New Rhizoctonia Diseases of Sugar Beets in Nebraska?
Robert M. Harveson, Extension Plant Pathologist University of Nebraska, Panhandle REC, Scottsbluff
Today the genus Rhizoctonia is comprised of a highly diverse group of sterile fungi that still share similar characteristics, namely they remain vegetative, producing no reproductive asexual spores but additionally possess several different sexually reproducing 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, some that routinely cause serious diseases in numerous plant species, and some that are more obscure, both in awareness and occurrence.
Rhizoctonia root and crown rot of sugar beets, caused by Rhizoctonia solani, is well-known to industry personnel throughout Nebraska and other areas of the central high plains, and was in fact, first reported in the U.S. on sugar beets from Iowa. The pathogen is a common soilborne resident in this area, and is generally considered to cause the most widespread, consistently damaging sugar beet disease in Nebraska (Figure 1). The same pathogen causes root diseases on a number of other plant species.
The first description of a disease that can be referred to with any certainty today as being caused by Rhizoctonia is that described by Henri-Louis Du Hammel de Monceau from southern France in 1728. He reported a disease of the crocus plant, Crocus sativa, (thus providing the species name of the fungus). This unusual plant is in the iris family and is the source of the spice
saffron. Diseased bulbs recovered from wilted plants were covered with a thick reddish-violet hyphal mat with knot-like swellings. The generic name of the pathogen, Rhizoctonia, is derived from the Greek, translating into English as "killer of roots" due to the fact that it rapidly attacks and kills the roots of its host plants. A similar root disease was found later causing a serious losses on carrots, mangolds (early sugar beets), and alfalfa, accompanied by a purple mold on subterranean plant parts. The disease shortly became well known throughout Europe and was eventually found afflicting 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. Because of the purple-colored hyphal mats found on below-ground plant parts (Figure 2), the disease became known by various names like violet Rhizoctonia root rot, red root rot, violet root felt disease, or violet root rot.
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 disease was first reported affecting alfalfa roots in 1890 from two different Nebraska locations by two different individuals (Roscoe Pound and W. J. Webber), and was found to be widespread throughout Nebraska on potatoes between 1960 and 1920. It has since been reported from Nebraska on sugar beets, but is still considered to be relatively rare in sugar beet production. Nevertheless, we have been finding this Rhizoctonia disease affecting sugar beets more regularly within the past two years.
Dry Rot Canker (DRC)
Another example of a largely unfamiliar Rhizoctonia disease is 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 an uncharacterized strain of Rhizoctonia and typified by localized, dry sunken lesions covering an internal brown spongy necrotic material that is readily differentiated from healthy beet tissue. The surface tissues of the cankers also produce a distinctive series of concentric circles, like a target board (Figure 3), which is easily contrasted with the typical brown to black lesions associated with the more familiar Rhizoctonia root rot (Figure 1). 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 has been identified repeatedly from Nebraska sugar beet fields (at least two dozen) since 2011.
Microscopical examination of the DRC isolates uncovered the presence of two nuclei within hyphal cells (a condition referred as binucleate). Furthermore, preliminary results from R.M. Harveson and M.D. Bolton, have indicated that small sections (sequences) from the ribosomal DNA, from the ITS (internal transcribed spacer) region of the dry rot canker isolates exhibited a significant homology (96% identity) with the sequence of a binucleate Rhizoctoniaspecies (AG-Fb). The ITS region is commonly used for molecular diagnostics due to a high degree of similarity shared among different organisms within this region, yet often will differ enough that distinctions can still be made between test subjects. Based on the different symptoms induced, nuclear condition, and DNA sequence similarities with binucleate Rhizoctonia species, the DRC isolates are definitely distinct from the multinucleate R. solanianastomosis groups (AGs) that typically cause root disease in sugar beet.
Summary and Conclusions
The implications of these "new" pathogens and their effects on potential disease losses to Nebraska sugar beet producers are presently unknown, but the recently increased incidence of both diseases is cause for some concern and will be the subject of future investigations. We need to determine the relationship of these fungi with Rhizoctonia solani-resistant varieties and whether the currently labeled fungicides are as effective in reducing disease problems as they are with the better-known Rhizoctonia root and crown rot. Additionally we need more information on how common (incidence and distribution) these fungal pathogens are in Nebraska soils, thus further surveys throughout the region are additionally being planned.