By Hafeez ur Rehman, M. Farooq & Dr Nazir Ahmad

The presence of weeds around rice field, rice stubbles, and ratoons of infected plants, warm temperature (25-30°C), high humidity, rain and deep water, severe winds and over fertilisation are factors favourable to the development of this disease.

Symptoms of the disease are observed at the tillering stage. The disease increases with plant growth and reaches its peak at the flowering stage. On seedling, the infection appears as tiny water soaked spot at the margin of the leaves. The spore enlarges to cover the whole leaf, which turn yellow and dry up. On leaf blade, the infection begins at the margin as water soaked stripe. The lesion enlarges in length and breadth resulting in the wavy margin and yellow leaf within few days. It can cover the whole leaf blade; turn white and later grayish contaminated with various saprophytic fungi. Leaves wilt and roll up and become grayish green to yellow during the seedling to the early tillering stage, resulting in a partial or total crop failure.

Seedling wilt can be observed in 1-3 weeks after transplanting. When infection occurs during panicle initiation or subsequently during post flowering stage, a severe impairment of grain development and a consequent increase with panicle sterility become unfilled but not stunted under severe conditions.

Irrigation water and splashing or windblown rain can disseminate the bacterium from plant to plant. Irrigation water is mainly considered to contribute to the spread of this disease. The bacterium or pathogen enters the leaf tissues through natural openings such as water pores on stomata on the leaf blade.

Once the bacterium enters the water pore or any opening on the leaf, it multiples in vessel in which it opens. When there is sufficient bacterial multiplication, some bacteria invade the vascular system and some ooze out from the water pore. The earlier the disease occurs, the higher the yield loss. However, infection at booting stages does not affect yield but results in poor quality and a high proportion of broken kernels.

The severity and significance of damages caused by infection necessitate the development of strategies to control and manage the disease to reduce crop loss. Bacterial blight disease management centers on methods that reduce the initial inoculum and subsequent development of the pathogen on host plants can be accomplished through chemical protection, host plant resistance, and biological control.

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, ploughing under rice stubble and straw following harvest are also management practices that can be followed. Proper application of fertiliser, 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. When different strains of bacteria are present, it is recommended to grow resistant varieties possessing field resistant genes. Seed treatment with bleaching powder (100µg/ml) and zinc sulfate (2 per cent) reduce bacterial blight. Control of the disease with copper compounds, antibiotics and other chemicals has not proven highly effective. The deployment of bio-control agents at the seedling stage may prevent early infection, resulting in increased levels of disease suppression.

However, a systematic search for bio-control agents that would bring about significant bacterial blight suppression in field conditions is necessary for establishment of biological control as a suitable strategy for resource-poor rice farmers.