colonized by fungal mycelium (CBG), agar plugs containing fungal mycelia (MAP), and sclerotia (SCL) for their ease of production and efficacy in inducing disease in sugar beet. First, the fungal growth rate and sclerotia production were compared on six types of media, clarified V8 [CV8], potato dextrose agar [PDA], metalaxyl benomyl vancomycin agar [MBV], yeast malt agar [YMA], corn meal agar [CMA], and oatmeal agar [OMA]. The fungus grew faster and produced more sclerotia in CV8 medium than in other media (P < 0.05). The rate of fungal growth from CBG, MAP, and SCL was evaluated. The in vitro rate of growth of R. solaniwas faster when originated from MAP than from SCL (P < 0.05) but equal to that from CBG. The different inoculum forms were then used to inoculate seeds at planting and 4-leaf stage sugar beet plants to evaluate the disease incidence and severity. R. solanion CBG caused greater severity. Overall, CBG was the best form of inoculum due to its ease of inoculum production, low cost, and ability to consistently cause severe disease symptoms on sugar beet plants.

North Dakota (ND) and Minnesota (MN) are two leading sugar beet producing states in the US jointly contributing 57% of domestic production, which results in over $5 billion in total economic activities. Sugar beet plant samples that had typical wilting and root rot symptoms were collected from Hickson, ND and Foxhome, MN in 2020. A fungus was isolated from diseased roots in potato dextrose agar media and was identified as Rhizopus arrhizus(synonym: R. oryzae) based on morphological features, microscopic visualizations, and the ribosomal Internal Transcribed Spacer DNA sequence identity. The pathogenicity of the R. arrhizusisolates was checked by seed and root inoculation in the laboratory and greenhouse. In vitro inoculation of sugar beet seeds with mycelial agar plugs in PDA plates showed reduced seedling growth and emergence. In the greenhouse, seeds inoculated with young growing mycelial agar plugs showed seedling rot, wilting and abnormal growth of cotyledons when evaluated at 10 days post inoculation (dpi). Sugar beet plants at four weeks old were inoculated with fungal spore suspension by the dipping method. At 21-dpi, inoculated roots showed characteristics root rot and wilting symptoms. Re-isolated fungus from inoculated seedlings with root rot symptom was found to be identical to pure culture of the isolates retrieved from the field samples. These findings added a new insight on and better understanding of the fungus causing seedlings and root rot of sugar beet; the information generated will direct further plan of action for its management to prevent crop loss.