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Canker a serious threat to citrus fruits
By Muhammad Amjad Ali

CITRUS stands as the second most important fruit worldwide after grapes in terms of area and production. Although citrus is very popular, its present status is threatened by a number of problems including low production due to pests. Of all the agricultural pests and diseases that threaten citrus crops, citrus canker is one of the most devastating diseases.

At present the world’s citrus yield is 105 million tones per annum. Pakistan occupies 13th position among the eminent citrus producing countries, with an area of about 200,000 hectares (2.25 per cent of the world), and a yield of two million tones of the fruit per year. In 2006, 22.57 million tones of citrus was produced in the country earning Rs5,394 million foreign exchange.

Citrus stands on top among the 30 fruits grown throughout the country with Punjab dominating in production. About 59 per cent of the total area and 64 per cent of the production among citrus is captured by kinno alone.

Citrus fruit has been reported to prevent liver, lungs and skin cancer, heart diseases, birth defects and contributes to a balance and healthy lifestyle. It is the best source of vitamin C, sugar, amino acids and other nutrients.

Citrus canker is a threatening disease for its growth. Many commercial citrus varieties are moderately to highly susceptible to the disease with kinno being the most vulnerable one. When the disease is severe, defoliation, dieback and fruit drop occur and infected fruits are less valuable or entirely unmarketable.

The disease, caused by the bacterium Xanthomonas axonopodis pv. citri, occurs in large areas of the world’s citrus-growing countries including Pakistan. This disease causes extensive damage to the fruit. The severity of the infection varies with different species and varieties and the prevailing climatic conditions.

Citrus canker is characterised by appearance of lesions on fruit, foliage, and young stems of susceptible cultivars of citrus. On leaves, first appearance is as oily looking, 2–10 mm, similarly sized, circular spots, usually on the abaxial surface. On leaves, stems, thorns and fruit, circular lesions become raised and blister-like, growing into white or yellow spongy pustules. These pustules then darken and thicken into a light tan to brown corky canker, which is rough to the touch (fig).

On stems, pustules may join together to split the epidermis along the stem length, and occasionally girdling of young stems may occur. Older lesions on leaves and fruit tend to have more elevated margins and are at times surrounded by a yellow halo (that may disappear) and a sunken centre. Sunken craters are especially noticeable on fruit, but the lesions do not penetrate far into the rind. Defoliation and premature abscission of affected fruit occurs on heavily infected trees.

In addition this disease results in poor quality and quantity of fruit. It also causes heavy economic losses to the growers. Citrus canker is a severe disease of several species of citrus and cultivars in many tropical and subtropical areas. In such areas, infected nursery plants constitute an important source of primary inoculums for newly established citrus groves. The majority of epidemiology studies on citrus canker have concentrated on local disease increase and spread of bacteria within citrus nurseries and commercial plantations. In citrus nurseries dissemination is primarily by splash dispersal.

Insofar as the control of this disease is concerned, in regions where citrus canker is endemic, integrated control measures rely most heavily on the planting of resistant varieties of citrus.

Local or regional eradication may be practiced to establish and maintain areas free of citrus canker for the planting of new orchards with cultivars of low to moderate susceptibility. Production of pathogen-free bud wood of citrus cultivars is the foundation of the integrated management programme for production of citrus cultivars with field resistance to citrus canker.

Cultural practices including windbreaks, and pruning or defoliation of diseased summer and autumn shoots, are recognised throughout the world as important measures for the management of citrus canker.

In case of chemical control, citrus canker is managed with preventive sprays of copper-based bactericides i.e. copper oxychloride. Such bactericides are used to reduce inoculums build up on new leaf flushes and to protect expanding fruit surfaces from infection. Effective suppression of the disease by copper-based sprays depends on several factors, such as the susceptibility of the citrus cultivar, environmental conditions, and adoption of other control measures.

The timing and number of copper-based sprays for effective control of the disease also depends on the same factors. In general, three to five sprays are necessary for effective control of citrus canker on citrus cultivars with intermediate levels of resistance, whereas, in years with weather that is highly conducive for epidemic development of citrus canker; up to six sprays may be recommended.

Moreover, spraying neem cake solution (50g/litre water) during rainy season at 15 days interval reduces the disease incidence and increases the shoot and leaf growth. The incidence of leaf miner, which is involved in spreading of the disease, should essentially be checked by means of insecticides. The infected twigs should be pruned and emerging flushes should be sprayed with one per cent Bordeaux mixture or streptomycin should be applied at 20-25 days interval. Antibiotics like agrimycin and streptomycin may also be used.

In orchard production areas like Sargodha, Faisalabad, Islamabad-Rawalpindi, Lahore, Sahiwal, Multan and Bahawalpur, there is need to prevent or reduce the risk of citrus canker epidemics through the establishment of windbreaks, construction of fences to restrict the bacterial access to the orchards, and the use of antibiotics and preventive copper based sprays.

Courtesy: The Dawn

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