Rice diseases caused by bacterial plant pathogens are difficult to diagnose and much harder to manage than are fungal diseases, mainly because there are fewer chemical control options. The wide range of rice diseases caused by fungal pathogens is managed by a correspondingly broad spectrum of fungicides. However, the only fungicides possessing the additional and complementary activity required to control bacterial diseases are the copper-containing fungicides and used successfully control bacterial diseases in rice.
Bacterial pathogens of rice spread rapidly in moist, humid conditions so it should be no surprise that paddy rice, whether rain-fed or irrigated, is ‘blighted’ by bacterial disease. The two most important and damaging bacterial diseases of rice are bacterial blight caused by (Xanthomonas oryzae pv. oryzae) and bacterial leaf streak caused by Xanthomonas oryzae pv. oryzicola. Both can be found on crops throughout most of the rice growing world including all of Asia where infection rates are inherently high. Crop losses of 50 per cent and 20 per cent have been recorded respectively for bacterial blight and bacterial leaf streak of rice.
Bacterial blight (Xanthomonas oryzae pv. oryzae)
Bacterial blight is most damaging in the early growth stages of the rice plant which literally dries up, wilts and dies and is commonly called the ‘Kresek’ or ‘Wilting’ stage of the disease. On older more established plants the disease begins as water-soaked stripes serval centimetres below the leaf tip or along the margin of the leaf. The affected area subsequently enlarges and becomes chlorotic (turning yellow) within just a few days and is appropriately called the ‘pale yellow leaf’ stage of the disease.
The host range of Xanthomonas oryzae pv. oryzae covers a number of wild grasses and sedges which makes the disease all more difficult to manage in rice crops. Wild host plants of include Leersia oryzoides (rice cutgrass), Zizania latifolia (Manchurian wild rice), Leptochloa chinensis (Asian sprangletop) and Cyperus rotundus (nutgrass or nutsedge)
Persistence and spread
The pathogen survives and persists on rice stubble and alternate weed hosts. It enters the leaves of young plants in the new rice crop through stomata, wounds, hydathodes, cracks at the base of the leaf sheaths and any available wounds or abrasions to the leaf.
Both irrigated and rain-fed lowland rice crops are affected by this disease. Heavy rain accompanied by strong wind is a significant factor in the spread of this bacterial pathogen and disease, and facilitating infection through wounds and abrasions inflicted by inclement weather condition to the plants and leaves. Bacterial exudates fall into the irrigation water and spread the disease to neighbouring fields. High temperature (25°C to 30°C) and high relative humidity (over 70 per cent) combine to increase disease development and spread with recorded crop losses of 50 per cent not uncommon.
Management and control
• Use rice varieties bred for resistance to bacterial blight
• Cultural control should include avoidance of excessive nitrogen fertilization, ensuring shallow water levels in rice nursery beds and the provision of good drainage especially during periods of flooding and also for rice plant nurseries.
• Plough in rice stubble and straw after harvest and keep rice fields as free as possible of alternative weed hosts.
• Allow fallow fields to dry out. This will suppress residual bacteria in the soil and on rice plant residues
• Sprays of copper-containing fungicides have been widely deployed in a prophylactic capacity to protect leaves from infection by bacterial leaf blight
Bacterial Leaf Streak (Xanthomonas oryzae pv. oryzicola
Translucent streaks of variable length and appearing between the leaf veins are the classic, initial foliar symptoms of bacterial leaf streak and becoming light brown in colour with age. Under moist, humid conditions yellowish-coloured droplets of bacterial ooze will appear on the lesions and will come to resemble ‘beads’ as the leaf surface dries out. All wild species of the genus Oryza (rice) are susceptible to infection and these plants will go on to serve as important reservoirs of inoculum.
Persistence and spread
Persistence of the pathogen is primarily on infected rice seed and straw but the bacteria may survive in irrigation water. Infection of rice leaves occurs mainly through stomata and leaf abrasions to establish infections with bacteria subsequently multiplying in the leaf parenchyma tissue. Optimum conditions for infection, disease development and spread are high rainfall and humidity (80 per cent plus) and temperatures over 80°C. Yield losses approaching 20 per cent are not unusual under conditions favourable to bacterial infection and disease development.
Spread of the bacteria occurs when fluid exudates from the leaf lesions are disseminated mainly by the splashing effect of raindrops on the leaf surface, by windblown water droplets but also by direct leaf-to-leaf contact and via contaminated irrigation water. Bacterial leaf streak occurs on rain-fed and irrigated rice cultivated in lowland rice paddy cultivations but also on rain-fed upland rice farming system. This disease is seed borne so infected rice seed, along with contaminated irrigation water supply, are the main vehicles for introduction of the pathogen into previously disease-free fields and areas.
Management and control
• Use rice varieties resistant to bacterial leaf streak
• Remove weed hosts and plough in rice stubble and straw, rice ratoons and volunteer seedlings all of which can be a source of infection for the following crop.
• Provide all essential nutrients in well-balanced fertilizer applications
• Ensure good drainage of fields and nurseries.
• Allow fallow fields to dry out to kill any bacteria in the soil and on plant residues
• Hot water treatment of the rice seed may kill this seed-borne pathogen. Temperature used is critical – high enough to destroy the pathogen but not so high that seed embryos are damaged and killed.
• Copper-based ‘fungicides’ including fixed copper compounds like cuprous oxide are used in a preventative capacity to protect rice crops from this bacterial disease. They have typically been applied as foliar sprays to protect rice plants and leaves from infection. In cases of severe infection, when yield may be affected, a copper-based fungicide applied as a foliar spray at the ‘heading’ stage of rice development can be effective in controlling the disease and minimising loss of grain yield.
Copper at the centre – past and present
There is a long history of copper fungicide use to control disease in rice with Bordeaux Mixture (contains copper sulphate) first used against the rice blast fungus disease over 100 years ago in 1914 in Japan. Reports suggest Bordeaux Mixture was first used around about the same time against bacterial blight of rice and also in Japan.
Sustained use of copper fungicide to control bacterial disease in rice over the last 50 years was somewhat side-lined at times by widespread use of antibiotics as foliar sprays in some East Asian countries and seen as a more effective means of control. However, use of antibiotics in this way is now firmly recognised as a ‘dangerous’ game by contributing to the development of resistance in bacterial populations to the action of antibiotics. And the serious implications of such when the bacteria concerned are closely related to livestock pathogens and even more so to human pathogens, especially if the antibiotics used to spray rice are in the same chemical grouping as those used in medicine.
There has been a more recent resurgence of interest in the use of copper fungicides to control bacterial disease in rice and especially bacterial leaf streak. Foliar sprays of fixed copper fungicide like cuprous oxide are recognised as a useful means of preventative control provided applications are made quite early in the crop cycle and no later than 40 to 50 days after sowing.
Attempts to control bacterial leaf streak with copper fungicides, or for that matter any prophylactic product, after this stage of growth and development is essentially a waste of time. This is because the bacterial pathogen is firmly established deep in the leaf tissue to escape the superficial preventative action of the copper fungicide on the leaf surface. Spraying should be carried out during dry conditions when the leaves are no longer wet from previous rainfall and are not likely to experience rainfall during or soon after spray application. This will minimise weathering pressure on the copper fungicide deposit thereby maximising its longevity on the leaf surface at levels which are adequate for extended protection of the leaves.
Current focus is on cuprous oxide applied as a solid delivery system (formulation) which adheres to rice seed as a ‘seed dressing’. Xanthomonas oryzae pv. oryzicola (bacterial leaf-streak) is a seed-borne pathogen which is killed by the copper-containing seed dressing as it attempts to infect the rice seedling during germination.