Xanthomonas oryzae pv. oryzae

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Xanthomonas oryzae pv. oryzae
Bacterial blight of rice.jpeg
Xanthomonas oryzae pv. oryzae, Bacterial blight of Rice
Scientific classification
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Gamma Proteobacteria
Order: Xanthomonadales
Family: Xanthomonadaceae
Genus: Xanthomonas
Species: X. oryzae
Variety: X. o. pv. oryzae
Trinomial name
Xanthomonas oryzae pv. oryzae

Xanthomonas oryzae pv. oryzae is a bacterium which causes a serious blight of rice, other grasses and sedges.[1]

Background[edit]

The genus Xanthomonas, which mostly comprises phytopathogenic bacteria, is a member of the family Xanthomonadaceae. Among xanthomonads, Xanthomonas oryzae pv. oryzae causes bacterial blight (BB) of rice which is one of the most important diseases of rice in most of the rice growing countries.[2]

Bacterial blight of rice has high epidemic potential and is destructive to high-yielding cultivars in both temperate and tropical regions especially in Asia. Its occurrence in the 70s in Africa and the Americas has led to concerns about its transmission and dissemination.[3]

Research on bacterial blight of rice was commenced in Japan as early as in 1901, and the efforts were focused mainly on ecological studies and chemical control. Since then, significant gains have been made in understanding BB through analysis of the interactions between X. oryzae pv. oryzae and rice at many levels, including studies focused on the epidemiology, population biology, physiology, cell biology, biochemistry, and molecular genetics of the host pathogen interaction. It is very notable that BB became the first case where the genome sequencing of both host plant and pathogen was completed.[4]

Hosts[edit]

There is a very large host range for Xanthomonas oryzae pv. oryza.[5]

Among the grasses, hosts include:

Among the Cyperaceae (Sedges), hosts include:

Symptoms[edit]

Symptoms appear on the leaves of young plants as pale-green to grey-green, water-soaked streaks near the leaf tip and margins. These lesions coalesce and become yellowish-white with wavy edges. The whole leaf may eventually be affected, becoming whitish or greyish and then dying. Leaf sheaths and culms of more susceptible cultivars may be attacked. Systemic infection results in wilting, desiccation of leaves and death, particularly of young transplanted plants.[6]

In older plants, the leaves become yellow and then die. In its advanced stages, the disease is difficult to distinguish from leaf blight caused by Xanthomonas oryzae pv. oryzicola, but lesion margins are wavy rather than linear as for the former. Damage is often associated with lepidopteran leaf rollers, leaf-folders and hispa beetles, since bacteria readily enter the damaged tissue caused by insect infestation.[6]

Disease Cycle[edit]

Rice plants become infected with Xanthomonas oryzae through rice seed, stem and roots that are left behind at harvest, as well as alternative weed hosts. X. oryzae lives on dead plants and seeds and probably moves plant-to-plant best through pattywater from irrigation or storms. Upon introduction to the host plant, the bacterium infiltrates the plant through natural openings (water pores and growth cracks on roots) and/or leaf and root wounds. X. oryzae grows in the plant and infects the plants leaf veins as well as the xylem causing blockage and plant wilting. Bacteria oozes from leaf lesions and is spread by wind or rain, especially when strong storms occur and cause wounds to plants. X. oryzae has a wide host range that includes Leersia sayanuka which acts as alternative host for the bacterium and are considered the most important source of primary inoculums, as well as a great mechanism for bacterium survival.[7]

Environment[edit]

Xanthomonas oryzae is endemic to Japan, but can also be found throughout the tropical rice producing countries of Asia. In the tropics the pathogen has the highest level of incidence during the rainy season when rain and wind wound crops. Rain and infected pattywater are the main dispersers of the disease therefore fields found in low, wet areas with poor drainage and susceptibility to flooding are areas of high incidence. The presence of Leersia sayanuka, is also key to the spread of disease because it is a naturally growing weed usually found around patties and has the ability to be infected by the bacterium and spread the bacterium through a rice patty.

The use of nitrogenous fertilizer has shown an increase in incidence but mainly because there is more plant growth and conditions stay more humid,[8] but does not have an effect on lesion size During drier weather bacterial ooze will secrete from leaf lesions in hopes of finding a new host. Ideal temperatures for X. oryzae growth are 26-30 °C; 20°C being the best temperature for initial growth. X. oryzae can live in soil with pH range from 4-8.8; optimum pH being 6-6.50.

Importance[edit]

Xanthomonas Oryzae causes a potentially devastating disease. Found worldwide in temperate and tropical regions, it can destroy up to 80 percent of a crop if the disease develops early. Even if it develops late, it can nonetheless severely diminish the quality and yield of the grain.

Bacterial leaf blight is a prevalent and destructive disease which affects millions of hectares throughout Asia.[9] In Japan alone, annual losses are estimated to be between 22,000 and 110,000 tons. In the Philippines, susceptible varieties lose up to 22.5% of the total harvest during wet seasons and up to 7.2% in the dry season. In resistant crops, these numbers are, respectively, 9.5% and 1.8%.[10]

Management[edit]

Management of bacterial leaf blight is most commonly done by planting disease resistant rice plants. PSB Rc82 is the standard variety of rice used in Southeast Asia, and the use of this cultivar enables the harvest of an estimated 0.8 million metric tons of rice per cropping season that would have otherwise been lost to bacterial leaf blight. Macassane, a new variety released in 2011, has been shown to have improved resistance to bacterial leaf blight and is being used currently in Mozambique.[11]

Traditional treatments, such as the applications of copper compounds or antibiotics, are largely ineffective in the control of bacterial leaf blight. Increasingly, rice is being genetically engineered for resistance to the disease, as treatment proves difficult. More than 30 genes have been identified as being associated with resistance to bacterial leaf blight, and have been given names Xa1 to Xa33.[12]

Biological control methods are relatively recent developments which are not currently in common use. They may be used in the future to reduce damage done by bacterial leaf blight, with experimental data showing up to a 64% reduction in damage.[13]

References[edit]

  1. ^ "Bacterial Leaf Blight". Rice Diseases Series(Part 1). Retrieved 24 October 2011. 
  2. ^ Hopkins, C.M.,White, F.F., Choi, S.-H., Guo, A., Leach, J.E., "Identification of a Family of Avirulence Genes from Xanthomonas oryzae pv. oryzae", Molecular Plant-Microbe Interactions, Vol. 5, No. 6, pp.451-495,1992.
  3. ^ AFFRC - Xanthomonas
  4. ^ AFFRC.go.jp - Genome Sequence of Host and Pathogen
  5. ^ a b c NAPPFAST: Xanthomonas oryzae hosts, NAPPFAST, 2007.
  6. ^ a b Invasive: Symptoms of Bacterial Blight, "USDA", May 04, 2010.
  7. ^ Ou, Shu H. “Rice Diseases”
  8. ^ Tagami, Y.; Mizukami, T. “Historical review of the researches on bacterial leaf blight of rice caused by Xanthomonas oryzae” Dowson. Special Report of the Plant Disease and Insect Pest Forecasting Service No. 10, 112
  9. ^ "Bacterial leaf blight affects paddy". The Hindu. 
  10. ^ "Xanthomonas Oryzae". European and Mediterranean Plant Protection Organization. 
  11. ^ "Dynamics of Xanthomonas oryzae pv. oryzae Populations in Korea and Their Relationship to Known Bacterial Blight Resistance Genes". Phytopathology 96: 867–875. doi:10.1094/phyto-96-0867. 
  12. ^ "Bacterial Blight". Disease- and pest-resistant rice. 
  13. ^ "Biological control of rice bacterial blight by plant-associated bacteria producing 2,4-diacetylphloroglucinol". Canadian Journal of Microbiology 52 (1): 56. January 2006. doi:10.1139/w05-106.