Robert M. Harveson, Extension Plant Pathologist University of Nebraska, Panhandle REC, Scottsbluff
Introduction
In contrast to diseases occurring on foliage, root diseases induced by soilborne pathogens often cause more serious losses because they are difficult to detect before substantial damage has already occurred. Therefore, root diseases in general, were slower to be recognized compared with the more easily visualized rusts and mildews.
In the mid-1800s, soilborne diseases became economically important in European crop production, particularly those caused by the sugar beet cyst nematode, and Rhizoctonia. In reality however, Rhizoctonia had been first reported almost 150 years prior to this and is now recognized as the first soilborne, root-infecting plant pathogen.
The Genus Rhizoctonia
Today the genus Rhizoctonia is comprised of a highly divergent group of sterile fungi that still share similar characteristics in their anamorphic (asexual) state, namely they remain vegetative, producing no asexual spores but additionally posses several different sexual forms that occur on rare occasions. Rhizoctonia species are effective saprophytes, allowing survival in soils for long periods. However, they also include several economically important plant pathogens that routinely cause serious diseases in numerous plant species. The two most important species from a plant disease standpoint are R. crocorum and R. solani. Over the years, a great deal of confusion has dominated the landscape between the two pathogens and their identities, in part because both species infected similar crops such as potatoes and root crops like sugar beets and carrots.
Rhizoctonia crocorum
The first description of a disease that can be referred to with any certainty today as being caused by Rhizoctonia is that of Henri-Louis Du Hammel de Monceau in 1728, describing a disease of the crocus plant (Crocus sativa), source of the spice saffron, in southern France. Diseased bulbs were covered with a thick reddish-violet hyphal mat with knot-like swellings that we know today as the overwintering structures called sclerotia. This proved to be the first confirmed, soilborne root pathogen ever identified.
The genus Rhizoctonia was then formally recognized in 1815 by the French mycologist Augustin Pyramus De Candolle. He additionally studied a similar disease of alfalfa, and afterward recognized two species – one on crocus (R. crocorum), and the other in alfalfa and clover that he named R. medicaginis. Both species characteristically produced a mat of violet mycelium on affected plant parts as originally described by Du Hamel. The fungus was named Rhizoctonia (meaning root-killer) because it so rapidly attacked and killed the roots of plants. The two species were later determined to be identical (not separate species) and the name R. corcorum was maintained for both pathogens. The disease they caused was additionally called “violet root rot” due to the purple-colored mycelium and sclerotia produced by the fungus.
Violet Root Rot in Europe
In the mid-1850s a disease was observed first in Germany causing a serious root rot on carrots, mangolds (early sugar beets), and also alfalfa, accompanied by a purple mold on underground plant parts. Infected plants became chlorotic before wilting and often dying rapidly. It was often localized in fields due to its clustered distribution, but still caused numerous plants in large areas of the field to die, resulting in substantial stand loss. This condition was referred to as “Fehlstellen”, translated into English as “dropouts”. The disease in Europe was eventually found capable of infecting at least 47 genera of plants in 21 families (mostly dicots), including alfalfa, asparagus, beets, carrot, clovers, crocus, fennel, geranium, oats, onion, pine, potato, and turnip.
Violet root rot in the U.S.
In the U.S., just as in Europe, the first Rhizoctonia disease to be reported was the violet root rot. Curiously this was reported simultaneously infecting alfalfa roots in 1890 from two different Nebraska locations by two different individuals (Roscoe Pound and W. J. Webber). In 1908, G. F. Freeman then identified the same fungus as a cause of an alfalfa root disease in Kansas called red root rot, which he thought could potentially become the most serious alfalfa problem in the state.
In 1915 it was first reported on potatoes in Oregon, and was additionally found infecting potatoes throughout Western Nebraska between 1916 and 1920, where it was described as “very prevalent” throughout the entire region. It has since been found sporadically in different far-flung areas of the U.S. on potatoes, sugar beets, alfalfa, carrots, and celery, but is still considered to be relatively rare in the U. S., unlike the situation in Europe.
Rhizoctonia solani
R. solani is well known as a soilborne, root-infecting species with an enormous host range and distributed throughout most if not all of the world’s arable landmass. Root diseases caused by this pathogen also tend to become more prevalent and severe when susceptible crops are used in a sequence that permits the build-up of high population densities of the pathogen within cultivated soils. This versatile organism is capable of infecting and causing a myriad of diseases, including seed decay, post- and pre-emergent damping-off, stem cankers, root rot, fruit decay, and in some instances, foliar blights. It is a quite remarkable organism by being so ubiquitous worldwide and incredibly damaging in so many crops, yet still maintaining the vigorous ability for saprophytic growth and survival.
Another long noted trait of the fungus has been its propensity for exhibiting a substantial degree of variation among miscellaneous strains. This wide range of
variability in morphology, pathogenicity, and biology has been a major force in creating the chaotic status of the taxonomic relationships observed among isolates, resulting in numerous separate strains that are associated with large geographic areas, down to different fields within smaller regions, to even different growing seasons with the same field.
Dry Rot Canker
A good example of one of these largely unexplored Rhizoctonia variants is the fungus inducing the dry rot canker disease of sugar beets. It was first reported by B. L. Richards in Utah in 1921, and has since been identified from California, Colorado, Minnesota, Montana, Nebraska, North Dakota, and Wyoming. The disease is caused by a largely uncharacterized strain of R. solani, and typified by localized, dry sunken lesions covering brown spongy material sharply delimited from healthy beet tissue. The surface tissues of the cankers also produce a distinctive series of concentric circles, like a target board.
The pathogen infects plants underground through the root and spreads upward, eventually destroying the crown. The disease still occurs very infrequently throughout the irrigated western United States, and does not require high levels of moisture. Little else is known about the pathogen or this form of root rot, primarily due to its rare appearances. However, it was recently identified from one field in Nebraska again in 2011 and more than a dozen fields in 2013 and 2014.
Several isolates are currently undergoing comparative molecular analyses with other R. solani isolates. Microscopical examination of these isolates uncovered the presence of two nuclei within in hyphal cells (binucleate). Furthermore, preliminary results from R.M. Harveson and M.D. Bolton, indicated that the sequences from the ribosomal DNA ITS region of the dry rot canker isolates exhibited a significant homology (96% identity) with the sequence of a binucleate Rhizoctonia species (AG-Fb). Based on the different symptoms induced, nuclear condition, and DNA sequence similarities with binucleate Rhizoctonia species, the DRC isolates are distinct from the multinucleate R. solani anastomosis groups (AGs) that typically cause root disease in sugar beet.
Rhizoctonia solani in America
The first reported disease attributed to R. solani in the U.S. was a destructive root rot of sugar beet by Louis H. Pammel from Iowa in 1891. Soon after this in 1892, Atkinson described damping-off and stem lesions on cotton caused by a sterile fungus in Alabama, a disease today called “sore shin” and now known to be caused by R. solani. The same pathogen was then reported by Lyon and Wianco a decade later as causing similar serious root rot problems in sugar beets from Nebraska, and it still persists as a problem for producers today.
Conclusions
Because R. solani and other Rhizoctonia fungi remain sterile and rarely produce identifying spores, the classification of these fungi often has been difficult because of a poorly defined taxonomy. There is a growing realization among mycologists now that Rhizoctonia (particularly R. solani) is actually a large fungal complex composed of numerous genetically distinct groups with disparate life histories.
In reality, the two species of Rhizoctonia discussed in this chapter are quite distinct and should not be confused with each other. Yet, a bewildering array of synonyms, erroneous names, and mistaken identities have been published since its first identification in 1728. Once the two species (R. solani and R. corcorum) were studied comparatively, a better understanding of the differences between them was achieved. It is also interesting to note how the two species differed in their incidence, distribution, and damaging effect on both the North American and European continents. In Europe, R. crocorum was predominant while R. solani took precedence in America. To a large extent this relationship is still in place, although R. solani has become increasingly more important throughout Europe over the last decade.