Rhizoctonia solani is a species of fungus in the orderCantharellales. Basidiocarps (fruit bodies) are thin, effused, and web-like, but the fungus is more typically encountered in its anamorphic state, as hyphae and sclerotia. The name Rhizoctonia solani is currently applied to a complex of related species that await further research. In its wide sense, Rhizoctonia solani is a facultativeplant pathogen with a wide host range and worldwide distribution. It causes various plant diseases such as root rot, damping off, and wire stem. It can also form mycorrhizal associations with orchids.
In 1858, the German plant pathologist Julius Kühn observed and described a fungus on diseased potato tubers and named it Rhizoctonia solani, the species epithet referring to Solanum tuberosum (potato). The disease caused was well known before the discovery and description of the fungus.[1] In 1956, Dutch mycologist M.A. Donk published the new name Thanatephorus cucumeris for the spore-bearing teleomorph of R. solani, based on the species Hypochnus cucumeris originally described from diseased cucumbers in Germany.[2]
Subsequent research has shown that Rhizoctonia solani is a complex of related species.[3] This was originally based on observing hyphal anastomosis (or lack of it) in paired isolates grown in culture. Successful anastomosis indicated that the isolates were genetically similar, whilst unsuccessful anastomosis indicated they were dissimilar and distinct.[4] As a result Rhizoctonia solani has been split into at least 25 different "anastomosis groups" (AGs) and sub-groups.[5] These AGs tend to be associated with different plant diseases.[4][6]
Following changes to the International Code of Nomenclature for algae, fungi, and plants, the practice of giving different names to teleomorph and anamorph forms of the same fungus was discontinued, meaning that Thanatephorus became a synonym of the earlier name Rhizoctonia.[9] In its current sense, therefore, Rhizoctonia solani includes both anamorphic and teleomorphic forms of the fungus. Thanatephorus cucumeris is part of the R. solani species complex, but since it is based on a different type species, it may not be a synonym of R. solanisensu stricto.
Hosts and symptoms
Rhizoctonia solanisensu lato causes a wide range of commercially significant plant diseases. It is one of the fungi responsible for brown patch (a turfgrass disease), damping off (e.g. in soybean seedlings),[10] black scurf of potatoes,[11] bare patch of cereals,[12]root rot of sugar beet,[13] belly rot of cucumber,[14] banded leaf and sheath blight in maize,[15]sheath blight of rice,[16] and many other pathogenic conditions. The fungus, therefore, has a wide host range and strains of R. solani may differ in the hosts they are able to infect, the virulence of infection, selectivity for a given host (which may range from nonpathogenic to highly virulent), the temperature at which infection occurs, the ability to develop in lower soil levels, the ability to form sclerotia, the growth rate, and survival in a certain area. These factors may not always be distinctive in every host that Rhizoctonia attacks or in every strain thereof.[4]
R. solani causing crown rot infection on Beta vulgaris, common beet
R. solani primarily attacks seeds of plants below the soil surface, but can also infect pods, roots, leaves, and stems. The most common symptom of Rhizoctonia is "damping off", or the failure of infected seeds to germinate. R. solani may invade the seed before it has germinated to cause this pre-emergent damping off, or it can kill very young seedlings soon after they emerge from the soil. Seeds that do germinate before being killed by the fungus have reddish-brown lesions and cankers on stems and roots.
Various environmental conditions put plants at higher risk of infection. The pathogen prefers warmer, wet climates for infection and growth. Seedlings are most susceptible to disease in their early stages.[3]
Cereals in regions of England, South Australia, Canada, and India experience losses caused by R. solani every year. Roots are killed back, causing plants to be stunted and spindly. Other non-cereal plants in those regions can experience brown stumps as another symptom of the pathogen. R. solani can also cause hypocotyl and stem cankers on mature plants of tomatoes, potatoes, and cabbages. Strands of mycelium and sometimes sclerotia appear on their surfaces. Roots turn brown and die after a period of time. The best known symptom of R. solani is black scurf on potato tubers, the scurf being the sclerotia of the fungus.
Symptoms on common beans, Rhizoctonia damping off, blight, and rot
Disease cycle
Rhizoctonia solani can survive in the soil for many years in the form of sclerotia. Sclerotia of Rhizoctonia have thick outer layers to allow for survival, and they function as the overwintering structure for the pathogen. In some rare cases (such as the teleomorph) the pathogen may also take on the form of mycelia that reside in the soil, as well. The fungus is attracted to the plant by chemical stimuli released by a growing plant and/or decomposing plant residue. The process of penetration of a host can be accomplished in a number of ways. Entry can occur through direct penetration of the plant cuticle/epidermis or by means of natural openings in the plant. Hyphae come in contact with the plant and attach to the plant by which through growth they begin to produce an appressorium which penetrates the plant cell and allows for the pathogen to obtain nutrients from the plant cell. The pathogen can also release enzymes that break down plant cell walls, and continues to colonize and grow inside dead tissue. This breakdown of the cell walls and colonization of the pathogen within the host forms the sclerotia. New inoculum is produced on or within the host tissue, and a new cycle is repeated when new plants become available. The disease cycle begins as such:
Sclerotia/mycelium overwinter in plant debris, soil, or host plants.
The young hyphae and fruiting basidia (rare) emerge and produce mycelia and rarely basidiospores.
The very rare production of the germinating basidiospores penetrate the stoma, whereas the mycelia land on the plant surface and secrete the necessary enzymes onto the plant surface to initiate invasion of the host plant.
After the mycelia successfully invade the host, necrosis and sclerotia form in and around the infected tissue which then leads to the various symptoms associated with the disease, such as soil rot, stem rot, damping off, etc. and the process begins all over again.[17]
Environment
The pathogen is known to prefer warm, wet weather, and outbreaks typically occur in the early summer months. Most symptoms of the pathogen do not occur until late summer, thus most farmers do not become aware of the diseased crop until harvest. A combination of environmental factors has been linked to the prevalence of the pathogen, such as presence of host plant, frequent rainfall/irrigation, and increased temperatures in spring and summer. In addition, poor drainage of the soil (whether caused by parent soil texture, or by compaction) is also known to create favorable environments for the pathogen.[18] The pathogen is dispersed as sclerotia, and these sclerotia can travel by means of wind, water, or soil movement between host plants.
Identification
R. solani infection on cucumber
Basidiocarps (fruit bodies) are thin, effused, web-like, corticioid, smooth, and ochraceous. Microscopically they have comparatively wide hyphae without clamp connections. Basidia bear 2 to 4 sterigmata. Basidiospores are ellipsoid to oblong, smooth, and colourless, 7 to 10 x 4 to 5.5 μm. They frequently produce secondary spores and germinate by hyphal tubes. The anamorphs consist of hyphae and occasionally sclerotia (small propagules composed of thick-walled hyphae).[6] The fungus produces white to deep brown mycelium when grown on an artificial medium and can often be recognized by the hyphae which are frequently monilioid (forming chains of swollen hyphal compartments), 4 to 15 μm wide, multinucleate, and tend to branch at right angles.
Management
Complete control of Rhizoctonia solani is not possible, but the severity of the pathogen can be limited. Successful control depends on characteristics of the pathogen, host crops, and the environment.[19] Controlling the environment, crop rotation, using resistant varieties,[4] and minimizing soil compaction are effective and non-invasive ways to manage disease. Planting seedlings in warmer soil and getting plants to emerge quickly helps minimize damage. Crop rotation also helps minimize the amount of inoculum that results in infection. A few resistant varieties with moderate resistance to R. solani can be used, but they produce lower yields and quantity than standard varieties. Minimizing soil compaction helps water infiltration, drainage, and aeration for the plants.
One specific chemical option is a chemical spray pentachloronitrobenzene (PCNB), which is known to be the best solution to reducing damping-off of seeds on host plants. To minimize this soil-borne disease, certified seed free of sclerotia can be planted. Although fungicides are not the most effective way to manage this pathogen, a few have been approved in the United States by the USDA for control of the pathogen.
As long as seed growers stay clear of wet, poorly drained areas while also avoiding susceptible crops, R. solani is not usually a problem. Diseases caused by this pathogen are more severe in soils that are moderately wet and a temperature range of 15–18°C (59–64°F).[20]
In the United States, Rhizoctonia solani can be found across all areas (environmental conditions permitting) where its host crops are located. The severity of infection can vary. Consequences include major yield losses (from 25% to 100%), increased soil tare (because the soil sticks to the fungal mycelium), and poor industrial quality of the crops based on increased levels of sodium, potassium, and nitrogen. Due to the number of hosts that the pathogen attacks, these consequences are numerous and detrimental to a variety of crops. Sheath blight caused by this pathogen is the second-most devastating disease after rice blast.[22]
Mycorrhizal association with orchids
Rhizoctonia solani is one of several Rhizoctonia species forming mycorrhizal associations with orchids. This association includes plant pathogenic strains of the fungus[23] as well as non-pathogenic strains.[24]
Genome
The draft genome of R. solani strain Rhs1AP covers 51.7Mbp, although the heterokaryotic genome of this strain was estimated at 86Mb, based on an optical map of the chromosomes. The discrepancy is explained by the aneuploid, highly repetitive genome of this species which prevented sequencing (or assembling) the complete DNA. The genome is predicted to encode 12,726 genes.[25] Another strain, AG1-IB 7/3/14, was recently sequenced too.[26]
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This is a glossary of some of the terms used in phytopathology.
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Pythium ultimum is a plant pathogen. It causes damping off and root rot diseases of hundreds of diverse plant hosts including corn, soybean, potato, wheat, fir, and many ornamental species. P. ultimum belongs to the peronosporalean lineage of oomycetes, along with other important plant pathogens such as Phytophthora spp. and many genera of downy mildews. P. ultimum is a frequent inhabitant of fields, freshwater ponds, and decomposing vegetation in most areas of the world. Contributing to the widespread distribution and persistence of P. ultimum is its ability to grow saprotrophically in soil and plant residue. This trait is also exhibited by most Pythium spp. but not by the related Phytophthora spp., which can only colonize living plant hosts.
Macrophomina phaseolina is a Botryosphaeriaceae plant pathogen fungus that causes damping off, seedling blight, collar rot, stem rot, charcoal rot, basal stem rot, and root rot on many plant species.
Stemphylium solani is a plant pathogen fungus in the phylum Ascomycota. It is the causal pathogen for grey leaf spot in tomatoes and leaf blight in alliums and cotton, though a wide range of additional species can serve as hosts. Symptoms include white spots on leaves and stems that progress to sunken red or purple lesions and finally leaf necrosis. S. solani reproduces and spreads through the formation of conidia on conidiophores. The teleomorph name of Stemphyllium is Pleospora though there are no naturally known occurrences of sexual reproduction. Resistant varieties of tomato and cotton are common, though the pathogen remains an important disease in Chinese garlic cultivation.
Ceratobasidium cornigerum is a species of fungus in the order Cantharellales. Basidiocarps are thin, spread on the substrate out like a film (effused) and web-like. An anamorphic state is frequently obtained when isolates are cultured. Ceratobasidium cornigerum is saprotrophic, but is also a facultative plant pathogen, causing a number of economically important crop diseases, and an orchid endomycorrhizal associate. The species is genetically diverse and is sometimes treated as a complex of closely related taxa. DNA research shows the species actually belongs within the genus Rhizoctonia.
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Colletotrichum coccodes is a plant pathogen, which causes anthracnose on tomato and black dot disease of potato. Fungi survive on crop debris and disease emergence is favored by warm temperatures and wet weather.
Rhizoctonia is a genus of fungi in the order Cantharellales. Species form thin, effused, corticioid basidiocarps, but are most frequently found in their sterile, anamorphic state. Rhizoctonia species are saprotrophic, but some are also facultative plant pathogens, causing commercially important crop diseases. Some are also endomycorrhizal associates of orchids. The genus name was formerly used to accommodate many superficially similar, but unrelated fungi.
Helicobasidium is a genus of fungi in the subdivision Pucciniomycotina. Basidiocarps are corticioid (patch-forming) and are typically violet to purple. Microscopically they have auricularioid basidia. Asexual anamorphs, formerly referred to the genus Thanatophytum, produce sclerotia. Conidia-bearing anamorphs are parasitic on rust fungi and are currently still referred to the genus Tuberculina.
Stromatinia cepivora is a fungus in the division Ascomycota. It is the teleomorph of Sclerotium cepivorum, the cause of white rot in onions, garlic, and leeks. The infective sclerotia remain viable in the soil for many years and are stimulated to germinate by the presence of a susceptible crop.
Collar rot is a symptomatically described disease that is usually caused by any one of various fungal and oomycete plant pathogens. It is present where the pathogen causes a lesion localized at or about the collet between the stem and the root. The lesions develop around the stem eventually forming a "collar". Observationally, collar rot grades into "basal stem rot", and with some pathogens is the first phase of "basal stem rot" often followed by "root rot". Collar rot is most often observed in seedings grown in infected soil. The pathogens that cause collar rot may be species or genera specific. But generalist pathogens such as Agroathelia rolfsii are known to attack over 200 different species. While bacteria caused collar rot is not common, trees infected with Fire blight may develop collar rot. Non-parasitic collar rot may be caused by winter damage.
Rice-sheath blight is a disease caused by Rhizoctonia solani, a basidiomycete, that causes major limitations on rice production in India and other countries of Asia. It is also a problem in the southern US, where rice is also produced. It can decrease yield up to 50%, and reduce its quality. It causes lesions on the rice plant, and can also cause pre- and post-emergence seedling blight, banded leaf blight, panicle infection and spotted seed.
Agroathelia rolfsii is a corticioid fungus in the order Amylocorticiales. It is a facultative plant pathogen and is the causal agent of "southern blight" disease in crops.
References
↑ [Parmeter, J. R. Rhizoctonia solani, Biology and Pathology. London, UK: University of California, 1970. Print.], University of California Biology and Pathology.
↑ Donk MA (1956). "Notes on resupinate fungi II. The tulasnelloid fungi". Reinwardtia. 3: 363–379.
↑ Ogoshi A (1996). "Introduction—the genus Rhizoctonia". Rhizoctonia Species: Taxonomy, Molecular Biology, Ecology, Pathology and Disease Control: 1–9.
1 2 Roberts P. (1999). Rhizoctonia-forming fungi. Kew: Royal Botanic Gardens. p.239. ISBN1-900347-69-5.
↑ Gonzalez D, Carling DE, Kuninaga S, Vilgalys R, Cubeta MA (2001). "Ribosomal DNA systematics of Ceratobasidium and Thanatephorus with Rhizoctonia anamorphs". Mycologia. 93 (6): 1138–1150. doi:10.1080/00275514.2001.12063247. S2CID196619800.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Sharon M, Sneh B, Kuninaga S, Hyakumachi M, Naito S (2008). "Classification of Rhizoctonia spp. using rDNA-ITS sequence analysis supports the genetic basis of the classical anastomosis grouping". Mycoscience. 49 (2): 93–114. doi:10.1007/S10267-007-0394-0. S2CID86120090.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ 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 (4): 763–776. doi:10.1007/s11557-013-0936-0. S2CID255319267.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Williamson B. Hadley G (1970). "Penetration and infection of orchid protocorms by Thanatephorus cucumeris". Pathology. 60: 1092–1096.
↑ Carling DE, Pope EJ, Brainard KA, Carter DA (1999). "Characterization of mycorrhizal isolates of Rhizoctonia solani from an orchid, including AG-12, a new anastomosis group". Phytopathology. 89 (10): 942–946. doi:10.1094/PHYTO.1999.89.10.942. PMID18944739.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Wibberg D, Rupp O, Jelonek L, Kröber M, Verwaaijen B, Blom J, Winkler A, Goesmann A, Grosch R, Pühler A, Schlüter A (2015). "Improved genome sequence of the phytopathogenic fungus Rhizoctonia solani AG1-IB 7/3/14 as established by deep mate-pair sequencing on the MiSeq (Illumina) system". J. Biotechnol. 203: 19–21. doi:10.1016/j.jbiotec.2015.03.005. PMID25801332.
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