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Crop yellowing (IRRI).
Bacterial leaf streak
Yellow streaks (IRRI)
Brown streaks (IRRI)
The linear streaks or narrow transparent streak can be seen against the sunlight. When the advancing part of the streaks are cut and placed in a glass with water, mass of bacterial cells would be seen coming out from the leaf making the water turbid after 5 minutes.
BLS is the only leaf spot disease with transparent narrow streaks as compared with other leaf diseases like brown spot, narrow brown spot, and bacterial blight.
At an early stage, the symptom looks similar to that of the narrow brown leaf spot. At a later stage, when the streaks coalesced, the symptoms of bacterial leaf streak look the same as those of bacterial blight.
Bacterial leaf streak can be distinguished from bacterial blight by its thinner translucent lesions with the yellow bacterial ooze.
The disease is transmitted through seeds to the next planting season. Planting of infected seeds, which are collected from diseased fields produce diseased seedlings. The bacteria, which is present in the water or those surviving in the debris left after harvest, are also sources of inoculum in the next planting season.
The bacterial cells in beads on leaves when moistened by dew or rain disperse and spread by wind cause new infection or damage on the same leaves or other leaves. High temperature and high humidity also favor new infection and development of lesions.
The disease usually occurs during the early stage of planting from maximum tillering to panicle initiation. Older plants are more resistant to the disease.
The bacteria causing the disease X. oryzae pv.oryzicola occur as rods. They are 1.2 x 0.3-0.5 µm in dimension. They are single, occasionally in pairs but not in chains. The bacteria have no spores and no capsules. They move with the aid of a single polar flagellum. They are Gram-negative and aerobic and can grow favorably at 28 °C.
The bacterial colonies on nutrient agar are pale yellow, circular, smooth, convex, and viscid and have an entire margin. Their growth on slant is filiform. Growth in nutrient broth is moderate with a surface ring growth without a definite pellicle.
Species of wild rice such as Oryza spontanea, O. perennis balunga, O. nivara, O. breviligulata, O. glaberrima, and Leersia hexandra Sw. (southern cutgrass) are alternate hosts of the disease.
The bacterium penetrates the leaf through natural openings (such as stomata and hydatodes), leaf injuries or artificial wounds due to wind, insect bites, or others. Initial infection is observed only on the parenchyma cells in between the veins of the leaf. Highest infection occurs at midday, during which the leaf stomata are fully opened. After gaining entrance, the bacterium multiplied in the substomatal cavity and progressed intercellularly in the parenchyma. Soon after lesions develop, bacterial exudates form on the surface of the lesions under moist conditions during the night. Under dry conditions, these exudates become small, yellow beads that eventually fall into the irrigation water. When the leaves are wet from dew or rain, with the aid of wind, the bacteria is carried from field to field by irrigation water.
The disease is usually observed during the tillering stage of the rice crop. The rice plant can easily recover at the later growth stages and grain yield losses are minimal.
Bacterial leaf streak is widely distributed in Taiwan, southern China, Southeast Asian countries, India, and West Africa. The disease is not reported to occur in temperate countries including Japan. Losses as high as 32.3% in 1000-grain weight due to BLS were reported.
At three disease intensities, the estimated yield losses were 8.3%, 13.5%, and 17.1% in the wet season and 1.5%, 5.9%, and 2.5% during the dry season.
The disease can be controlled by proper application of fertilizers and proper planting spacing, the use of resistant varieties, and hot water treated seeds.
Practicing field sanitation is important. Ratoons, straws and volunteer seedlings left after harvest can be destroyed to minimize the initial inoculum at the beginning of the season. Providing good drainage system especially in seedbeds can also manage this disease.
Planting of resistant varieties, which are available at IRRI and at National Research Institute, is the most effective method of controlling bacterial leaf streak. Fallow field and allowing to dry thoroughly is also recommended.
Suparyono, JLA Catindig, FA dela Peña, and IP Oña