Rhizoctonia solani (Black scurf and stem canker in potato)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Basidiomycota
Subphylum	Agaricomycotina
Class	Agaricomycetes
Order	Cantharellales
Family	Ceratobasidiaceae
Genus	Rhizoctonia

Scientific Name

Rhizoctonia solani

Scientific Name Synonyms

Moniliopsis aderholdii

Corticium solani

Rhizoctonia napaeae

Thanatephorus cucumeris

Common Name

Rhizoctonia damping-off, blight and rot

Author: Tatiana Sanchez, University of Florida

Reviewed by: Jeffrey Rollins, University of Florida

Pathogen

The type species for the genus Rhizoctonia was initially the species Rhizoctonia crocorum described by DeCandolle in 1815. It was later changed to R. solani by the International Code of Botanical Nomenclature (8). Rhizoctonia solani, described by Kuhn in 1858, is the most economically important species of the genus Rhizoctonia. Following the description of R. solani, a number of studies were conducted on this fungus and anastomosis groups (AGs) (sometimes also referred as intraspecific groups –ISGs), investigating the ability of different isolates to fuse together interchanging genetic material (7, 8). A total of 13 anastomosis groups had been described for this species (5). Many studies conducted from 1960 to the 1970’s, exemplified the wide geographical distribution of R. solani as well as its broad host range (8). The lack of distinctive morphological structures of this anamorph, combined with a lack of technology, lead to an incorrect description of numerous isolates that were all merged into the species complex R. solani.

Taxa belonging to the Rhizoctonia solani species complex have been rearranged and divided into three groups: 1) multinucleate Rhizoctonia with hyphae of approximately 6-10 µm in diameter with teleomorph in the genus Thanatephorus Donk; 2) binucleate Rhizoctonia with hyphae between 4-7 µm in diameter with teleomorphs in the genus Ceratobasidium Rogers; and 3) multinucleate Rhizoctonia with teleomorph in the genus Waitea Warcup & Talbot (7). This article will focus on R. solani (Thanatephorus cucumeris teleomorph) on potato causing black scurf and stem canker. Most of the R. solani isolates that infect potato belong primarily to the anatomosis group 3 (AG-3) although, there are also pathogenic isolates in other AGs for this crop. Isolates belonging to the AG-3, cause stem canker and black scurf on potato (5).

Symptoms and Signs

Symptoms can be observed on above and below ground plant parts. Symptoms observed above ground early in the season include necrosis at the tips of the sprouts (which may eventually cause the emerging plant to die) and sunken lesions on stolons, roots, and stems. Later in the season, sclerotia are produced in the tubers creating a sign called black scurf which is simply, sclerotized mycelium (5, 9). Stems with cankers can become girdled, resulting in stunted plants. Leaves of infected plants develop a purplish and chlorotic coloration. In severe infections, green tubers develop above the ground. Affected tubers are deformed and can produce sclerotia on the surface (9).

Ecology and Spread

Rhizoctonia solani is a soil pathogen and thus is affected by soil conditions. Black scurf and stem canker are more severe in soils that are cool and moist. Development of these diseases is favored by soil temperatures between 16 to 23 °C, while soil temperatures above 25°C reduce the severity of canker (1). The disease also tends to be more severe on dry light soils.The sexual stage of this pathogen can be found in infected potato plants but is not clear the role of basidiospores in the epidemiology of stem canker and black scurf in potato (7). It is believed that R. solani sclerotia (i.e., resistant structures) are the structures responsible for long distance dispersal (2).

Life cycle of R. solani on potato: Production of asexual spores is not observed in R. solani (3). Sclerotia formed at the end of the cropping season, allow the fungus to overwinter. The fungus can also survive on infested tubers left on the ground, soil, and debris. During the spring, potato plants from any stage can become infected by mycelia of the pathogen that spread underground from plant to plant.

Geographic Distribution

Rhizoctonia solani is a cosmopolitan fungus and infects a wide variety of plant families (4). Most of the isolates responsible for potato stem canker and black scurf are found ubiquitously and within the AG-3 (9).

Management

Management of the disease should not rely solely on chemical control. It must be achieved through the combination of multiple management strategies:

• Conduct a soil test prior to establishment of the crop to know nutrient content and soil pH. Low pH is not conducive for disease development.
• Use disease-free soil. Avoid areas with history of potato production or history of potato scurf and stem canker.
• Use certified potato seed and assure disease-free propagation material. If seed is not certified, it should be treated with antagonists or fungicides before planting.
• Perform long rotations (i.e., 3 or more years) to produce a significant reduction on the inoculum of R. solani. Rotations with canola, barley, or sweet corn have been recommended in the literature for reduction of Rhizoctonia inoculum and enhancement in potato quality.
• Separate tubers from crop residues after destruction of the stem to prevent an increase in black scurf.
• Plant less susceptible cultivars. There are differences in susceptibilities among potato cultivars, however resistant cultivars have not been developed yet.
• Consult your local extension specialist for legal and efficacious fungicide products available in your state. Remember, the label is the law and the product applicator is responsible for reading and following all chemical labeling.

Diagnostic Procedures

Typical characteristics of this genus include brown pigmented hyphae and constrictions at the base of the branching hyphae, which are oriented at a right angle from the main hypha. The hyphae are broad and multinucleate, lack clamp connections, and are usually fast growing. Hyphae septa have dolipores with discontinuous parenthesomes. The hyphae are monilioid and sclerotia are produced in culture (3, 6). The lack of diagnostic morphological characters at the species level makes morphological differentiation among different species difficult. However, isolates can be classified by hyphal interaction, biochemical characteristics, pathogenicity and ultimately, by molecular techniques using genetic markers. Biological tests for pathogenicity are time consuming and require space. Molecular tests can be expensive but can provide a more accurate diagnosis. One of the most appropriate methods for diagnosis of Rhizoctonia to the species level is by ribosomal DNA internal transcribed spacer (ITS) sequence analysis (9).

Resources and References

1. ANDERSON, N. A. 1982. The genetics and pathology of Rhizoctonia-solani. Annual Review of Phytopathology, 20, 329-347.

2. DAS, S., SHAH, F. A., BUTLER, R. C., FALLOON, R. E., STEWART, A., RAIKAR, S. & PITMAN, A. R. 2013. Genetic variability and pathogenicity of Rhizoctonia solani associated with black scurf of potato in New Zealand. Plant Pathology, n/a-n/a.

3. GARCIA, V. G., ONCO, M. A. P. & SUSAN, V. R. 2006. Review. Biology and systematics of the form genus Rhizoctonia. Spanish Journal of Agricultural Research, 4, 55-79.

4. FARR, D.F., & ROSSMAN, A.Y. Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved April 2, 2014, from http://nt.ars-grin.gov/fungaldatabases

5. LEHTONEN, M. J., AHVENNIEMI, P., WILSON, P. S., GERMAN-KINNARI, M. & VALKONEN, J. P. T. 2008. Biological diversity of Rhizoctonia solani (AG-3) in a northern potato-cultivation environment in Finland. Plant Pathology, 57, 141-151.

6. OBERWINKLER, F., RIESS, K., BAUER, R., KIRSCHNER, R. & GARNICA, S. 2013. Taxonomic re-evaluation of the Ceratobasidium-Rhizoctonia complex and Rhizoctonia butinii, a new species attacking spruce. Mycological Progress, 12, 763-776.

7. OGOSHI, A. 1987. Ecology and pathogenicity of anastomosis and intraspecific groups of Rhizoctonia-solani KUHN. Annual Review of Phytopathology, 25, 125-143.

8. SNEH B, J.-H. S., NEATE S, DIJST G 1996. Rhizoctonia Species: Taxonomy, Molecular Biology, Ecology, Pathology and Disease Control, Dordrecht, Netherlands, Kluwer Academic.

9. TSROR, L. 2010. Biology, Epidemiology and Management of Rhizoctonia solani on Potato. Journal of Phytopathology, 158, 649-658.

Acknowledgments

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)