The following points highlight the two main bacterial diseases in plants. The bacterial diseases are: 1. Citrus Canker 2. Potato Scab.
Bacterial Disease # 1. Citrus Canker:
Fawcett and Jenkins (1933) reported that citrus canker disease is originated in India and Java because they have detected canker lesions on the oldest citrus herbaria kept at Royal Botanical Gardens in Kew. England (Citrus medica collected in India between 1827-1831, citrus aurantifolia collected in Java between 1842-1844).
Now the disease known to occurs almost in all citrus growing countries of the world. This disease affects cultivars and hybrids of citrus and citrus relatives including orange, grape-wine, pummeto, mandarin, lemon, lime, tangerine, sour orange and rough lemon. Because of its rapid spread, high potential damage and impact on export and domestic sales, the disease is a significant threat to all citrus growing countries.
Plants infected with citrus canker disease have characteristic lesions on leaves, stems and fruits with raised brown, water soaked margins develop around the necrotic tissues (Fig. 3). A characteristic symptom of the disease on the leave is the yellow halo that surrounds lesion. These lesions start as pin point spots and attain a maximum size of 2-10 mm diameter.
Lesions became visible about 7-10 days after infection on the lower surface of leaves and soon appear on the upper surface. The lesion persists on twigs and branches for several years and support long term survival of the of the bacterium. Severely infected fruits can drop prematurely, leading to reduced yield. The internal quality of mature fruit with lesions is unaffected and is still edible and usable for juice.
(a) Causal organism Xanthomonas axonopodis pv. citri and Xanthomonas axonopoids pv. aurantiofolii.
Xantotnonas is small, aerobic, rod shaped mobile bacterium. It is 1.5-2.0 x 0.5-.75 microns in size and has single polar flagellum. It forms chains and capsules but no spores.
Infected twigs having old lesions are the main source of infection. These lesion ooze bacteria which are blown away by the wind or dispersed by rain. Healthy plants are infected by these bacteria which enters through stomata or wounds (caused by citrus leaf miner Phyllocnistis citrella in feeding activity).
The bacteria, on penetration into the host multiplies in the intercellular spaces, dissolves the middle lamella and gets established into the cortex. Lesion on healthy plants become visible about 7 to 10 days after infection on the underside of the leaves and soon thereafter on the upper surface.
Man is also an important agent of dissemination through infected nursery stock. High mean temperature (20-30°C) coincide with high rain fall favours the development of disease.
1. Eradication of infected trees and burning them.
2. Applying preventive sprays of copper based bacterisides e.g., Kocide 3000.
3. Pruning of infected twigs and leaves during the dry season and then spraying the trees with 1% Bordeaux mixture.
4. Spraying the antibiotics for e.g., streptomycin sulphate and phonomycin.
5. Strictly applying quarantine methods.
6. Replacing susceptible citrus cultivars with resistant cultivars.
7. Growing wind breaks to hinder inoculum dispersal.
8. Many areas of new infections of citrus canker are linked to human and mechanical transmission.
9. Developing transgenic plants for e.g., a Xanthomonas resistance gene from the rice has been transferred into a sweet orange.
Bacterial Disease # 2. Potato Scab:
It is a common bacterial disease of potato tubers. The disease occurs throughout the potato growing regions of the world. The pathogen affects beets, radish and other root crops.
It is a cosmetic disease with no or little effect on the yield. The major loss from the scab is lower market quality because tubers are unsightly and have poor customer appeal. Severe scab reduces the quality of the usable tubers, as more peeling is required.
The first symptoms of the disease, are the appearance of small usually circular, brownish specks or spots (lesions) on the young tubers. These spots soon enlarge, darken, and become corky.
Certain spots may merge to form large scabby areas. These spots may be so numerous as to give a russeted appearance to entire tuber. Pitted scab develops where lesions develop up to half inch deep. These deep spots are dark brown to black in colour.
Causal organism: Streptomyces scabies.
It is a saprophytic bacterium. It is represented by branched mycelium with few or no cross walls. The mycelium produces spores on specialised, spiral sporogenous hyphae. The spores produce one or two germ tubes (Fig. 4).
The disease spreads through soil water, by wind blown soil and infected seed tubers. The pathogen overwinters in the soil on infected plant tissues. The pathogen can also survive in the digestive tract of animals.
The bacterium produces spores which penetrate the tissues of young tuber directly, through lenticels, wounds, stomata and form a mycelium. After penetration the pathogen feeds on the peridermal layers causing death of the cells.
Living cells surrounding this tissue divide rapidly and produce several layers of cork cells that isolate the pathogen and other plant cells. These corky layers pushes the infected area outwards. As the potato periderm breaks, a scab is formed.
The vegetative mycelium produce spiral sporogenous hyphae which break into spores. These spores spread the disease. As the first cork layer is penetrated a new one forms below repeating the cycle and resulting in the development of large scab lesions. 5.2 to 8 soil pH favours the spreading of disease (Fig. 4).
1. Use of certified scab free seed potatoes.
2. Keeping soil pH at or below 5.2 will suppress scab.
3. Avoid light textured soil.
4. Planting resistant cultivars, Variety Nooksack is highly resistant to disease.
5. Crop rotation for e.g., rotation of potato crop with alfalfa reduces the scab severity.
6. Keeping soil moist during early tuber development.
7. Chemical pentachloronitrobenzene (PCNB) also known as Blocker (Amvac) or Maneb-zinc dust can be used for seed treatment.
8. Biological control of scab with streptomyces phage.