Although virtually unknown in sugarbeet production, researchers recently determined dry rot canker was caused by a largely uncharacterized species of Rhizoctonia, referred to as "binucleate."

B.L. Richards reported a previously undescribed root disease of sugarbeet near Cornish, Utah, in 1920, calling it dry rot canker (DRC). The disease was characterized by localized, dry sunken lesions that penetrated deeply into the taproot interior, causing the decaying tissue to rapidly dry out as infection continued inward.

Surface tissues of the cankers produced a distinctive series of concentric circles, like a target board. The lesions resulted in cavities filled with a dry pithy material consisting of both fungal hyphae and decayed host matter. These symptoms were distinct from those of the well-known rhizoctonia root and crown rot disease (RRCR) caused by R. solani.

Richards said he learned through correspondence with G.L. Peltier, a University of Nebraska-Lincoln plant pathologist, that a similar disease was observed in Nebraska in 1920. Based on preserved herbarium specimens, the disease's presence could be traced to Utah as early as 1915.

DRC has since occurred infrequently throughout other irrigated production areas of the western U.S., and until recently, little else was known about this disease, primarily due to its rare occurrences.

New appearances
In mid-September 2011, a field near Bridgeport in Morrill County had a high incidence of plants (10% to 15%) exhibiting wilting and yellowing symptoms suggestive of RRCR. However, root symptoms were different, consisting of sunken lesions with concentric circles, and a rot penetrating deep into root tissues like those described by Richards for DRC.

SUNKEN LESIONS: Dry rot canker is characterized by dry sunken lesions that create cavities of dry pithy material.

Between 2013 and 2015, more than a dozen additional sugarbeet fields in Morrill and Scottsbluff counties in western Nebraska were identified with plants displaying identical symptoms. Isolations from diseased root tissues all yielded fungal cultures strongly resembling R. solani.

Diagnostic and field studies
Molecular analyses of DNA sequences revealed the DRC isolates had high degrees of similarity (96% identity), with sequences of a binucleate Rhizoctonia species. This is a group that possess two nuclei within their cells in contrast to the more familiar, multinucleate root and crown root rot pathogen of R. solani.

Researchers confirmed this characteristic with microscopic examinations. All subsequent DRC isolates examined to date (more than 20), possess near identical DNA sequences. Based on the different symptoms, unique nuclear condition and DNA sequence similarities with binucleate Rhizoctonia species, it was demonstrated that the DRC isolates were distinct from the R. solani pathogen associated with RRCR.

UNL researchers have also conducted recent field studies showing the DRC pathogen is sensitive to current fungicides and disease-tolerant cultivars in the same manner as R. solani, which is good news for disease management. However, this work also discovered that DRC isolates caused significantly greater yield reductions and disease severity compared with those induced by R. solani when employing cultivars lacking disease resistance.

Richards, the original investigator, presumed that the Rhizoctonia isolates inducing the dry rot canker disease were different than typical R. solani isolates based on different symptoms. This observation was made long before the availability of current, more sensitive molecular tools. Studies by researchers in the Nebraska Panhandle have confirmed those suspicions, proving the pathogen is a distinct Rhizoctonia species, and documented the re-emergence of DRC for the first time in almost 100 years.

Harveson is a plant pathologist at University of Nebraska-Lincoln.