First report of Binucleate Rhizoctonia (Ceratobasidium) Ag F causing dry rot canker of sugar beet in Idaho

Authors: WOODHALL, JAMES - University Of Idaho; BROWN, LARA - University Of Idaho; HARRINGTON, MIRANDA - University Of Idaho; HERBERT, KEVIN - University Of Idaho; KEITH, SHELIA - University Of Idaho; Webb, Kimberly; NEHER, OLIVER - Amalgamated Sugar Company

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/28/2020
Publication Date: 7/9/2020
Citation: Woodhall, J.W., Brown, L., Harrington, M., Herbert, K., Keith, S., Webb, K.M., Neher, O. 2020. First report of Binucleate Rhizoctonia (Ceratobasidium) Ag F causing dry rot canker of sugar beet in Idaho. Plant Disease. 104(9). https://doi.org/10.1094/PDIS-06-19-1328-PDN.
DOI: https://doi.org/10.1094/PDIS-06-19-1328-PDN

Interpretive Summary: Sugar beet roots that had symptoms for a disease not seen in Idaho before were sent to the University of Idaho Parma Diagnostic lab to determine what could be causing the problems. Foliar symptoms were similar to diseases known to be caused by Rhizoctonia, but had different symptoms on the roots which included 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. To determine the pathogen causing the symptoms fungal cultures were isolated from the lesions, and identified as Binucleate Rhizoctonia (AG F) based on morphological and molecular characteristics. These isolates were then re-inoculated onto sugar beet, and caused the same symptoms as were initially seen. To confirm that the Binucleate Rhizoctonia (AG F) was the causal agent the fungus was re-isolated from the inoculated sugar beet roots. As far as we know, this is the first report from Idaho of Binucleate Rhizoctonia (AG F) causing disease on sugar beet.

Technical Abstract: In August 2018, three separate samples of sugar beet were received by the University of Idaho Parma Diagnostic lab for diagnosis with 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 3-5 mm in size were removed from the tissue and plated onto semi-selective media to isolate suspect pathogens. Rhizoctonia-like colonies were consistently observed and were transferred to fresh plates of Potato Dextrose Agar (PDA) for isolation. After two weeks, colonies were tan to light brown with several sclerotia present. Representative colonies for each sample were selected for sequencing of the rDNA ITS region. The resulting DNA sequences were 100% identical to a Binucleate Rhizoctonia (BNR) AG F isolate previously associated with dry rot in Nebraska (GenBank KC842197). Based on sequence analysis and colony morphology isolates from all samples were identified as BNR AG F. To complete Koch's postulates two-week-old cultures of a suspect isolate were used to inoculate 15-day-old sugar beet roots. After 48 days, lesions were observed on inoculated roots whilst 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 core rot canker in Idaho.