| CITRUS TECHNICAL | |||
In India, gene pool of citrus totaling more than 500 varieties or species and genera are collected and conserved in field gene bank at different locations. Citrus varieties or species were evaluated for nucellar embryony. Number of embryos/seed and number of seedlings vary from species to species and genera to genera. Mandarins, acid lime, grapefruit (C. paradisi Macf.) have high polyembryony, whereas pummelo [C. grandis (L.) Osbeck] citron and lemon are monoembryonic. Through the evaluation of germplasm degree of sterility, incompatibility in gene pool was identified. Germplasm was screened against insect pest and disease to identify the source of resistance. Resistance to leaf miner is associated with phenology of flushing and presence of phenolic compounds. From the screening of more than 180 accessions against citrus nematode (Tylenchulus semipenetrans) trifoliate orange (Poncirus trifoliata Rafin) and their hybrids were found resistant. High degree of variability for the resistance against Phytophthora sp was observed in gene pool of citrus. Trifoliate orange and sour orange exhibited high degree of resistance against phytophthora root rot (Phytophthora nicotianeae var parasitica). Among Trifoliages, high degree of resistance was observed in Flying Dragon and Argentina trifoliate. None of the cultivars was found resistant to greening, although degree varied. Differential uptake of nutrients among the cultivars of citrus was also observed. Tolerance against salt is associated with differential uptake. The tolerant species had capacity to exclude chloride uptake from the system. Rangpur lime and Cleopatra mandarin were most tolerant to salt. Rangpur lime exhibited high tolerance to drought. The cultivars were assessed for shelf-life and various processed products. Advanced research on citrus in India has led to the adoption of excellent cultivars. It is the result of introductions and testing that Kinnow mandarin, Jaffa and Valencia oranges have gained so much popularity in different regions of the country. Various selections have been made especially in lemon, which is being commercialized. These are Pant lemon, Baramasi and PAU selection. Systematic approach for the selection of better clones in acid lime was initiated in Marathwada, Tamil Nadu and Karnataka. The 3 promising selections, viz. Vikram, Pramalani and Sai Sarbati, were released in Maharashtra and PKM (Jai Devi) in Tamil Nadu. Tenali is highly promising in Andhra Pradesh. Aurangabad Seedless in lemon and Mudkhed Seedless in mandarin were identified. At the IIHR, Bangalore, attempts made to combine canker resistance through hybridization have resulted in selection of promising hybrids having resistance to canker. The following superior cultivars were released: Citrus need adequate nutrients for better productivity, as the fruits are adapted to a wide range of soil types under varying climatic conditions. Lack of nutrients leads to decline. Leaf analysis of citrus orchards in different regions of the country was extensively done to understand the capability of soil to supply nutrients and also to use the leaf-nutrient status as an index for monitoring fertilizer use. The zinc is most critical nutrient among the micronutrients, although nutrients like manganese and iron are found deficient and need attention. In heavy rainfall area calcium and nitrogen nutrition need check. Thus, nitrogen, potassium, calcium and zinc nutrition is an integral part of nutritional management. Assessment of nutritional requirement for mandarin, sweet orange and acid lime under different agro-climatic conditions indicates that citrus require 600-800 g N, 300-400 g P2O5 and 600-800 g K2O plant/year for bearing trees. Experts advise that these nutrients should be applied in the form of cake and inorganic nutrients. Research findings have revealed that application of 20 kg farmyard manure + 7.5 kg Neem cake and 50% dose of inorganic nutrients enhanced the yield and quality of fruits. Most appropriate time of foliar zinc application appears to be the stage when leaves are completing its expansion, i.e. 30-45 days old shoots. Response to manganese, iron, copper and borax was also observed but the results are not consistent which may depend upon the status of these nutrients in the soil. It is invariably observed that trifoliate orange as rootstock, is less efficient in nutrient absorption than Rangpur lime or rough lemon. Troyer citrange and carrizo citrange accumulate less manganese and appear to be useful in acid soils. The maximum root activity is found within the depth of 25 cm and at radial distance of 120 cm in mandarin. Similarly, in Kagzi lime also 80% roots are confined to radial distance of 120 cm. Thus application of fertilizer in this zone is beneficial for better fertilizer-use efficiency. Application of nutrients coinciding with time of flushing is better than the application when the plants are dormant. According to the Department of Agricultural Research and Education, ICAR, Government of India, in 1999-2000, application of 1,200 g N, 400 g each of P2O5 and K2O/plant in mandarin recorded highest yield at Akola. A dose of 800 g N, 200 g P2O5 and 300 g K2O/plant gave a yield of 23.9 kg/tree/year in acid lime. In sweet orange, a dose of 800 g N, 300 g P2O5 and 600 K2O/plant/year gave an yield of 68.6 kg/tree/year at Tirupati. Intercropping should be done to utilize the vacant land to generate income till the citrus plants become productive. However, orchards may start to decline if exhaustive intercropping is used indiscreetly. The choice of intercrop depends upon local conditions and climate. Based on benefit: cost analysis potato, cabbage and peas were found beneficial in mandarin orchards in Darjeeling. Under Coorg conditions, mixed cropping with Robusta coffee was found to be more remunerative then monoculture, though plant vigour of Coorg mandarin was comparatively suppressed. Citrus plants responds well to irrigation. Water management constitutes an important input in citrus orchard, which determines productivity and longevity. Normally no irrigation is practiced in heavy rainfall areas of North-Eastern states. Plants that receive 1 or 2 irrigation during dry period have healthy growth and produce heavier fruits in these areas. At the same time consecutive irrigation without definite period of soil-moisture stress is detrimental to crop yield, as such as situation leads to more vegetative growth. Monitoring of soil moisture is effective for regulating flowering and fruiting in acid lime under tropical situation. Of late drip irrigation is becoming very popular in water-scarce area of Maharashtra, parts of Tamil Nadu and Andhra Pradesh. Many young citrus orchards were planned and planted wit drip system of irrigation. The economic analysis of these 2 systems favoured for the use of drip irrigation, which is viable. The system improved the yield, uniformity in fruits and gave brilliant quality of fruits in respect of shape, colour and edible quality. The growers apply the nitrogenous fertilizer through the drip which further economize on the fertilizer use. Heavy weed infestation depletes the soil and competes for moisture and harbours pest and diseases during rainy season, which cause anoxia resulting in decline of the tree. Mechanical weeding using hoe or spade or plough is common but to economize on use of manpower; experts advise integrated weed management Although a large number of pests and diseases attack citrus crops, only a few are important and cause regular heavy loss and require control measures. Psylla, Diaphornia citri, 2 species of blackfly, viz. Dialeurodes citri and Aleurocanthus woglumi, scales and mealy bugs are the major pests of citrus in India. For avoiding most pest problems, conditions that lead to stress on plants should be avoided such as close planting and water logging. The affected plant parts should be pruned and destroyed. Excessive use of nitrogenous fertilizer and irrigation should also be avoided. Good orchard sanitation, removal of weeds and destruction of ants which help in protecting harmful sap sucking insects have been found useful in reduction of pest infestation. Field release of predatory ladybird beetle, Cryptoleamus montrouzieri @ 10 beetles/plant and inoculative release of exotic parasite, Leptomastrix dactylopii have been recommended in coccid infestation is high. Among nematodes, citrus nematode (Tylenchlus semipenetrans), Paratylenchus coffee, Hopolaimus indicus and Melodogyne javanica are pathogenic to citrus. Out of more than 15 species present infesting citrus orchards, citrus nematode (Tylenchulus semipenetrans) appeared widely distributed in different regions. Application of neem-cake recorded 35.7% reduction in nematode population compared with the control. A fungal parasite, Paecilomyces lilacinus greatly reduces the nematode population under glasshouse conditions. During last few decades diseases caused by fungi, bacteria, virus or viorids or mycoplasma have assumed in alarming proportion in different agro-ecological regions. Among the fungal diseases, foot rot, root rot, gummosis, leaf fall and fruit rot (Phytophothora spp), twig blight (Colletotrichum gloesporioides, Diplodia natelansis and Fusarium sp), powdery mildew (Acrosporium tingitanium), pink disease (Pellicularia salmonicolour), leaf spot (Alernaria citri), scab (Elsinoe fawcetti) and sooty mould (Capnodium citri) have been reported from the different parts of the country with varying intensity. Root rot is one of the serious diseases occurring in India. The fungus associated with root rot is Phytophthora palmivora, P. citrophthora and P. parasitica. However, isolates from the Coorg were identified as Phytophthora nocotiance var parasitica. Use of tolerant rootstock is recommended to combat the disease, although chemical control has also been found effective. Drenching with Ridomil and Foltaf effectively reduces the incidence of the disease. The bacterial canker, caused by Xanthomonas campestris pv citri, is one of the most serious diseases of acid lime prevalent all over the country. The disease is highly infectious spread from tree to tree through the water splashes and affects all the aerial parts of plants (leaves, twigs, petioles, branches, fruit stalks, fruits and thorns). Affected fruit crack is liable to be damaged during the transit. Due to the presence of cankerous tissue the fruit has poor appeal to the consumers. To prevent the disease, only disease-free planting material should be used. Removal of leaf and spraying with copper oxychloride is also beneficial. In India more than 15 virus like diseases occur, of which tristeza and greening are widespread and responsible for significant losses. Transmission citrus tristeza virus (CTV) and host-vector relationship was studied but control of CTV could not be achieved till mild strain was found. Mild strain protects the severe strains of the citrus tristeza virus on acid lime. Presence of mild and severe strains in few mild strain-inoculated plants, infection of uninoculated plants and severe stem pitting in cross-protected plants raised the doubt on the usefulness of cross protection. The effectiveness of mild CTV strain in protecting the plants against the severe strain was not proved in the country, but the benefit of technique is successful in foreign countries. However, this technique shows significance in protecting citrus crop from the citrus tristeza virus. Thus the cross protection using mild strain appears to be a practical method to reduce the losses. Presently the techniques were developed for the identification of mild strain using the sero and molecular diagnostic techniques. It would definitely clarify the situation and help in selection of suitable strains. Another fastidious disease is greening, caused by bacterium, causes substantial loss in production. The antibiotic injection could not control this disease. But chemo- and thermothereaphy were successful up to some extent. Use of disease-free planting material is advocated, but the presence of vector (Diaphorina citri) does not reduce the risk. The work on cross protection against greening was initiated at Ludhiana where inoculum and vectors were present. Considering the problem of virus and virus like diseases and their management, supply of virus-free planting material through the ICAR-sponsored schemes was started. There are evidences of definite yield advantages, prolonged bearing life and increased longevity of plants from virus-free budwood. New orchards using virus-free planting material have greater advantages. Foundation stock of disease-free budwood source and cleaning of virus from time to time are certainly basic requirements of virus-disease management in citrus. Though virus-free planting material may also get infected in field in due course, nevertheless a stage at which the first infection takes place has a definite bearing on the losses caused by virus diseases. Once the plant is established. It can sustain the losses in a good system of management while infected plants may not attain the stage. Micropropagation technique using shoot-tip grafting standardized may help for production of virus-free planting material through effective monitoring system. Maturity of citrus fruits depends upon the climatic condition, scion rootstock and management practices. The extensive work was done on influence of climate, nutrition and rootstock on quality of fruit. The period of maturity is shortest in acid lime (5-6 months), longest in mandarin and sweet oranges (9 to 10 months) which is further influenced by heat unit and moisture. Attempt made to develop maturity standard based on TSS: acid ratio and external colour of fruit are index for harvesting of fruits which goes with the experience of the orchardist. Pre-harvest sprays of chemicals and growth-regulator increase the shelf life of fruits. Calcium chloride or nitrate or spray of hormones (2, 4, 5-T) reduces weight loss and enhances the TSS of fruits in storage, when applied 2 weeks before harvesting. Post-harvest diseases, caused by Penicillium digitatum, P. italicum, Phomopsis citri, Diplodia natalensis and Alernaria citri were found to be major cause for deterioration of fruits during post-harvest handling and storage. Treatment of fruits with 1,000 ppm Bavistin can reduce post-harvest loss appreciably and extend the shelf life to 25-26 days even at ambient temperature. Waxol along with Benomyl or growth-regulators was found beneficial to extend the shelf life of Kinnow. Wrapping of fruits with grease-proof paper or perforated polyethylene bags, packing in wooden crate and storage at 0-3°C temperature and 86-90% relative humidity were found optimum. Best storage temperature for Darjeeling mandarin was also found 1.5-3°C. Degreening at 26-28°C and 90-95% relative humidity with 500 ppm ethylene was achieved in Nagpur santra without affecting internal quality. Shelf-life of Nagpur santra was extended by 3 weeks on wrapping the fruits with heat-shrinkable polythene and Cryovate film (D9 55 and BDF 2001). These developments helped in export of mandarin. Only small percentage of fruits is being processed and there is an immense scope for the rapid development of processing industries with increasing growth of citriculture vis-à-vis. The economy of citrus processing can be improved by waste utilization and the citrus growers can get their due share. There is a scope for high production despite the challenge for which concerted efforts are on the way. |
