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Bacterial leaf blight

Causal organisms: Bacteria
Important species:

  • Bacterial blight of bean (Xanthomonas axonopodis pv. phaseoli)
  • Bacterial blight of cotton; Angular leaf spot of cotton (X. axonopodis pv. malvacearum)
  • Bacterial leaf blight of rice (X. oryzae pv. oryzae)
  • Bacterial leaf streak of rice (X. oryzae pv. oryzicola)
  • Bacterial leaf blight of tomato and pepper (X. campestris pv. vesicatoria)
  • Cassava bacterial blight (X. campestris pv. manihotis)
  • Cassava leaf spot (X. cassavae)
  • Bacterial blight of soybean (Pseudomonas savastanoi pv. glycinea; P. syringae pv. glycines)

    Host plants

    Beans, cassava, cotton, cucurbits, rice, tomato, pepper, plantain, and many other secondary host crops

    Affected plant stages

    All growth stages

    Affected plant parts

    Leaves, pods, and fruits

    Symptoms

    Bacterial blight of bean

    Infected seed is wrinkled and shrivelled with its hilum (the scar or eye of a bean or seed that marks its attachment to the stalk) being discolored and has a very poor germination rate. Infected seedling has injured growing tips. The primary leaves have angular watersoaked spots. Infected leaf has watersoaked spots with lemon-yellow or bright-yellow colored margins. As the disease progresses, the spots turn brown and the leaf may fall down prematurely. Infected pod has watersoaked spots with reddish-brown edges. When the infected tissue dries out, a bacterial crust is formed on the surface of the older pods lesions as a result of the drying of the bacterial discharges. A diseased-field shows plants with burnt appearance.

    Bacterial blight of cotton; Angular leaf spot of cotton

    Infected leaf has angular, dark-green watersoaked spots with red to brown margin that will eventually turn dark-brown or black due to death of the infected tissues. Severe infestation leads to premature falling of leaves (defoliation). As the disease progresses, the leaf petiole and stem may become infected resulting in premature defoliation. An infected stem is girdle with black lesions (black arm syndrome) causing it to die and break. An infected boll has round watersoaked spots causing it to rot.

    Bacterial leaf blight of rice

    An infected leaf has yellow watersoaked lesions at the margin of its leaf blade. The lesions run parallel along the leaf and when they join together may cover the whole leaf. Bacterial discharge appears on young lesion early in the morning that looks like a milky dewdrop. As the disease progresses, the leaf dries-up with white lesions and the leaf blade has wavy margins.

    Bacterial leaf streak of rice

    Infected leaf has narrow, dark-green, watersoaked streaks of various lengths initially found on the leaf blade during tillering and booting growth stages. As the disease progresses, the streaks turn to yellowish-gray and transparent. Bacterial discharge appears as numerous yellow beads on the surface of the lesion. In its advance stage, the lesions turn brown to grayish-white and they remain linear on the leaf margins.

    Bacterial leaf blight of tomato and pepper

    Infected tomato leaf has dark watersoaked circular spots which are about 3 mm in sizes but may become larger when the temperature is right. The spots become angular and turn brown-black. Eventually, the centre of the spots may dry and leaf may drop prematurely. Infected young fruit has small black spots. As the bacteria further develop, the spots turn brown, slightly sunken, scabby, and sometimes surrounded by rings. Severe infestation gives the plant the appearance of blight.

    Infected pepper leaf has small, circular pale-green raised spots. Eventually, the spots become chocolate-brown with a paler-brown center on the lower leaf surface. During severe infestation, the plant drops most of its leaves leaving its fruits exposed to direct sunlight. Infected fruit has circular green spots. As the bacteria progresses, the spots turn dark-brown to black and with raised, cracked, scabby surfaces.

    Cassava bacterial blight

    Infested leaf has angular watersoaked spots along its veins, margin, and tip. The infected leaf blade turns brown with the typical watersoaked symptom at the leading edge of the brown patch. As the disease further develops, the spots join together into large patches killing the leaf blade as they expand. The leaf eventually dries and falls down.

    Cassava leaf spot

    The damage is similar to cassava leaf blight wherein infected leaf has watersoaked angular leaf spots that often extend along the veins but without the formation of the small secondary spots progressing into the blighted areas. Larger dead tissues only develop on the leaf blade when several angular spots joined together. As the bacteria mature, the center of the spot turns dark-brown covered with small yellow discharges and becomes surrounded by a narrow watersoaked line and a yellow ring.

    Bacterial blight of soybean

    Infected leaf has small angular watersoaked lesions that are surrounded by yellow-green rings. As the lesions enlarge, they joined together forming dark-brown or black dead areas with yellow margins. Eventually the lesions dry up and tear down giving the leaf ragged and tattered appearance.

    Conditions that favor development

    1. Warm temperature, frequent rain, and high humidity
    2. Over-crowded plants with poor air flow and low sunlight penetration among plants
    3. Improper soil nutrient and irrigation management
    4. Poor soil drainage
    5. Diseased-seeds and planting materials

    Preventive control

    1. Crop rotation with crops that are not susceptible to the bacteria
    2. Use of diseased-free seeds, diseased-free planting materials and cuttings, and use of resistant cultivars.
    3. Hot water seed treatment
    4. Proper fertilization and water management
    5. Proper land preparation for better drainage
    6. Proper plant spacing for proper air circulation and sunlight penetration within plants
    7. Insect pest control as they may serve as the carrier of the bacteria
    8. Weed control
    9. No farm activities when plants are wet
    10. Clean farm tools
    11. Field sanitation
    12. Removal and proper disposal of infected plant parts
    13. Deep plowing to bury plant debris and followed by fallowing the area
    14. Lemongrass extract
    15. Mint extract



    • External links


    References


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