Plum ranks fourth among important temperate fruits like apple, peach, pear, apricot etc.Plum fruits are used as table fruits as well as in making jelly, jam and preserve. It is cultivated successfully in the moderately high altitude areas of Nagaland, Manipur, Meghalaya and Arunachal Pradesh.
Composition & Uses
Plums are rich in sugars and carotenes. Plum pits which constitute about 2-7 % of the fruit yield up to 26.7 per cent of kernels. The kernels are bitter in taste. A fatty oil can be extracted from the kernels which resembles the expressed almond oil and can be used for lubricating, cooking, as an illuminant and for hair dressing. The aroma of plum blossoms is due to benzaldehyde.The gum present on the fruit surface and that exuding from the bark is composed of D- galactose, D-mannose, L-arabinose, D-xylose, L-rhamnose, glucuronic acid and hexuronic acid.
Plums are used either as fresh desert fruit or cooked. They are also canned, dried and made into jam. Plums containing a high proportion of solids, particularly sugars, are preferred for drying and are known as prunes. Prunes are demulcent, mildly laxative and refrigerant and often added to cathartic decoctions to improve their flavour and make them more effective. Prunes are also canned and used for the preparation of prune pulp and prune beverage. The prune pulp is used in ice-cream mix, confectionery products and meat sauces.
The plum is not very fastidious in its soil requirements. Generally deep sandy loam soils with good internal drainage and freedom from alkalinity and salinity are considered best. European plums, however, have been reported to perform better on heavy loam, rich clayey soils. For prunes, soils particularly rich in potassium should be preferred, as these are heavy potash feeders. In other words, Plum requires deep, fertile and well drained loamy soils.
Plum requires more or less similar climate and soil condition as those for pear. It needs sufficient cold during the winter for proper rest, but the plantation site should be free from spring frost. European plum (Prunus domestica) is cultivated at higher elevations and Japanese plum (Prunus saliciana) at lower elevations in sub mountainous areas. Japanese plum is successfully grown in the hills of Meghalaya, Nagaland, Manipur and Arunachal Pradesh from an altitude of 1000 to 1500 metres above mean sea level. In higher altitudes of Arunachal Pradesh, European plums can thrive well.
Plum is generally propagated by shield –budding during summer, or by tongue grafting during winter, on seedlings of wild apricot, peach and plum. The most important root stocks are myrobalan plum and wild apricot, because these are resistant to nematodes. Plum root stocks are also preferred because of their general graft compatibility and adaptability to many soil types. Only Japanese plums do better on peach rootstocks.
One year old budded plants are generally planted in pits at a distance of 6 X 6 metres during the later part of winter. Pits are prepared and filled earlier during the beginning of monsoon in a manner similar to peach.
Sod mulch system of cultivation practice is generally adopted in most areas, although it grows even if much care is not taken. Remove of weeds in orchard plantations has become very expensive, and moreover, labour is not available. So the recent trends have been shifted to chemical co troll of weeds. Experiments conducted at Himachal Pradesh Krishi Viswa Vidyalaya, Palampur have shown that simazine and atrazine at the rate of 6 kg/ha along with grammaxone at the rate of 2 kg/ha, applied after emergence of weeds, have orchards and can be used on a commercial scale.
Mulching together with herbicidal soil management practices (simazine applied at the rate of 6 kg/ha with post emergence treatment of grammaxone and dry hay 7.5 to 10 cm thick give better growth and yields with optimum fruit weight and size.
Training & Pruning
Plum varieties vary in respect of growth and bearing behaviour. Plum trees normally remain in production for many years and bear heavy crops, which tend to break down the branches due to a heavy load of fruits under the open centre system of training.Therefore; modified central leader system of training is recommended. Several varieties of European plums bear on spurs and require negligible pruning.Consequently, only diseased and crossed branches are removed. The Japanese varieties require more pruning to induce more vegetative growth, and to increase the fruiting surface, because they bear fruits on newly developed shoots of 30 to 50 cm length.
Thinning of Fruits
Fruit thinning is also done in most of the varieties. It not only increases uniformity of fruit size and colour but may also reduce breakage of branches by more evenly distributing the load. Besides, regularity of bearing is accomplished by thinning in April-May, with a spacing of 5-7 cm between fruits, before the pits become hard. Generally the Japanese varieties require more thinning than the European varieties. Of course, the degree of response varies from one variety to another in relation to fruit thinning. For example, beauty needs severe thinning while Santa Rosa requires only light thinning. Best blossom thinning can be obtained with a spray of 200 ppm ethepon.
Manures & Fertilizers
It is important to apply sufficient amount of fertilizers to ensure moderate new growth, especially for the Japanese varieties which bear fruits on one year old branches.However, heavy fertilization delays colour formation and maturity, which are especially important for the early varieties. A fertilizer dose consisting of 500 g nitrogen, 250g of phosphorus and 500 g potassium should be given 20-25 days before flowering to fully bearing trees annually. Farm Yard Manure (FYM) of 35-40 kg of farm yard manure is also incorporated in the soil 25-30 days before the application of fertilizers.
Plum trees come into bearing 5-6 years after planting, and continue to produce for a period of 35-40 years. Flowering starts around the months of February-March and harvesting done from June to August. Fruits should be harvested when almost mature to get the best colour, quality and maximum sugar content. For distant markets, harvesting of fully mature fruits is done, when 50 percent of the surface has developed a red colour and the fruit contains 16-17 percent soluble solids in case of santa rosa variety. Spraying of 500 ppm ethepon (1.250 ml in one litres water) about a week before harvest gives excellent colour and quality of plum fruits. Fruit firmness in Santa Rosa plums can be significantly increased by application of 10 ppm TIBA (1 g in 100 litres water).125 ppm boric acid (1 g in 8 litres of water) is effectively in increasing the weight of the fruits.
Plum tree must have adequate amount of water available throughout the growing season in order to attain proper fruit size and induce sufficient new growth and flower bud differentiation. It is, however, not necessary to maintain soil moisture far above the permanent wilting point, as no advantage in terms of greater proportion of large sized fruit is obtained.If, however, readily available moisture becomes a limiting factor before the fruits are fully grown then the proportion of large sized fruits may be decreased. The plums require readily available moisture through harvest with decreasing amount for the rest of the season. Farmers tend to over-irrigate plum trees which is though not of any particular benefit, some tree losses can occur, especially if peach rootstock has been used. On the other hand, young trees are particularly sensitive to improper scheduling of irrigation. The root system of such plant is very limited and may experience dry condition in the small volume of soil occupied. Even though surrounding soil is wet. Such desiccations for short periods will result in erratic growth flushes and less total growth for the season. The irrigation requirements of plum trees have been estimated to be about 36 acre-inches.
Diseases and Pest
Plum suffers less from diseases and pests in comparison to other temperate fruits.
It is the most common disease of plum in India. It is caused by the bacterium Pseudomonas syringae.The organism enters through damaged portions or wounds caused by pruning. Water lesions develop and cause exudation of gum from bark, outer sap wood and fruits during high atmospheric humidity. It can be prevented by mixing dihydrostreptomycin sulphate in Bordeaux paste used for covering pruning wouns.Spraying of Streptocycline ( 3 g/100 litres water ) before the onset of rains, and copper oxychloride fungicide (300 litres of water) after leaf fall constitute the control measures.
Plum should be picked when mature enough to continue it’s ripening process off the tree. The mature fruit is not eaten ripe at time of harvest, but it can attain the desirable quality following a post harvest ripening period. The appropriate maturity standards vary with the cultivar and the intended use of the fruit. Plums grown for distant markets should be picked when firm ripe. At this stage the fruit is less likely to be injured during handling and storage. Optimum picking maturity varies from cultivar to cultivar.
Some of the indices of maturity that are considered more reliable are:
Per cent fruit surface colouration: In cv. Santa Rosa, fruit colour and firmness were found to be the best indices for picking maturity. With Hary Pickstone fruit colour was found only reliable index and for Sangold colour and TSS.With Gaviota colour, firmness and TSS were found equally reliable indices. Spectrophotometric method: This has been successfully applied to plums.Borsheim (1980) found a strong correlation between visual assessment of fruit maturity and assessment by colorimetric determination. Fruit flesh firmness as determined by a standard pressure tester: Recommended pressures for plums range between 4.1 to 0.1 kg varying with cultivar. Total Soluble Solids (TSS) content of the juice has also been suggested as a reliable index for determining fruit maturity.However, the TSS values varied widely among cultivars. It has been suggested that harvesting of Kala Amritsari plum when the TSS value is around 13.0 per cent. TSS/Acid ratio: The ratio between the two has been considered more reliable criteria for determining fruit maturity than either of them alone.
Plums are very perishable and delicate and hence must be picked with care. They are picked by hand into buckets or baskets with padded liners. So far as possible stems should be allowed to remain attached to the fruit. Mechanical harvesting as yet has not been found feasible with plum. Precaution must be taken not to puncture the fruit with finger nails, etc.The pouring of fruit from one container into another should also be done very carefully and slowly with minimum damage. Care should also be taken to prevent direct exposure of the fruit to sun.
Plums are packed in shallow crates about 4 to 5 inches deep, packing not more than three layers of fruit. After packing, the fruit should be cooled immediately to 0°C.which would stop the ripening process in plums for a period of approximately 12 days.