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Bacterial leaf blight (BB)
In the field, diseased leaves can be collected and cut near the lower end of the lesions. The cut diseased leaves can be placed in a test tube with water for a few minutes. The cut portion can be observed against the light to see the bacterial ooze streaming out from the cut ends into the water. After 1-2 hours, the water becomes turbid.
To distinguish kresek symptoms from stem borer damage, the lower end of the infected seedling can be squeezed between the fingers. Yellowish bacterial ooze may be seen coming out of the cut ends.
Infected plants show kresek, which resemble rice stem borer damage.
The presence of weeds around the field, the rice stubbles, and ratoons of infected plants sustains survival of the disease. They become sources of initial inoculum. Likewise, the bacteria in the rice paddy and irrigation canals encourage new infection on leaves.
Warm temperature (25-30° C), high humidity, rain and deep water favor the disease. Wetland areas also encourage the presence of the disease. Severe winds, which cause wounds, and over fertilization are suitable factors for the development of the disease.
Irrigation water and splashing or windblown rain can disseminate the bacterium from plant to plant. The use of trimming tools for transplanting and by handling during transplanting can also trigger new infection. For example, the kresek symptom is associated with seedling infection, which was damaged during transplanting operations.
The bacteria causing the disease are rods, 1.2 x 0.3-0.5 µm. They are single, occasionally in pairs but not in chains. They are Gram negative, non-spore-forming, and devoid of capsules. Their colonies on nutrient agar are pale yellow, circular, and smooth with an entire margin. They are convex and viscid.
Leersia sayanuka Ohwi, L. oryzoides (L.) Sw., L. japonica, and Zizania latifolia are alternate hosts of the disease in Japan. In the tropics, the disease is found to infect Leptochola chinensis (L.) Nees, L. filiformis (Lam.) P. Beauv., and L. panicea (Retz.) Ohwi. Cyperus rotundus L. and C. difformis L. are recorded as alternate hosts of the disease in India. In Australia, the disease is known to survive on wild rice, Oryza rifopogon and O. australiensis.
Leaf tips of seedlings cut before transplanting and leaf injuries serve as important sources of inoculum especially for kresek.
The bacterium or pathogen enters the leaf tissues through natural openings such as water pores on hydathodes or stomata on the leaf blade, growth cracks caused by the emergence of new roots at the base of the leaf sheath, and on leaf or root wounds. Once the bacterium enters the water pore or any opening, it multiples in the epitheme, into which the vessel opens. When there is sufficient bacterial multiplication, some bacteria invade the vascular system and some ooze out from the water pore.
Bacterial blight is one of the most serious diseases of rice, which is known worldwide. It is common in both tropical and temperate countries. Strains in tropical areas are more virulent than that of in the temperate region.
Yield loss due to this disease corresponds to the plant growth stages at which the rice plants were infected. The earlier the disease occurs, the higher the yield loss. Infection at booting stages does not affect yield but results in poor quality and a high proportion of broken kernels.
Bacterial blight is reported to have reduced Asia's annual rice production by as much as 60%. For example, in Japan, about 300,000 to 400,000 hectares of rice were affected by the disease in recent years. There were 20% to 50% yield losses reported in severely infected fields. In Indonesia, losses were higher than those reported in Japan. In India, millions of hectares were severely infected, causing yield losses from 6% to 60%.
Practicing field sanitation such as removing weed hosts, rice straws, ratoons, and volunteer seedlings is important to avoid infection caused by this disease. Likewise, maintaining shallow water in nursery beds, providing good drainage during severe flooding, plowing under rice stubble and straw following harvest are also management practices that can be followed. Proper application of fertilizer, especially nitrogen, and proper plant spacing are recommended for the management of bacterial leaf blight.
The use of resistant varieties is the most effective and the most common management practices adopted by farmers in most growing countries in Asia. When different strains of bacteria are present, it is recommended to grow resistant varieties possessing field resistant genes. Fallow field and allow to dry thoroughly is recommended.
Seed treatment with bleaching powder (100µg/ml) and zinc sulfate (2%) reduce bacterial blight. Control of the disease with copper compounds, antibiotics and other chemicals has not proven highly effective.
Suparyono, JLA Catindig, FA dela Peña, and IP Oña