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Rice Diseases Series(Part 1) – Bacterial Leaf Blight

The cultivated Asian Rice(Oryza sativa L.), being a highly domesticated crop, has several major pathogens due to its long history of dependence on humans. Early breeding efforts during the green revolution focused more on grain yield, and rightly so because of pressing worldwide problem on poverty and population growth. In the past two or three decades however, disease resistance has been included as one of the major objectives of breeding programs worldwide.

Bacterial leaf blight is well known worldwide and is one of the most serious diseases of rice, affecting crops in tropical and temperate regions. Yield reduction may range from 20 to 80 percent depending on the crop stage when infection occurs.
Generally, infection at an earlier stage translates to a higher yield loss, while late infection at booting stage may not be significant but results in poor grain quality and very low head rice recovery. Yield is affected due to the reduction of the plant’s photosynthetic capacity as a result of reduced leaf area index. The lesions on the leaves significantly reduce the leaf area required for photosynthetic activity.

Causal organism and mechanism of damage
The causal organism of the disease is Xanthomonas oryzae pv. oryzae, a rod-shaped, Gram negative, nonspore forming bacterium occurring singly or in pairs. The bacterium infects the plant by entering leaf tissues through natural openings such as stomata, growth openings at the base of leaf sheaths caused by emergence of new roots, and through leaf or root wounds caused by certain management practices particularly during transplanting. The bacteria multiply inside the leaf tissues and invade the vascular system. Some ooze out from water pores.

Factors contributing to infection and development of the disease
The disease is usually prevalent during the wet season due to strong winds and splashing or windblown rain, which may contribute to wounding of plants and favor dissemination of the bacterium from plant to plant. High humidity and warm temperature are also some of the factors favoring the disease.

There are many sources of initial inoculum of the disease. Weeds around the field, rice stubbles, rice straws, and ratooned plants are good sources of inoculum. Surviving bacteria may also be found in the paddy and irrigation canal.

Some crop management practices also contribute to infection. Excessive use of nitrogen fertilizer increases the incidence of the disease. During transplanting, cutting the seedling tips encourages infection through the cut tissue. In fact, breeders and pathologists screen rice varieties for BLB resistance by cutting leaf tissues using a scissor dipped in a suspension of XX oryzae. Flag leaf cutting may also contribute to BLB infection. In hybrid rice seed production, flag leaves of the female plants are cut to increase out-crossing rate but the practice also makes the females vulnerable to BLB infection.

Symptoms
Bacterial leaf blight is very easy to identify in the field. It is categorized into the “leaf streak” diseases because of its characteristic lesion which runs along the length of the leaf blade. The lesions are water-soaked and yellowish, starting on leaf tips and later increases in length and width. Lesions usually have a wavy margin but later coalesce during severe infection to totally occupy the leaf blade. The lesions may turn white as the disease advances. Growth of various species of saprophytic fungi may be evident as greyish spots. Severely infected leaves dry quickly. Bacterial ooze may appear as milky aqueous drops on young lesions early in the morning.

In seedlings, infection is usually a result of handling during transplanting. Symptoms include kresek or seedling wilt, which is similar to deadhearts, a sign of stemborer infestation. Kresek is observed 1-3 weeks after transplanting. Leaves wilt and roll up and become yellowish. In severe infections, the entire seedling completely wilts. Infection at reproductive stage results in unfilled panicles.

BLB can be confirmed by the following method. In the field, collect diseased leaves and cut them near the lower end of the lesions. Place these in a small transparent tube or glass_ 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. The water becomes turbid after a couple of hours.

Prevention and management
The most common and most effective management practice is to plant resistant varieties. Proper field sanitation should also be observed such as eliminating weeds that can serve as alternate hosts, rice stubbles, rice straws and ratoons. Check with your local municipal and barangay ordinances on the proper disposal of rice straws. Burning of rice straws is a violation of the Clean Air Act.

Ratoons, stubbles, and volunteer seedlings can be plowed under during land preparation. Allowing the field to dry thoroughly after harvest is highly recommended. During planting season. maintain shallow water in seedbeds and provide an efficient drainage system for the seedbed and the field. Seed treatment with bleaching powder (100 grams per liter) and 2 percent zinc sulphate may help reduce infection.

Other chemicals such as copper compounds and antibiotics have not been proven to significantly control BLB infection. Farmers in a community may also practice synchronized planting to reduce the occurrence of inoculum over an extended period of time. Proper fertilizer management particularly nitrogen application should be considered as well as proper plant spacing.