|HARVESON, ROBERT - University Of Nebraska|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2013
Publication Date: 11/1/2013
Citation: Harveson, R.M., Bolton, M.D. 2013. First evidence of a binucleate Rhizoctonia as the causal agent of dry rot canker of sugar beet in Nebraska. Plant Disease. 97:1508.
Interpretive Summary: The fungal pathogen Rhizoctonia solani causes a common disease of sugar beet called Rhizoctonia root and crown rot (RRCR). A field in Nebraska was identified as having typical foliar symptoms of RRCR, but taproot symptoms not typically associated with that disease. The pathogen was isolated from diseased taproots and shown to be a species of binucleate Rhizoctonia, which is distinct from the much more common R. solani. We inoculated plants with the isolated pathogen, which caused similar disease as those found in the field. The pathogen was re-isolated from diseased plants and reconfirmed to be binucleate Rhizoctonia. The symptoms are more closely related to a rare disease called dry rot canker, which hasn’t been reported on sugar beet in more than 75 years. Therefore, our studies suggested that dry rot canker is caused by binucleate Rhizoctonia.
Technical Abstract: Sugar beet (Beta vulgaris L.) is the primary source of domestic sucrose in the United States. In 2011, a sugar beet field in Morrill County NE was noted with wilting and yellowing symptoms suggestive of Rhizoctonia root and crown rot (RCRR), an important disease of sugar beet caused by Rhizoctonia solani anastomosis group 2-2. Root symptoms consisted of localized, dry sunken lesions covering brown spongy tissue penetrating deeply into taproots. The surface tissues of the cankers distinctively produced a series of concentric circles. These symptoms are inconsistent with RRCR, but are suggestive of a rarely occurring disease known as dry rot canker (DRC). To investigate the etiology of this disease, the pathogen was isolated from diseased taproots. Colonies were tan to light brown, with large amounts of floccose, aerial hyphae. The internal transcribed spacer (ITS) region of rDNA was sequenced and found to be 100% identical between the four isolates and 96% identical to binucleate Rhizoctonia and Ceratobasidium sp. AG-F. Hyphal cells were observed to be binucleate after staining. Therefore, these isolates were identified to be a binucleate Rhizoctonia group AG-F based on morphological and molecular characteristics. To determine pathogenicity, 1 and 2 month old greenhouse-grown sugar beet plants were inoculated. After approximately three weeks, Koch’s postulates were fulfilled after pathogenicity tests resulted in expected root lesions and isolates were recovered from diseased plants as described above. To our knowledge, this communication represents the first confirmed report of DRC on sugar beets in more than 75 years. The original investigator suspected that the isolates he found inducing this disease were different than typical R. solani isolates based on different symptoms. Our results, also based on different symptoms but also with distinct molecular, biological, and pathogenicity traits validate those suspicions while also strongly suggesting the identity of a binucleate Rhizoctonia species pathogenic to sugar beet that is distinct from the more common R. solani.