Reported Disease Distribution
Back in 1921, B. L. Richards with the Utah Agricultural Experiment Station described a root disease with different symptoms than those traditionally associated with RRCR, including those noted above. He referred to it as dry rot canker (DRC). Fifteen years later, another similar sugarbeet root disease was also reported widely distributed throughout Minnesota and Colorado in the summers of 1936-1938. It has since been reported from additional western sugarbeet-growing states including California, Montana, Nebraska, North Dakota and Wyoming.
* Robert M. Harveson is extension plant pathologist, University of Nebraska Panhandle Research & Extension Center, Scottsbluff. Melvin D. Bolton is research plant pathologist with the USDA-ARS Northern Crop Science Laboratory, Fargo. N.D.
Additional Affected Fields
In early September 2013, another field was found in western Nebraska with dead and dying plants exhibiting symptoms identical to those from the 2011 Morrill County field. However this field was located in Scotts Bluff County, northwest of Mitchell, Neb. Over the next three weeks, an additional half-dozen fields throughout Scotts Bluff and Morrill counties were identified with similar disease symptoms, and root samples were collected. In all cases, isolation from symptomatic root tissues yielded fungal cultures strongly resembling Rhizoctonia.
Pathogenicity tests for the new isolates were then conducted in both the greenhouse and field, and compared with the typical RRCR isolates. Inoculated plants exhibited symptoms identical with the distinctive lesions characteristic of the DRC disease under both greenhouse and field environments. Correspondingly, the RRCR isolates further produced the expected symptoms of small oval, superficial lesions that coalesced over time to cover large areas of the root surface. This proved that the two types of isolates were both capable of causing root disease, but also suggested a substantially different pathology based on development of different symptoms.
The new isolates were analyzed by Melvin Bolton with the USDA-ARS, Fargo, N.D., utilizing complex molecular tests that differentiate isolates genetically. The results of these tests have convincingly proven that the DRC isolates are distinct from the typical Rhizoctonia pathogen associated with RRCR. Furthermore, all DRC isolates examined to date (>20 isolates), from multiple fields throughout the North Platte Valley, have been shown to be nearly identical in genetic makeup. These isolates have not yet been discovered from growing areas outside the North Platte Valley, but a major goal for 2014 is to survey all areas of Nebraska intensively for incidence of this Rhizoctonia variant.
B. L. Richards, the original investigator, suspected that the Rhizoctonia isolates that he found inducing the dry rot canker disease in Utah were different than typical R. solani isolates based on different symptoms. This remarkable observation was made decades before the availability of our current more sensitive molecular tools.
Our data from 2013 confirm those suspicions while also strongly suggesting the presence of a new but distinct Rhizoctonia species, pathogenic on sugarbeets. The implications of this new pathogen residing in production fields and its effects on Rhizoctonia-resistant cultivars is presently unknown, but the comprehensive survey planned for 2014 will help to at least determine the extent if the pathogen’s presence and distribution in Nebraska.
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