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Rhizoctonia solani/ Thanatephorus cucumeris

R. solani was described by Julius Kühn on potato in1858

 

Characteristics and Identification

The vegetative mycelium of R. solani is colorless when young bout brown colored as they grow mature. The mycelium consists of hyphae portioned into individual cells by a septum containing a dough-nut shape pore. This septal pore often allows for the movement of cytoplasm, mitochondria, and nuclei from cell to cell. The hyphae often branch at 90° angles and usually possess more than three nuclei per hyphal cell.  

The anatomy of the septal pore and the cellular nuclear number has been used extensively to differentiate R. solani from other Rhizoctonia fungi. Because R. solani and other Rhizoctonia fungi do not produce conidia and only rarely produce basidiospores the classification of these fungi often has been difficult. Today they are identified by hyphal anastomosis. The concept implies that isolates of Rhizoctonia that have the ability to recognize and fuse (anastomose) with each other are genetically related. The species complex is composed of many genetically distinct anastomosis groups (in total 13) with very diverse life histories. These groups represent independent evolutionary groupings within R. solani. The anastomosis group AG-1 can be further subdivided into three itraspecific groups based on disease symptoms, cultural characteristics, rDNA similarity and isozymes

 

The taxonomy of the species complex is based on nuclear condition, anastomosis grouping, morphological characters (type of sclerotia, basidia and basidiospores), biology of the organism (host range) and phylogenetics (ITS rDNA)

 

Reproduction and Host Range

R. solani is a Basidiomycet and reproduces maily asexual (by mitosis) reproduction. The asexual spores are called conidia.    Basidiospores (dikaryon) are not enclosed in a fruiting body or mushroom. R. Solani is  a major pathogen infecting Poaceae with a great host spectrum (rice, maize, sorghum).  R. solani Ag-1 IA causes for example rice sheath blight.

 

Dispersal, Attraction and Injection

Sprores cannot be dispersed by wind because slerotia are too heavy. Large distance transport of water or with infected material. As a result in upland rice there are infection in spots and which are then distributing. In paddy rice there is random infection. R. solani as a soilborn fungi primarily attacks below ground plant parts such as seeds, hypocotyls and roots, but it is also capable of infecting above ground plant parts. The fungus is attracted to the plant by chemical stimulants released by actively growing plant cells and decomposing plant residue. As the attraction process proceeds, the fungal hypha come in contact with the plant and become attached to its external surface (The mycelium/sclerotia of the fungus are close to or splashed on the plant tissue). After attachment, the fungus continues to grow on the external surface of the plant and will cause disease by producing a specialized infection structure (appressorium) that penetrates the plant cell and releases nutrients for continued fungal growth and development. The infection process is promoted by the production of extracellular enzymes that degrade the plant cell wall (cellulose, cutin, pectin). As the fungus kills the plant cells, the hyphae continue to grow and colonize dead tissue, often forming sclerotia.

R. solani prduces sclerotia in soil and on plant tissue which survive for many years.  The rounded bodies are able to survive periods of adverse environment and germinate when conditions are more favourable.     Some rice pathogens of R. solani have evolved the ability to produce sclerotia with a thick outer layer that allows them to float and survive in water.   R. solani also survives as mycelium by colonizing soil organic matter as a saprophyte.

 

Symptoms

The most common symptom is damping-off characterized by non-germination of severely infected seed. Infected seedlings are not killed by the fungus but often have cankers, which are reddish-brown lesions on stem and roots.

 

Resistance breeding

Breeding resistance to leaf/ sheath blight into crops (maize, ice, soybean) is difficult because resistance is expressed quantitatively. Thus the knowledge of the population biology of the pathogen is needed to implement more effective control strategies.

July 31st, 2009
Topic: Crop health management, Crop Science Tags: None

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