Give symptoms, etiology and control of Citrus Canker.

Ans. Citrus canker is a widespread disease in all the citrus growing areas of the world. It is stated to have originated from China and has spread to Europe and the U.S.A. In U.S.A the disease became so severe that mass eradication of diseased plants, often the entire orchards, had to be undertaken to prevent its spread. The disease is still quite serious in India, China, Japan and Java.

Symptoms: The disease occurs on leaves, twigs, thorns, older branches and fruits. Leaf lesions first appear as small, watery, translucent spots. They are raised and become yellowish brown. They first develop on the lower surface of the leaf and then on both the surfaces. As the disease advances the surface of the spots becomes white or grayish and finally ruptures in the centre giving a rough, corky, and crater-like appearance. The spots increase in size (1 mm to 1 cm in diameter) and may coalesce to form elongated lesions on fruits and twigs. The rough lesions are surrounded by a yellowish brown to green raised margin. Spots occurring on petioles and midrib cause premature defoliation. On larger branches the necrotic lesions (cankers) are irregular, rougher, and more prominent. Cankers on fruits are similar to those on leaves and crater-like depression in the centre is more prominent. The injury to fruits is only skin deep and no effect on the pulp or juice is noticed. Cankers on twigs cause them to break.

The causal organism: Xanthomonas citri (Hasse) Dows. Syn. Pseudomonas citri Hasse; Bacterium citri (Hasse) Doidge; Phytonnonas citri (Hasse) Bergey et al.

The bacterium is rod, 1.5 – 2.0 x 0.5-0.75 microns in size. It forms chains and capsule but no spores and is motile by one polar flagellum (monotrichous). It is Gram-negative and aerobic. Colonies on beef agar are circular, straw yellow to amber yellow, slightly raised and glistening. Nitrates are not reduced.

The bacterium enters the host through natural openings (stomata) and through wounds such as those caused by insects, movement of thorns, etc. It multiplies rapidly in the inter-cellular spaces, dissolves the middle lamella and establishes in the cortical region. The disease is favoured by mild temperature and wet weather. Temperatures between 20°C and 30°C with good evenly distributed rains are most suitable. Presence of free moisture on the host surface for at least 20 minutes is essential for successful infection.

The organism does not survive in the soil or in infected plants parts fallen on the ground. This has been shown to be due to antagonism of other soil-microbes to this bacterium under natural conditions. Attacked twigs bearing old lesions on the tree are the main source of perennation of the pathogen. The bacteria from the cankers are mostly disseminated by driving rains and by insects such as citrus leaf miners (Phyllocnistis citrella and Thosconyrsa citri). However, the chief agent of dissemination and introduction into new localities is man himself who transfers the disease through infected nursery stock.

Control measures: The disease is so serious that the only practical method of control is complete destruction of the diseased plants by burning them. Though costly, this method is most effective. In India, however, the disease is well established in most of the orchards and eradication of trees will not be feasible. The usual recommendations made to check the disease are : (i) use of disease-free nursery stock for planting in new orchards, (ii) spraying the plants before planting in new orchards with one per cent Bordeaux mixture, and (iii) in old orchards pruning of the affected twigs and spraying with one percent Bordeaux mixture at periodic intervals, especially during rainy season. The dropped off canker affected leaves and twigs should be collected and burnt. The vigour of the plant should always be maintained by proper irrigation and fertilization. Proper care should be taken to minimize the attack of leaf miners. Quarantine regulations prohibiting movement of diseased stock should be rigidly followed.

Rangaswamy, et al. have shown that the disease can be controlled by antibiotic sprays. Streptomycin sulphate or crude agricultural preparations of streptomycin in concentrations of 500 to 1,000 p.p. m. sprayed at 15 days interval effectively checked the disease on 48 year old lime trees while Bordeaux mixture was not effective. Phytomycin also was effective. Nirvan has also confirmed the above results in respect of streptomycin sulphate. The antibiotics are absorbed by leaves and translocated into the plant system thus functioning as a systemic bactericide.

Vaheeddudin, et al. have reported that spraying of neem-cake at the rate of about 160 lbs. per acre is highly effective in checking citrus canker as well as leaf miner. Fifty pounds of cake is soaked in 20 gallons of water and allowed to decompose for about a week. It is then sprayed without filtration. Some of the cake falls on the ground and becomes manure. Several sprayings are required to produce good results. In experiments, two sprayings at 3 weeks interval during August-September reduced the disease intensity from 5.8 per cent in unsprayed plots to 2.5 in sprayed plots. Bordeaux mixture was not so effective.

Control through resistant varieties is also possible as different species of Citrus show different degrees of susceptibility to the disease. However, no such work has yet been done in India.

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