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

Authors: BHUIYAN, ZIAUR - North Dakota State University; Lakshman, Dilip; MOSHER, PRESLEY - North Dakota State University; KHAN, MOHAMED - North Dakota State University

Submitted to: Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2021
Publication Date: 10/15/2021
Citation: Bhuiyan, Z.M., Lakshman, D.K., Mosher, P., Khan, M.F. 2021. Identification of Rhizopus arrhizus (Fisher) causing root rot in sugar beet in North Dakota and Minnesota, USA. Journal of Plant Pathology. 104:357-362. https://doi.org/10.1007/s42161-021-00967-2.
DOI: https://doi.org/10.1007/s42161-021-00967-2

Interpretive Summary: 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 economic activities. A disease of sugar beet with typical wilting and root rot symptoms was detected in growers’ fields of ND and MN in 2020. The disease-causing agent was pathologically, morphologically, and molecularly characterized and identified as a soilborne fungus, Rhizopus arrhizus (synonym: R. oryzae). This is the first report on the occurrence of this disease in ND and MN. Our findings have implications on sugar beet disease management in the respective states. The information generated in this manuscript will be useful for mycologists, plant pathologists and agricultural extension workers.

Technical Abstract: 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 characteristic 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.