Author’s note:  This is the second article in a series focusing on different root rot diseases caused by the genus Rhizoctonia.

SCOTTSBLUFF, Neb. – Rhizoctonia is the generic name for a group of diverse, soil-residing fungi that are responsible for causing economically damaging root diseases on a wide range of crops. In fact, the name “Rhizoctonia” translates from Greek into English as “killer of roots.” 

Rhizoctonia solani

Rhizoctonia solani is the most well-known root member of this group, and is widely distributed throughout soils worldwide. This species causes a myriad of diseases, including seed decay, post- and pre-emergent damping-off, stem cankers, root and crown rots, fruit decay, and in some instances, foliar diseases. You may be surprised to learn that R. solani was not the first Rhizoctonia pathogen discovered.  Did you know that a lesser-known species had been reported causing root a disease called violet root rot in the first quarter of the 1700s?

 Rhizoctonia crocorum

In 1728, Henri-Louis Du Hammel de Monceau described a disease of the crocus plant in southern France (Crocus sativa), now recognized to be caused by Rhizoctonia. This strange plant is the source of the spice saffron and a member of the iris family. Infected bulbs were covered with a thick reddish-violet hyphal mat.

The report also mentioned that the fungus moved spatially underground through the soil, infecting and spreading disease from bulb to bulb. The genus Rhizoctonia was formally created almost a century later in 1815 by the French mycologist Augustin Pyramus De Candolle to accommodate the crocus root pathogen, calling it R. crocorum. 

Violet root rot in Europe

In the mid-19th century the disease became problematic throughout agricultural production in Europe, causing root disease on many crop species, including almost 50 genera of 21 plant families. It was known as violet root rot, red root rot, or violet root felt disease.

The disease was particularly well known in Germany after Julius Kuhn published an account of serious root rots on alfalfa, carrots, and mangolds (early sugar beets) accompanied by a purple mold on all below-ground plant parts. Disease symptoms were characterized by wilting, followed by rapid death of plants from localized, randomly distributed clusters within fields, a condition referred to as “Fehlstellen,” translated into English as “dropouts” or “defects.” 

Violet root rot in the U.S.

In a similar manner to Europe, the first Rhizoctonia disease discovered in the United States was violet root rot on alfalfa. Reports were made independently in 1890 from two sites in Nebraska by two different individuals (Roscoe Pound and W. J. Webber). Both remarked that the disease was rare, of little consequence and “seemed to be harmless.” 

However, two decades later, G. F. Freeman conversely reported in 1908 that the red root disease of alfalfa was spreading so rapidly that he feared it threatened the industry in Kansas, and every effort should be made to find a practical means of control. Violet root rot was first reported on potatoes in Oregon, and was additionally described as “very prevalent” throughout Western Nebraska potato fields between 1916 and 1920.

Violet root rot today

Since the early 1900s, the disease has been identified sporadically in the United States on potatoes, carrots, sugar beets, alfalfa, and celery, but is nevertheless still considered relatively rare. Due to its infrequent occurrences, the pathogen is still poorly understood, making its potential for causing economic damage possible but uncertain in the United States while still remaining more common and problematic in Europe.

Next time: an even more obscure Rhizoctonia root disease affecting sugar beets.

This article is based on material from these sources:

Duggar, B. M.  1915.  Rhizoctonia crocorum (Pers.) DC and R. solani Kuhn (Corticum vagum B. & C.) with notes on other species.  Ann. Missouri Bot. Gardens 2: 403-458.

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

Peltier, G. L.  1916.  Parasitic Rhizoctonias in America.  Illinois Agric. Exp. Sta. Bull. 189: 283-390.

Zadoks, J. C.  1981.   Mr. Duhamel’s 1728 treatise on the violet root rot of saffron crocus: “Physical explanation of a disease that perishes several plants in the Gastinois [France], and saffron in particular”.  Mededelingen van de Landbouwhogeschool, Wageningen
 81-7: 1-31.