Identification of Rhizopus arrhizus (Fisher) causing root rot in sugar beet in North Dakota and Minnesota, USA

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. arrhizus isolates 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.