Xanthomonas oryzae pv. oryzae is a bacterial pathovar that causes a serious blight of rice, other grasses, and sedges.
Background
The genus Xanthomonas, which mostly comprises phytopathogenic bacteria, is a member of the family Xanthomonadaceae. Among xanthomonads, X. o. 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.
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.
Research on bacterial blight of rice was begun in Japan as early as in 1901, and those 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.
Hosts
There is a very large host range for Xanthomonas oryzae pv. oryzaa.
Among the grasses, hosts include: Cenchrus ciliaris (Buffelgrass) Cynodon dactylon (Bermuda grass) Echinochloa crus-galli (Barnyard grass) Leersia hexandra (Southern cut grass) Leersia oryzoides (Rice cutgrass) Leersia sayanuka (Rice cutgrass) Leptochloa chinensis (Chinese sprangletop) Oryza (species of rice), including Oryza sativa (Rice) Panicum maximum (Guinea grass) Paspalum scrobiculatum (Ricegrass paspalum) Poaceae (Meadow grasses) Urochloa mutica (Tall panicum) Zizania aquatica (Annual wildrice) Zizania palustris (Northern wild rice (USA)) Zoysia japonica (Zoysiagrass)
Among the Cyperaceae (Sedges), hosts include: Cyperus difformis (Small-flowered nutsedge) Cyperus rotundus (Purple nutsedge)
Symptoms
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. 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 X. o. 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.
Disease cycle
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 plant's 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.
Environment
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.