Panhandle Perspectives - April 17, 2018

Did you know? Dry rot canker – an uncommon Rhizoctonia disease of sugarbeets

Robert M. Harveson, Extension Plant Pathologist, Panhandle R&E Center, Scottsbluff

Author’s note:  This is the third and final article in a series focusing on different sugar beet root rot diseases caused by the genus Rhizoctonia.

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Bob Harveson

Last time I related a brief summary of the rarely occurring root disease, violet root rot, its causal agent, Rhizoctonia crocorum, and its relationship with the well-known root and crown rot pathogen, R. solani. This article will highlight an even more infrequently appearing Rhizoctonia disease of sugar beets called dry rot canker.

Although it is generally unfamiliar to many people involved in sugar beet production, we have recently demonstrated this disease to be caused by a largely uncharacterized species of Rhizoctonia, referred to as “binucleate.”

 New Disease

Plant Pathologist B.L. Richards reported a previously undescribed root disease of sugar beet near Cornish, Utah, in August 1920, calling it dry rot canker (DRC). The disease was characterized by localized, dry sunken lesions penetrating into the taproot’s interior. This caused decaying tissues to rapidly dry out as infection continued inward, resulting in cavities filled with a dry pithy material consisting of both fungal hyphae and decayed host matter.

Surface tissues of the cankers also produced a distinctive series of target board-like concentric circles.  These disease signs and symptoms were quite distinct from those of the more recognized Rhizoctonia root and crown rot disease (RRCR) caused by R. solani.

Richards mentions that he learned through correspondence with the early University of Nebraska-Lincoln plant pathologist G. L. Peltier that a similar disease was also observed from Nebraska in 1920.  Furthermore, it was determined that the disease’s presence could be traced to having occurred in Utah as early as 1915, based on preserved herbarium specimens. Since that time, DRC has occurred infrequently throughout other irrigated production areas of the western United States, and until recently, little else was known about this disease, primarily due to its rare appearances. 

New Appearances

In mid-September 2011, a field near Bridgeport, in Morrill County, Neb., was found with a high incidence of plants (10-15 percent) exhibiting wilting and yellowing symptoms suggestive of Rhizoctonia root and crown rot. However, the root symptoms were distinctly different, consisting of sunken lesions with concentric circles and a rot penetrating deep into root tissues similar to those for DRC described by Richards.

Between 2013 and 2015, more than a dozen additional sugar beet fields in Morrill and Scotts Bluff Counties of western Nebraska were identified and surveyed containing plants displaying identical symptoms.  Isolations from diseased root tissues all yielded fungal cultures strongly resembling Rhizoctonia solani, causal agent of the root and crown rot disease.

Diagnostic Tests

After conducting molecular analyses of DNA sequences, it was determined that the DRC isolates possessed a high degree of similarity (96 percent identity) with sequences of a binucleate Rhizoctonia species. Binucleates are a group of Rhizoctonia species that have two nuclei within their cells, in contrast to the more familiar, multinucleate root and crown root rot pathogen (R. solani).

We also confirmed this characteristic trait with microscopic examinations. All subsequent DRC isolates examined to date (more than 20), have near identical DNA sequences. Based on the production of different symptoms, unique nuclear condition, and DNA sequence similarities with other binucleate Rhizoctonia species, we have conclusively demonstrated that the DRC isolates obtained from diseased sugarbeet roots are unmistakably different from the R. solani pathogen associated with RRCR.

Field Studies 

We have also conducted recent field studies over two years confirming that the DRC pathogen is sensitive to current fungicides and responds to disease tolerant and susceptible cultivars in the same manner as R. solani.  This is good news for us in providing practical options for future disease management strategies.  However, this work also observed that the DRC isolates actually caused significantly greater yield reductions and disease severity ratings when compared with infections by R. solani using disease susceptible cultivars.

Conclusions

Richards, the original investigator, concluded that the Rhizoctonia isolates inducing the dry rot canker disease were different than typical R. solani isolates based on different symptoms. This remarkably prescient observation was made long before the availability of our current more sensitive molecular tools.  Our studies have confirmed those presumptions, proving that the pathogen is a distinct Rhizoctonia species, while also documenting the reemergence of DRC epidemics for the first time in almost 100 years. 

This article is based on material from these sources:

Harveson, R. M.  2015.  The bacterium of many colors.  APS Press, St Paul, MN, 288 pp.

Harveson, R. M., and Bolton, M. B.  2013.  First evidence of a binucleate Rhizoctonia as the causal agent of dry rot canker of sugar beet in Nebraska. Plant Dis. 97: 1508.

LeClerg, E. L.  1939.  Studies on dry-rot canker of sugar beets. Phytopathology 29: 793-800.

Richards, B. L.  1921.  A dryrot canker of sugar beets.  J. Agr. Res. 22: 47-52.