Rice-sheath blight is a disease caused by ''

Rhizoctonia solani ''Rhizoctonia solani'' is a species of fungus in the order Cantharellales. 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 ''Rhi ...

'' (

teleomorph In mycology, the terms teleomorph, anamorph, and holomorph apply to portions of the life cycles of fungi in the phyla Ascomycota and Basidiomycota: *Teleomorph: the sexual reproductive stage (morph), typically a fruiting body. *Anamorph: an ase ...

is '' Thanetophorus cucumeris''), a

basidiomycete Basidiomycota () is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya (often referred to as the "higher fungi") within the kingdom Fungi. Members are known as basidiomycetes. More specifically, Basi ...

, 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.

Disease cycle

Infected plants develop circular or oblong lesions, usually green-gray and water-soaked, on their lower leaves, normally in the late tillering or early internode elongation stage of growth. Under favorable conditions of high humidity and low sunlight, the lesions spread and reach the upper part of the plant using runner hyphae. When there is leaf dieback and sun can penetrate and dry the lesions, they turn tan with a brown border. The sclerotia are produced near the infection in about 6 days before falling off. They then survive in the soil, and can be spread as the field is prepared or when it’s flooded for irrigation, enabling them to infect other plants. Both the

sclerotia A sclerotium (; (), is a compact mass of hardened fungal mycelium containing food reserves. One role of sclerotia is to survive environmental extremes. In some higher fungi such as ergot, sclerotia become detached and remain dormant until favor ...

and

mycelia Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates ...

of ''

'' overwinter in plant debris and in tropical environments where they can survive in weed hosts.

Management

Biological control

The use of plant growth promoting rhizobacteria (PGPRs) has been proposed as a form of biological control. They have been used to promote plant growth and control other soil-residing bacteria, and have been seen to control bacterial pathogens by competing for space and nutrients and activating plant defense mechanisms. Studies show that some strains of bacteria, when applied to the rice seed before planting, decrease the viability of the sclerotia of ''R. solani'', reducing instances of disease and increasing yield. Some species of antagonists that could become biocontrol agents are a few strains of ''

Pseudomonas fluorescens ''Pseudomonas fluorescens'' is a common Gram-negative, rod-shaped bacterium. It belongs to the ''Pseudomonas'' genus; 16S rRNA analysis as well as phylogenomic analysis has placed ''P. fluorescens'' in the ''P. fluorescens'' group within the genu ...

'' that inhibit the mycelial growth and sclerotia germination.

Chemical control

The main control method of sheath blight is the use of systemic and nonsystemic fungicides, of systemic are considered more effective. This method produces fewer instances of disease, less inoculum and better yields. One commonly used chemical controls is

azoxystrobin Azoxystrobin is the ISO common name for an organic compound that is used as a fungicide. It is a broad spectrum systemic active ingredient widely used in agriculture to protect crops from fungal diseases. It was first marketed in 1996 using the b ...

, a

QOI Qo inhibitors (QoI), or quinone outside inhibitors, are a group of fungicides used in agriculture. Some of these fungicides are among the most popular in the world. QoI are chemical compounds which act at the quinol ''outer'' binding site of the ...

that prevents the respiration of fungi. As resistant plant cultivars have not been found, and cultural controls are impractical, chemical control is currently considered the best option for controlling the disease.

Pathogenesis

Once the rice sheath has been inoculated, the pathogen forms an appressorium and infection cushions. Both intercellular and intracellular

hyphae A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...

are formed in the epidermal and mesophyll cells. The pathogen then releases many cell wall degrading enzymes (CWDEs) that contribute to lesion formation and spread, including polygalacturonase, cellulase, pectin methylgalacturonase, and polygalacturonic acid trans-eliminase. The ShB pathogen also produces toxins that inhibit rice radicle growth and cause wilting of leaves. The main contributors to

pathogenesis Pathogenesis is the process by which a disease or disorder develops. It can include factors which contribute not only to the onset of the disease or disorder, but also to its progression and maintenance. The word comes from Greek πάθος ''pat ...

by the ShB pathogen are the secreted hormones Cytochrome P450s, a family of enzymes involved in the biosynthesis of plant hormones) and growth hormones.

References

{{Reflist Cantharellales Rice diseases Fungal plant pathogens and diseases