Sheath blight of rice

Rice-sheath blight is a disease caused by Rhizoctonia solani (teleomorph is Thanetophorus cucumeris), a basidiomycete, that causes major limitations on rice production in India and other countries of Asia.[1] It is also a problem in the southern US, where rice is also produced.[2] It can decrease yield up to 50%, and reduce its quality.[3] 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.[4]

A rice leaf exhibiting typical watermark lesions associated with sheath blight disease

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.[3] 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.[5] Both the sclerotia and mycelia of Rhizoctonia solani overwinter in plant debris and in tropical environments where they can survive in weed hosts.[6]

Management

Cultural control

Breeding resistant cultivars has not been successful. Few wild species of rice have the necessary resistance genes. It's widely believed that resistance may be found in multiple genes, or that it is controlled by quantitative trait loci; however, these resources have yet to be found, making resistance breeding difficult.[7]

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.[7] 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.[1] Some species of antagonists that could become biocontrol agents are a few strains of Pseudomonas fluorescens that inhibit the mycelial growth and sclerotia germination.[1]

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, a QOI that prevents the respiration of fungi.[8]

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.[9]

Pathogenesis

Once the rice sheath has been inoculated, the pathogen forms an appressorium and infection cushions. Both intercellular and intracellular hyphae 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 by the ShB pathogen are the secreted hormones Cytochrome P450s, a family of enzymes involved in the biosynthesis of plant hormones[10]) and growth hormones.[11]

References
  1. Devi, T. Vasantha; Vizhi, R. Malar; Sakthivel, N.; Gnanamanickam, S. S. (Oct 1989). "Biological control of sheath-blight of rice in india with antagonistic bacteria". Plant and Soil. 119 (2): 325–330. doi:10.1007/BF02370425. ISSN 0032-079X. S2CID 24890314.
  2. Jones, R. K. (1989). "Characterization and Pathogenicity of Rhizoctonia spp. Isolated from Rice, Soybean, and Other Crops Grown in Rotation with Rice in Texas". Plant Disease. 73 (12): 1004. doi:10.1094/pd-73-1004. ISSN 0191-2917.
  3. Lee, Fleet N. (1983). "Rice Sheath Blight: A Major Rice Disease". Plant Disease. 67 (7): 829. doi:10.1094/pd-67-829. ISSN 0191-2917.
  4. Kumar, Dharmendra; Amaresh Gouda, S. (July 2018). "Evaluation of mycoparasitic efficacy of nematode-trapping fungi against Rhizoctonia solani inciting sheath blight disease in rice (Oryza sativa L.)". Biological Control. 122: 31–40. doi:10.1016/j.biocontrol.2018.04.003. ISSN 1049-9644.
  5. G., R.V.; Webster, Robert K.; Gunnell, Pamela S. (Nov 1992). "Compendium of Rice Diseases". Mycologia. 84 (6): 953. doi:10.2307/3760308. ISSN 0027-5514. JSTOR 3760308.
  6. Vijay Krishna Kumar, K.; Yellareddygari, S.K.R.; Reddy, M.S.; Kloepper, J.W.; Lawrence, K.S.; Zhou, X.G.; Sudini, H.; Groth, D.E.; Krishnam Raju, S.; Miller, M.E. (Mar 2012). "Efficacy of Bacillus subtilis MBI 600 Against Sheath Blight Caused by Rhizoctonia solani and on Growth and Yield of Rice" (PDF). Rice Science. 19 (1): 55–63. doi:10.1016/s1672-6308(12)60021-3. ISSN 1672-6308.
  7. "Rice Sheath Blight: A Review of Disease and Pathogen Management Approaches". Journal of Plant Pathology & Microbiology.
  8. "Rice Sheath Blight". Rice Sheath Blight. Retrieved 2019-12-15.
  9. Hashiba, Teruyoshi; Kobayashi, Takashi (1996), "Rice Diseases Incited by Rhizoctonia Species", Rhizoctonia Species: Taxonomy, Molecular Biology, Ecology, Pathology and Disease Control, Springer Netherlands, pp. 331–340, doi:10.1007/978-94-017-2901-7_30, ISBN 978-90-481-4597-3
  10. Kim, G.-T.; Tsukaya, H. (2002). "Regulation of the biosynthesis of plant hormones by cytochrome P450s". Journal of Plant Research. 115 (3): 169–77. doi:10.1007/s102650200022. PMID 12579366. S2CID 1267602.
  11. Zheng, Aiping; Lin, Runmao; Zhang, Danhua; Qin, Peigang; Xu, Lizhi; Ai, Peng; Ding, Lei; Wang, Yanran; Chen, Yao; Liu, Yao; Sun, Zhigang (2013-01-29). "The evolution and pathogenic mechanisms of the rice sheath blight pathogen". Nature Communications. 4 (1): 1424. Bibcode:2013NatCo...4.1424Z. doi:10.1038/ncomms2427. ISSN 2041-1723. PMC 3562461. PMID 23361014.
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