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The apple is one of the most economically and culturally significant fruits in the world today. The actual origin of apples is not known, it is likely the apple tree originated in Kazakhstan, in central Asia east of the Caspian Sea. The capital of Kazakhstan, Alma Ata, means “full of apples.” By 1500 BC apple seeds had been carried throughout Europe. The Greeks, Etruscans, and Romans cultivated apples. To the latter goes the credit, during the early centuries of the Christian era, for having carried apple seeds and trees to the British Isles. Apples arrived also in America with the early settlers of the United States brought apple seeds with them. Nowadays, apple is grown in all temperate zones. The leading producer country is China, followed by the United States. Among the countries of the northern hemisphere, Turkey, Poland, Italy, and India follow while in the Southern Hemisphere, the largest producer is Chile.
The apple tree is a tree plant of medium-high dimensions that it can reach the height of 8-10 meters, however with the use of dwarfing rootstocks, in modern cultivation, there is a marked reduction the size of the tree around 2 m. Tree size is also heavily dependent training system.
It presents creeping and shallow roots, the leaves are alternate, elliptical, with serrate margins, dark green with light pubescence on underside. Gems are manifested both in wood than mixed (wood buds and fruit buds).
The flowers are white with shades pink, gathered in an inflorescence called corymb of 4-9 flowers. The center flower opening first is often called the “King bloom” and has the potential to produce a larger fruit than other flowers. The fruit is a false fruit, called pome. The bulk of the fleshy edible portion derives from the hypanthium or floral cup, not the ovary. It can be of different color and size according to the varieties. The background color of the peel, in ripe fruit, can be green, yellow, red or two-tone; the color of the pulp varies from white to white greenish, cream-white and yellowish. Seeds are relatively small and black, and mildly poisonous.
Most cultivars are commercially self-unfruitful and therefore require entomophilic cross-pollination. For the establishment of an apple orchard in the field, it is therefore necessary that it is composed of several cultivars, of which at least one is a good pollinator. It must produce a rather long flowering, at least contemporary or slightly earlier than the cultivar to be pollinated.
The annual cycle of apple comprises several phenological stages. Apple require a considerable period of dormancy, after that with warmer temperature buds unfold to start the leaf development. The fruiting cycle begins with the flower induction, continue with flowering, fruit-set, fruit development until fruit ripening.
The apple tree is a plant that has a high chill requirement (vernalization), it thrives in areas having a distinct winter period, generally from latitude 30 ° to 60 °, both north and south. This winter chilling requirement, or minimum chill requirement, is usually defined as the number of hours per year where the temperature should be below about 7°C (45°F), but above freezing. Periods when the temperature is substantially below freezing are not thought to be as useful for counting towards chill hours as the period when the temperature is just above freezing. Most apple varieties have a chill requirement of about 1,000 hours or more.
Withstands winter temperatures even below 10-15 °C (50-59 °F) . Temperatures above 30 ° C (86 °F) are tolerated with difficulty. It prefers sub-acid to alkaline soils, up to 7/8 pH value, greater resistance to active limestone than the vine, so it has no major problems. The soils in which apple trees grow must be well drained and fertile. It has medium tolerance to salinity. The depth of the soil is not a very important parameter also because dwarfing rootstocks are used, which do not deepen that much.
The nutrition is probably the most important factor for a successful orchard operation, and it can be controlled through a proper fertilization program. The contribution of macro and micro-nutrients, both at the level of the root system and at the level of the crown of apple trees, has multiple purposes: to favor the early entry into production, to improve production from both quantitative and qualitative standpoint, keeping these parameters constant over the years of production of the plant, making the management of the apple orchard sustainable. The quantities of nutrients removed from the orchard change according to the genotype, the age of the plant, the vegetative development, the pedo-climatic conditions, the productivity, and the fruit growing technique applied.
Nitrogen is the nutrient most used in fruit trees and is usually the first element to be considered in an orchard fertilization program, as shoot growth depends highly on nitrogen content. Annual nitrogen applications are necessary to maintain sufficient nitrogen reserves in the tree. The apple tree is a species capable of re-mobilizing in spring considerable quantities of nitrogen reserves previously accumulated in the roots, in the woody organs and in the buds. After the fall of the petals and in correspondence with the beginning of the rapid growth of the shoots, the amount of nitrogen absorbed by the root system increases and then decreases later, at the time of flowering. The dose of N can be calculated based on fruits removal, of the expected yields and of the availability of nitrogen present in the soil.
Phosphorus is removed to a much lesser extent than nitrogen and potassium. The addition of phosphorus must be carried out in particular at the beginning of the vegetative activity; in this phase, the presence of adequate Phosphorus levels increase the growth of young rootlets, improve the assimilation of other nutrients, promote the development of leaves and improve the nutrition of the reproductive organs.
Potassium is of paramount importance for apple not only for tree growth and development but also for the size and quality of fruit yield. The apple plant’s demand for potassium varies, along with the progression of phenological phases, during the growing season. The potassium demand peaks during ripening of fruits featuring relatively high concentration of potassium comparable to that of the leaves. The mainstream method of apple tree potassium fertilization is through application of the fertilizer to the soils to improve potassium uptake by the roots. During ripening, foliar application of Potassium can increase sugar content and specifically on red apple, intensify the color especially if the nutritional action is linked to a biostimulant action.
Other nutrients very important for apple growth and development are:
The crop also benefits from the application of products with a biostimulant action to improve the availability of nutrients in the soil, to increase the yield from a quantitative and qualitative standpoint and to reduce the negative impact of climatic stresses. The application of biostimulants increases the environmental and economic sustainability of the production system.
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