Bacterial leaf blight (BB)
Xanthomonas oryzae pv. oryzae
Signs and symptoms
• leaf blight
o Water-soaked to yellowish stripes on leaf blades or starting at leaf tips then later increase in length and width with a wavy margin
o Appearance of bacterial ooze that looks like a milky or opaque dewdrop on young lesions early in the morning
o Lesions turn yellow to white as the disease advances
o Severely infected leaves tend to dry quickly
o Lesions later become grayish from growth of various saprophytic fungi
• seedling wilt or kresek
o Observed 1-3 weeks after transplanting
o Green water-soaked layer along the cut portion or leaf tip of leaves as early symptom
o Leaves wilt and roll up and become grayish green to yellow
o Entire plant wilt completely
• yellow leaf or pale yellow mature plants
o Youngest leaf is uniform pale yellow or has broad yellow stripe
o Older leaves do not show symptoms
• Panicles sterile and unfilled but not stunted under severe conditions
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.
Factors favoring disease development
• presence of weeds
• presence of rice stubbles and ratoons of infected plants
• presence of bacteria in the rice paddy and irrigation canals
• warm temperature, high humidity, rain and deep water
• over fertilization
• handling of seedlings at transplanting
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.