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Symptoms appear on leaves of young plants, after planting out, 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, known as kresek, results in wilting, desiccation of leaves and death, particularly of young transplanted plants. In older plants, the leaves become yellow and then die. In later stages, the disease may be difficult to distinguish from bacterial leaf streak (X. oryzae pv. oryzicola). (From EPPO/CABI, 1992; further information is given by Ou, 1985.)
Phytosanitary control measures are important (see Phytosanitary Risk): seed from infested areas is either prohibited or should come from pathogen-free crops and be subject to further testing for the pathogen.
For control in the field, use of seed from uninfected plants (or seed treatments), resistant varieties (as available) and careful attention to crop management (for example, by water control, avoidance of damage to seedlings) are most important. Awoderu et al. (1991) also place importance on restricting nitrogen fertilizer applications to about (80-100 kg N)/ha.
There is intensive research on mechanisms of host resistance to bacterial leaf blight and intensive breeding work. Chemical control is being attempted by the use of bactericides and antibiotics, particularly in Japan, but has not yet reached the stage of general recommendations. There are possibilities for biological control by fluorescent pseudomonads (Anuratha and Gnanamanickam, 1987) and bacteriocinogenic strains of X. oryzae pv. oryzae (Sakthivel and Mew, 1991).
Bacterial leaf blight is the most damaging disease of rice in South and South-East Asia and Japan, particularly since the introduction of dwarf high-yielding varieties. In Japan, where figures are available, up to 400,000 ha may be affected annually, with losses of 20-30% and up to 50%. Elsewhere, in more tropical regions, where the kresek form occurs, losses are thought to be even higher. Severe infection results in poor grain development, broken rice and deterioration in chemical and nutritional composition (Ou, 1985). In Africa, losses of 2.7-41% in grain yield have been found (Awoderu et al., 1991).
A yield loss simulation (YLS) model was used to simulate the effect of bacterial leaf blight on the growth and yield of rice cv. IR-64. Simulation results of the YLS were then compared to those of the blight model. The YLS was a multipest model in which damage mechanisms were simulated for foliar diseases-bacterial leaf blight/sheath blight, stem borer and weeds. The damage mechanisms of the bacterial leaf blight considered in the model were the reduction in leaf area index and leaf weight. The blight model was based on the Oryza-1 model, and simulates the effect of bacterial leaf blight/sheath blight on rice growth and yield. The major damage mechanisms of bacterial leaf blight considered in the model affect the characteristics of the photosynthesis light response curve. The stem and the storage organ weights were simulated better by the YLS than the blight. The simulated reduction in the total dry matter due to bacterial leaf blight with the YLS and the blight varied by 17.6-22.1 and 6.6-15.5%, respectively, compared to the observed reduction of 11.9-21.4% in different disease treatments. The effect of the disease was thus overestimated by the YLS whereas it was underestimated slightly by the blight. Both the models need further improvement in the quantification of damage mechanisms of bacterial leaf blight (Subhash Chander et al., 2002).