Apple (tree)

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The apple is the pomaceous fruit of the apple tree, species Malus domestica in the rose family Rosaceae. It is one of the most widely cultivated tree fruits. Standard trees may reach up to 40 feet (12 m) tall, with a broad, often densely twiggy crown. Most commercial apple orchards use dwarf or semi-dwarf trees produced by grafting onto rootstocks bred for dwarfing and other characteristics. The leaves are alternately arranged simple ovals 3 to 5 in (5–12 cm) long and 1-3 in (3–6 cm) broad on a 1/2 - 2 in (2–5 cm) petiole with an acute tip, serrated margin and a slightly downy underside. Flowers are produced in spring simultaneous with the budding of the leaves. The flowers are white with a pink tinge that gradually fades, five petaled (rarely six), 1-1 1/2in (2.5–3.5 cm) in diameter. The fruit matures in autumn, and is typically 2-4in (5–9 cm) in diameter. The center of the fruit contains five carpels (uncommonly six) arranged in a five-point star, each carpel normally containing two seeds.

The tree originated from Asia, where its wild ancestor is still found today. There are more than 7,500 known cultivars of apples resulting in range of desired characteristics. It should be noted however, that cultivars vary in their yield and the ultimate size of the tree, even when grown on the same rootstock.

At least 60 million tons (55 million tonnes) of apples were grown worldwide in 2005, with a value of about $10 billion. China produced about two-fifths of this total. The United States is the second leading producer, with more than 7.5% of the world production. Turkey, France, Italy and Iran are among the leading apple exporters.

Botanical information

The wild ancestor of Malus domestica is Malus sieversii. It has no common name in English, but is known in Kazakhstan, where it is native, as alma; in fact, the region where it is thought to originate is called Almaty, or "reach of the apples". This tree is still found wild in the mountains of Central Asia in southern Kazakhstan, Kyrgyzstan, Tajikistan, and Xinjiang, China.

For many years, there was a debate about whether M. domestica evolved from chance hybridization among various wild species. Recent DNA analysis by Barrie Juniper, Emeritus Fellow in the Department of Plant Sciences at Oxford University and others, has indicated, however, that the hybridization theory is probably false. Instead, it appears that a single species still growing in the Ili Valley, on the northern slopes of the Tien Shan mountains at the border of northwest China and Kazakhstan, is the progenitor of the apples we eat today. Leaves taken from trees in this area were analyzed for DNA composition, which showed them all to belong to the species M. sieversii, with some genetic sequences common to M. domestica.

Other species that were previously thought to have made contributions to the genome of the domestic apples are Malus baccata and Malus sylvestris, but there is no hard evidence for this in older apple cultivars. These and other Malus species have been used in some recent breeding programmes to develop apples suitable for growing in climates unsuitable for M. domestica, mainly for increased cold tolerance.

History

The center of diversity of the genus Malus is the eastern Turkey, southwestern Russia region of Asia Minor. The apple tree was perhaps the earliest tree to be cultivated, and its fruits have been improved through selection over thousands of years. Alexander the Great is credited with finding dwarfed apples in Asia Minor in 300 BC; those he brought back to Greece might have been the progenitors of dwarfing rootstocks. Apples were brought to North America with colonists in the 1600s,and the first apple orchard on the North American continent was said to be near Boston in 1625. Winter apples, picked in late autumn and stored just above freezing, have been an important food in Asia and Europe for millennia, as well as in Argentina and in the United States since the arrival of Europeans.

Apple cultivars

There are more than 7,500 known cultivars of apples. Different cultivars are available for temperate and subtropical climates. Reputedly the world's biggest collection of apple cultivars is housed at the National Fruit Collection in England. Most of these cultivars are bred for eating fresh (dessert apples), though some are cultivated specifically for cooking or producing cider. Cider apples are typically too tart and astringent to eat fresh, but they give the beverage a rich flavor that dessert apples cannot.

Commercially popular apple cultivars are soft but crisp. Other desired qualities in modern commercial apple breeding are a colorful skin, absence of russeting, ease of shipping, lengthy storage ability, high yields, disease resistance, typical "Red Delicious" apple shape, long stem (to allow pesticides to penetrate the top of the fruit), and popular flavor.

Modern apples are generally sweeter than older cultivars, as popular tastes in apples have varied over time. Most North Americans and Europeans favor sweet, subacid apples, but tart apples have a strong minority following. Extremely sweet apples with barely any acid flavor are popular in Asia. As an example, the U.S. state of Washington made its reputation for apple growing on Red Delicious. In recent years, many apple connoisseurs have come to regard the Red Delicious as inferior to cultivars such as Fuji and Gala due to its merely mild flavor and insufficiently firm texture.

Old cultivars are often oddly shaped, russeted, and have a variety of textures and colors. Some find them to have a better flavor than modern cultivators, but may have other problems which make them commercially unviable, such as low yield, susceptibility to disease, or poor tolerance for storage or transport. A few old cultivars are still produced on a large scale, but many have been kept alive by home gardeners and farmers that sell directly to local markets. Many unusual and locally important cultivars with their own unique taste and appearance exist; apple conservation campaigns have sprung up around the world to preserve such local cultivars from extinction. In the United Kingdom old cultivars such as Cox's Orange Pippin and Egremont Russett are still commercially important even though by modern standards they are low yielding and disease prone.

Apple production

Apple breeding

Like most perennial fruits, apples ordinarily propagate asexually by grafting. Seedling apples are different from their parents, sometimes radically. Most new apple cultivars originate as seedlings, which either arise by chance or are bred by deliberately crossing cultivars with promising characteristics[1]. The words 'seedling', 'pippin', and 'kernel' in the name of an apple cultivar suggest that it originated as a seedling. Apples can also form bud sports (mutations on a single branch). Some bud sports turn out to be improved strains of the parent cultivar. Some differ sufficiently from the parent tree to be considered new cultivars.

Breeders can produce more rigid apples through crossing. For example, the Excelsior Experiment Station of the University of Minnesota has, since the 1930s, introduced a steady progression of important hardy apples that are widely grown, both commercially and by backyard orchardists, throughout Minnesota and Wisconsin. Its most important introductions have included 'Haralson' (which is the most widely cultivated apple in Minnesota), 'Wealthy', 'Honeygold', and 'Honeycrisp'.

Pollination

Apples are self-incompatible; they must cross-pollinate to develop fruit. During the flowering each season, apple growers usually provide pollinators to carry the pollen. Honey bee hives are most commonly used. Orchard mason bees are also used as supplemental pollinators in commercial orchards. Bumble bee queens are sometimes present in orchards, but not usually in enough quantity to be significant pollinators.

There are four to seven pollination groups in apples depending on climate:

One cultivar can be pollenized by a compatible cultivar from the same group or close (A with A or A with B but not A with C or D).

It is important that pollinators make sufficient visits to the blossoms to deliver enough pollen grains to fertilize the majority of incipient seeds. If too few seeds are produced, the apples may abort (excessive June drop), or be small and deformed. In cases where large blocks of single varieties were mistakenly planted without pollination planning, pollenizers may be imported in the form of drum bouquets of crabapple flowers, or with grafted crabapple branches on evenly spaced trees. Crabapples provide abundant, and highly viable pollen, so they are excellent pollenizers for any apple cultivar that blooms at the same time.

Apples bloom in clusters of flowers; normally there are five to a cluster, though a few cultivars have six. The first to bloom is the "king bloom" and it tends to make the highest quality apple. Also its growth will tend to suppress any other apples in the same cluster, causing June drop whether they are well pollinated or not. Apple trees that set too heavy a crop and do not naturally thin by June drop need to be helped by manual or chemical thinning, else the tree may not yield in the following season, slipping into a pattern of biennial bearing.

If a king bloom is not well pollinated due to inadequate bees, poor pollinating weather, lack of pollenizers, or if it is frosted during bloom, the next three blossoms provide a second opportunity for a crop. However, if all three set, no one of these will suppress the others and chemical thinning is very difficult; expensive manual thinning may be required. If frost or poor pollination causes the loss of the first four blossoms, the final blossom, which is the weakest, may not produce the best quality apple, but it is desired to get it set anyway, since it will help keep the tree from biennial bearing and excessive woody growth in the absence of fruit.

Maturation and harvest

Cultivars vary in their yield and the ultimate size of the tree, even when grown on the same rootstock. Some cultivars, if left unpruned, will grow very large, which allows them to bear a great deal more fruit, but makes harvest very difficult. Mature trees typically bear 40–200 kg of apples each year, though productivity can be close to zero in poor years. Apples are harvested using three-point ladders that are designed to fit amongst the branches. Dwarf trees will bear about 10–80 kg of fruit per year.

Timing of harvest is managed by testing the conversion of starch to sugar in the fruit, adjusted to whether the fruit will go immediately to fresh market or processing, or whether it will be stored.

Storage

Commercially, apples can be stored for some months in controlled-atmosphere chambers to delay ethylene-induced onset of ripening. Ripening begins when the fruit is removed.

Pests and diseases

Ladybug larva eating wooly apple aphids

The trees are susceptible to a number of fungal and bacterial diseases and insect pests. Modern commercial orchards use Integrated Pesticide Management to reduce the use of synthetic pesticides, but still must use some chemicals to maintain high fruit quality, tree health, and high yields.

A trend in orchard management is the use of organic methods. These use a less aggressive and direct methods of conventional farming. Instead of spraying potent chemicals, often shown to be potentially dangerous and maleficent to the tree in the long run, organic methods include encouraging or discouraging certain cycles and pests.

To control a specific pest, organic growers might encourage the prosperity of its natural predator instead of outright killing it, and with it the natural biochemistry around the tree. Organic apples generally have the same or greater taste than conventionally grown apples, with reduced cosmetic appearances.

A wide range of pests and diseases can affect the plant; three of the more common are mildew, aphids and apple scab.

  • Mildew: which is characterized by light grey powdery patches appearing on the leaves, shoots and flowers, normally in spring. The flowers will turn a creamy yellow color and will not develop correctly. This can be treated in a manner not dissimilar from treating Botrytis; eliminating the conditions which caused the disease in the first place and burning the infected plants are among the recommended actions to take.
  • Aphids: There are five species of aphids commonly found on apples: apple grain aphid, rosy apple aphid, apple aphid, spirea aphid and the woolly apple aphid. The aphid species can be identified by their colour, the time of year when they are present and by differences in the cornicles, which are small paired projections from the rear of aphids. Aphids feed on foliage using needlelike mouthparts to suck out plant juices. When present in high numbers, certain species may reduce tree growth and vigor.
  • Apple scab: Symptoms of Scab are olive-green or brown blotches on the leaves. The blotches turn browner as time progresses. Then brown scabs form on the fruit. The diseased leaves will fall early and the fruit will become increasingly covered in scabs - eventually the fruit skin will crack.

Among the most serious disease problems are fireblight, a bacterial disease; and Gymnosporangium rust, and black spot, two fungal diseases.

Young apple trees are also prone to mammal pests like voles and deer, which feed on the soft bark of the trees, especially in winter. Where snows are deep, and voles are not sufficiently suppressed before winter, entire orchards may be girdled by voles, working unseen under the snow. Where a large part of the diameter of the tree is damaged, bridge grafting may be used to save the tree.

Commerce

China produces about two-fifths of the world's total. United States is the second leading producer, with more than 7.5% of the world production. Turkey, France, Italy and Iran are among the leading apple exporters.

In the United States, more than 60% of all the apples sold commercially are grown in Washington state. Other important apple states are Michigan, New York, and Pennsylvania. Apples require cold winters to produce fruit buds, and only a few specially bred "low chill" cultivars can be grown in the southernmost states.

Imported apples from New Zealand and other more temperate areas are competing with US production and increasing each year.

Most of Australia's apple production is for domestic consumption. Imports from New Zealand have been disallowed under quarantine regulations for fire blight since 1921.
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