The apple tree (Malus pumila, commonly and erroneously called Malus domestica) is a deciduous tree in the rose family best known for its sweet, pomaceous fruit, the apple. It is cultivated worldwide as a fruit tree, and is the most widely grown species in the genus Malus. The tree originated in Central Asia, where its wild ancestor, Malus sieversii, is still found today. Apples have been grown for thousands of years in Asia and Europe, and were brought to North America by European colonists. Apples have religious and mythological significance in many cultures, including Norse, Greek and European Christian traditions.
Apple trees are large if grown from seed. Generally apple cultivars are propagated by grafting onto rootstocks, which control the size of the resulting tree. There are more than 7,500 known cultivars of apples, resulting in a range of desired characteristics. Different cultivars are bred for various tastes and uses, including cooking, eating raw and cider production. Trees and fruit are prone to a number of fungal, bacterial and pest problems, which can be controlled by a number of organic and non-organic means. In 2010, the fruit's genome was sequenced as part of research on disease control and selective breeding in apple production.
The apple is a deciduous tree, generally standing 6 to 15 ft (1.8 to 4.6 m) tall in cultivation and up to 30 ft (9.1 m) in the wild. When cultivated, the size, shape and branch density are determined by rootstock selection and trimming method. The leaves are alternately arranged dark green-colored simple ovals with serrated margins and slightly downy undersides.
Blossoms are produced in spring simultaneously with the budding of the leaves, and are produced on spurs and some long shoots. The 3 to 4 cm (1.2 to 1.6 in) flowers are white with a pink tinge that gradually fades, five petaled, with an inflorescence consisting of a cyme with 4–6 flowers. The central flower of the inflorescence is called the "king bloom"; it opens first, and can develop a larger fruit.
The fruit matures in late summer or autumn, and cultivars exist with a wide range of sizes. Commercial growers aim to produce an apple that is 2 3⁄4 to 3 1⁄4 in (7.0 to 8.3 cm) in diameter, due to market preference. Some consumers, especially those in Japan, prefer a larger apple, while apples below 2 1⁄4 in (5.7 cm) are generally used for making juice and have little fresh market value. The skin of ripe apples is generally red, yellow, green, pink, or russetted although many bi- or tri-colored cultivars may be found. The skin may also be wholly or partly russeted i.e. rough and brown. The skin is covered in a protective layer of epicuticular wax. The exocarp (flesh) is generally pale yellowish-white, though pink or yellow exocarps also occur.
The original wild ancestor of Malus pumila was Malus sieversii, found growing wild in the mountains of Central Asia in southern Kazakhstan, Kyrgyzstan, Tajikistan, and Xinjiang, China. Cultivation of the species, most likely beginning on the forested flanks of the Tian Shan mountains, progressed over a long period of time and permitted secondary introgression of genes from other species into the open-pollinated seeds. Significant exchange with Malus sylvestris, the crabapple, resulted in current populations of apples being more related to crabapples than to the more morphologically similar progenitor Malus sieversii. In strains without recent admixture the contribution of the latter predominates.
In 2010, an Italian-led consortium announced they had sequenced the complete genome of the apple in collaboration with horticultural genomicists at Washington State University, using 'Golden Delicious'. It had about 57,000 genes, the highest number of any plant genome studied to date and more genes than the human genome (about 30,000). This new understanding of the apple genome will help scientists in identifying genes and gene variants that contribute to resistance to disease and drought, and other desirable characteristics. Understanding the genes behind these characteristics will allow scientists to perform more knowledgeable selective breeding. The genome sequence also provided proof that Malus sieversii was the wild ancestor of the domestic apple—an issue that had been long-debated in the scientific community.
The center of diversity of the genus Malus is in eastern present-day Turkey. 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 Kazakhstan in 328 BCE. Winter apples, picked in late autumn and stored just above freezing, have been an important food in Asia and Europe for millennia.
Of the many Old World plants that the Spanish introduced to Chiloé Archipelago in the 16th century, apple trees became particularly well adapted. Apples were introduced to North America by colonists in the 17th century, and the first apple orchard on the North American continent was planted in Boston by Reverend William Blaxton in 1625. The only apples native to North America are crab apples, which were once called "common apples". Apple cultivars brought as seed from Europe were spread along Native American trade routes, as well as being cultivated on colonial farms. An 1845 United States apples nursery catalogue sold 350 of the "best" cultivars, showing the proliferation of new North American cultivars by the early 19th century. In the 20th century, irrigation projects in Eastern Washington began and allowed the development of the multibillion-dollar fruit industry, of which the apple is the leading product.
Until the 20th century, farmers stored apples in frostproof cellars during the winter for their own use or for sale. Improved transportation of fresh apples by train and road replaced the necessity for storage. Controlled atmosphere facilities are used to keep apples fresh year-round. Controlled atmosphere facilities use high humidity, low oxygen, and controlled carbon dioxide levels to maintain fruit freshness. They were first used in the United States in the 1960s.
Society and culture
In Norse mythology, the goddess Iðunn is portrayed in the Prose Edda (written in the 13th century by Snorri Sturluson) as providing apples to the gods that give them eternal youthfulness. English scholar H. R. Ellis Davidson links apples to religious practices in Germanic paganism, from which Norse paganism developed. She points out that buckets of apples were found in the Oseberg ship burial site in Norway, and that fruit and nuts (Iðunn having been described as being transformed into a nut in Skáldskaparmál) have been found in the early graves of the Germanic peoples in England and elsewhere on the continent of Europe, which may have had a symbolic meaning, and that nuts are still a recognized symbol of fertility in southwest England.
Davidson notes a connection between apples and the Vanir, a tribe of gods associated with fertility in Norse mythology, citing an instance of eleven "golden apples" being given to woo the beautiful Gerðr by Skírnir, who was acting as messenger for the major Vanir god Freyr in stanzas 19 and 20 of Skírnismál. Davidson also notes a further connection between fertility and apples in Norse mythology in chapter 2 of the Völsunga saga when the major goddess Frigg sends King Rerir an apple after he prays to Odin for a child, Frigg's messenger (in the guise of a crow) drops the apple in his lap as he sits atop a mound. Rerir's wife's consumption of the apple results in a six-year pregnancy and the Caesarean section birth of their son—the hero Völsung.
Further, Davidson points out the "strange" phrase "Apples of Hel" used in an 11th-century poem by the skald Thorbiorn Brúnarson. She states this may imply that the apple was thought of by Brúnarson as the food of the dead. Further, Davidson notes that the potentially Germanic goddess Nehalennia is sometimes depicted with apples and that parallels exist in early Irish stories. Davidson asserts that while cultivation of the apple in Northern Europe extends back to at least the time of the Roman Empire and came to Europe from the Near East, the native varieties of apple trees growing in Northern Europe are small and bitter. Davidson concludes that in the figure of Iðunn "we must have a dim reflection of an old symbol: that of the guardian goddess of the life-giving fruit of the other world."
Apples appear in many religious traditions, often as a mystical or forbidden fruit. One of the problems identifying apples in religion, mythology and folktales is that the word "apple" was used as a generic term for all (foreign) fruit, other than berries, including nuts, as late as the 17th century. For instance, in Greek mythology, the Greek hero Heracles, as a part of his Twelve Labours, was required to travel to the Garden of the Hesperides and pick the golden apples off the Tree of Life growing at its center.
The Greek goddess of discord, Eris, became disgruntled after she was excluded from the wedding of Peleus and Thetis. In retaliation, she tossed a golden apple inscribed Καλλίστη (Kalliste, sometimes transliterated Kallisti, "For the most beautiful one"), into the wedding party. Three goddesses claimed the apple: Hera, Athena, and Aphrodite. Paris of Troy was appointed to select the recipient. After being bribed by both Hera and Athena, Aphrodite tempted him with the most beautiful woman in the world, Helen of Sparta. He awarded the apple to Aphrodite, thus indirectly causing the Trojan War.
The apple was thus considered, in ancient Greece, to be sacred to Aphrodite, and to throw an apple at someone was to symbolically declare one's love; and similarly, to catch it was to symbolically show one's acceptance of that love. An epigram claiming authorship by Plato states:
I throw the apple at you, and if you are willing to love me, take it and share your girlhood with me; but if your thoughts are what I pray they are not, even then take it, and consider how short-lived is beauty.— Plato, Epigram VII
Atalanta, also of Greek mythology, raced all her suitors in an attempt to avoid marriage. She outran all but Hippomenes (also known as Melanion, a name possibly derived from melon the Greek word for both "apple" and fruit in general), who defeated her by cunning, not speed. Hippomenes knew that he could not win in a fair race, so he used three golden apples (gifts of Aphrodite, the goddess of love) to distract Atalanta. It took all three apples and all of his speed, but Hippomenes was finally successful, winning the race and Atalanta's hand.
Though the forbidden fruit of Eden in the Book of Genesis is not identified, popular Christian tradition has held that it was an apple that Eve coaxed Adam to share with her. The origin of the popular identification with a fruit unknown in the Middle East in biblical times is found in confusion between the Latin words mālum (an apple) and mălum (an evil), each of which is normally written malum. The tree of the forbidden fruit is called "the tree of the knowledge of good and evil" in Genesis 2:17, and the Latin for "good and evil" is bonum et malum.
Renaissance painters may also have been influenced by the story of the golden apples in the Garden of Hesperides. As a result, in the story of Adam and Eve, the apple became a symbol for knowledge, immortality, temptation, the fall of man into sin, and sin itself. The larynx in the human throat has been called the "Adam's apple" because of a notion that it was caused by the forbidden fruit remaining in the throat of Adam. The apple as symbol of sexual seduction has been used to imply human sexuality, possibly in an ironic vein.
There are more than 7,500 known cultivars of apples. Cultivars vary in their yield and the ultimate size of the tree, even when grown on the same rootstock. Different cultivars are available for temperate and subtropical climates. The UK's National Fruit Collection, which is the responsibility of the Department of Environment Food and Rural Affairs, includes a collection of over 2,000 cultivars of apple tree in Kent. The University of Reading, which is responsible for developing the UK national collection database, provides access to search the national collection. The University of Reading's work is part of the European Cooperative Programme for Plant Genetic Resources of which there are 38 countries participating in the Malus/Pyrus work group.
The UK's national fruit collection database contains a wealth of information on the characteristics and origin of many apples, including alternative names for what is essentially the same "genetic" apple cultivar. Most of these cultivars are bred for eating fresh (dessert apples), though some are cultivated specifically for cooking (cooking apples) 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, common apple shape, and developed 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 and especially Indian Subcontinent .
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 cultivars, but they may have other problems which make them commercially unviable—low yield, disease susceptibility, poor tolerance for storage or transport, or just being the "wrong" size. A few old cultivars are still produced on a large scale, but many have been preserved 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 Russet' are still commercially important even though by modern standards they are low yielding and susceptible to disease.
In the wild, apples grow readily from seeds. However, like most perennial fruits, apples are ordinarily propagated asexually by grafting. This is because seedling apples are an example of "extreme heterozygotes", in that rather than inheriting DNA from their parents to create a new apple with those characteristics, they are instead significantly different from their parents. Triploid cultivars have an additional reproductive barrier in that 3 sets of chromosomes cannot be divided evenly during meiosis, yielding unequal segregation of the chromosomes (aneuploids). Even in the case when a triploid plant can produce a seed (apples are an example), it occurs infrequently, and seedlings rarely survive.
Because apples do not breed true when planted as seeds, grafting is generally used to produce new apple trees. The rootstock used for the bottom of the graft can be selected to produce trees of a large variety of sizes, as well as changing the winter hardiness, insect and disease resistance, and soil preference of the resulting tree. Dwarf rootstocks can be used to produce very small trees (less than 3.0 m (10 ft) high at maturity), which bear fruit earlier in their life cycle than full size trees. Dwarf rootstocks for apple trees can be traced as far back as 300 BC, to the area of Persia and Asia Minor. Alexander the Great sent samples of dwarf apple trees to Aristotle's Lyceum. Dwarf rootstocks became common by the 15th century, and later went through several cycles of popularity and decline throughout the world. The majority of the rootstocks used today to control size in apples were developed in England in the early 1900s. The East Malling Research Station conducted extensive research into rootstocks, and today their rootstocks are given an "M" prefix to designate their origin. Rootstocks marked with an "MM" prefix are Malling-series cultivars later crossed with trees of 'Northern Spy' in Merton, England.
Most new apple cultivars originate as seedlings, which either arise by chance or are bred by deliberately crossing cultivars with promising characteristics. 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.
Since the 1930s, the Excelsior Experiment Station at the University of Minnesota has introduced a steady progression of important apples that are widely grown, both commercially and by local orchardists, throughout Minnesota and Wisconsin. Its most important contributions have included 'Haralson' (which is the most widely cultivated apple in Minnesota), 'Wealthy', 'Honeygold', and 'Honeycrisp'.
Apples have been acclimatized in Ecuador at very high altitudes, where they can often, with the needed factors, provide crops twice per year because of constant temperate conditions year-round.
Apples are self-incompatible; they must cross-pollinate to develop fruit. During the flowering each season, apple growers often utilize pollinators to carry pollen. Honey bees are most commonly used. Orchard mason bees are also used as supplemental pollinators in commercial orchards. Bumblebee 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:
- Group A – Early flowering, 1 to 3 May in England ('Gravenstein', 'Red Astrachan')
- Group B – 4 to 7 May ('Idared', 'McIntosh')
- Group C – Mid-season flowering, 8 to 11 May ('Granny Smith', 'Cox's Orange Pippin')
- Group D – Mid/late season flowering, 12 to 15 May ('Golden Delicious', 'Calville blanc d'hiver')
- Group E – Late flowering, 16 to 18 May ('Braeburn', 'Reinette d'Orléans')
- Group F – 19 to 23 May ('Suntan')
- Group H – 24 to 28 May ('Court-Pendu Gris' – also called Court-Pendu plat)
One cultivar can be pollinated by a compatible cultivar from the same group or close (A with A, or A with B, but not A with C or D).
Cultivars are sometimes classified by the day of peak bloom in the average 30-day blossom period, with pollenizers selected from cultivars within a 6-day overlap period.
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 much more fruit, but makes harvesting very difficult. Depending on the tree density (number of trees planted per unit surface area), mature trees typically bear 40–200 kg (90–440 lb) 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. Trees grafted on dwarfing rootstocks will bear about 10–80 kg (20–180 lb) of fruit per year.
Farms with apple orchards may open them to the public, so consumers may themselves pick the apples they will purchase.
Crops ripen at different times of the year according to the cultivar. Cultivar that yield their crop in the summer include 'Gala', 'Golden Supreme', 'McIntosh', 'Transparent', 'Primate', 'Sweet Bough', and 'Duchess'; fall producers include 'Fuji', 'Jonagold', 'Golden Delicious', 'Red Delicious', 'Chenango', 'Gravenstein', 'Wealthy', 'McIntosh', 'Snow', and 'Blenheim'; winter producers include 'Winesap', 'Granny Smith', 'King', 'Wagener', 'Swayzie', 'Greening', and 'Tolman Sweet'.
Commercially, apples can be stored for some months in controlled atmosphere chambers to delay ethylene-induced ripening. Apples are commonly stored in chambers with higher concentrations of carbon dioxide and high air filtration. This prevents ethylene concentrations from rising to higher amounts and preventing ripening from occurring too quickly. Ripening continues when the fruit is removed from storage.
For home storage, most cultivars of apple can be held for approximately two weeks when kept at the coolest part of the refrigerator (i.e. below 5 °C). Some can be stored up to a year without significant degradation.[dubious ][verification needed] Some varieties of apples (e.g. 'Granny Smith' and 'Fuji') have more than three times the storage life of others.
Pests and diseases
Apple trees are susceptible to a number of fungal and bacterial diseases and insect pests. Many commercial orchards pursue a program of chemical sprays to maintain high fruit quality, tree health, and high yields. A trend in orchard management is the use of organic methods. These prohibit the use of synthetic pesticides, though some older pesticides are allowed. Organic methods include, for instance, introducing its natural predator to reduce the population of a particular pest.
A wide range of pests and diseases can affect the plant; three of the more common diseases/pests 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 color, 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 needle-like mouth parts to suck out plant juices. When present in high numbers, certain species reduce tree growth and vigor.
- Apple scab: Apple scab causes leaves to develop olive-brown spots with a velvety texture that later turn brown and become cork-like in texture. The disease also affects the fruit, which also develops similar brown spots with velvety or cork-like textures. Apple scab is spread through fungus growing in old apple leaves on the ground and spreads during warm spring weather to infect the new year's growth.
Among the most serious disease problems are fireblight, a bacterial disease; and Gymnosporangium rust, and black spot, two fungal diseases. Codling moths and apple maggots are two other pests which affect apple trees. Young apple trees are also prone to mammal pests like mice and deer, which feed on the soft bark of the trees, especially in winter. The larvae of the apple clearwing moth (red-belted clearwing) burrow through the bark and into the phloem of apple trees, potentially causing significant damage.
|Apple production – 2014|
|Country||(millions of tonnes)|
World production of apples in 2014 was 84.6 million tonnes, with China producing 48% of the world total (table). Other major producers with 6% or less of the world total were the United States, Poland, Turkey, and Italy.
|Nutritional value per 100 g (3.5 oz)|
|Energy||218 kJ (52 kcal)|
|Dietary fiber||2.4 g|
|Vitamin A equiv.||
|Pantothenic acid (B5)||
|Percentages are roughly approximated using US recommendations for adults.|
The whole fruit including the skin, except for the seeds, is suitable for human consumption. The core, containing the seeds, is usually not eaten and is discarded.
Apples can be consumed various ways: juice, raw in salads, baked in pies, cooked into sauces, and other baked dishes.
Several techniques are used to preserve apples and apple products. Apples can be canned, dried or froze. Canned or frozen apples are eventually baked into pies or other cooked dishes. Apple juice or cider is also bottled. Apple juice is often concentrated and froze.
Apples are often eaten raw. Cultivars bred for raw consumption are termed dessert or table apples.
- In the UK, a toffee apple is a traditional confection made by coating an apple in hot toffee and allowing it to cool. Similar treats in the U.S. are candy apples (coated in a hard shell of crystallized sugar syrup), and caramel apples (coated with cooled caramel).
- Apples are eaten with honey at the Jewish New Year of Rosh Hashanah to symbolize a sweet new year.
Apples are an important ingredient in many desserts, such as apple pie, apple crumble, apple crisp and apple cake. When cooked, some apple cultivars easily form a puree known as apple sauce. Apples are also made into apple butter and apple jelly. They are often baked or stewed and are also (cooked) in some meat dishes. Dried apples can be eaten or reconstituted (soaked in water, alcohol or some other liquid).
Apples are milled or pressed to produce apple juice, which may be drunk unfiltered (called apple cider in North America), or filtered. Filtered juice is often concentrated and frozen to be reconstituted later and consumed. Apple juice can be fermented to make cider (called hard cider in North America), ciderkin, and vinegar. Through distillation, various alcoholic beverages can be produced, such as applejack, Calvados, and apfelwein.
Sliced apples turn brown with exposure to air due to the conversion of natural phenolic substances into melanin upon exposure to oxygen. Different cultivars vary in their propensity to brown after slicing and the genetically engineered Arctic Apples do not brown. Sliced fruit can be treated with acidulated water to prevent this effect. Sliced apple consumption tripled in the US from 2004 to 2014 to 500 million apples annually due to its convenience.
Organic apples are commonly produced in the United States. Due to infestations by key insects and diseases, organic production is difficult in Europe. The use of pesticides containing chemicals, such as sulfur, copper, microorganisms, viruses, clay powders, or plant extracts (pyrethrum, neem) has been approved by the EU Organic Standing Committee to improve organic yield and quality. A light coating of kaolin, which forms a physical barrier to some pests, also may help prevent apple sun scalding.
Apples are a rich source of various phytochemicals including flavonoids (e.g., catechins, flavanols, and quercetin) and other phenolic compounds (e.g., epicatechin and procyanidins) found in the skin, core, and pulp of the apple; they have unknown health value in humans.
Phenolic compounds, such as polyphenol oxidase, are the main driving force behind browning in apples. Polyphenol oxidase catalyzes the reaction of phenolic compounds to o-quinones causing the pigment to turn darker and therefore brown.
Phlorizin is a flavonoid that is found in apple trees, particularly in the leaves, and in only small amounts if at all in other plants, even other species of the Malus genus or related plants such as pear trees.
One form of apple allergy, often found in northern Europe, is called birch-apple syndrome, and is found in people who are also allergic to birch pollen. Allergic reactions are triggered by a protein in apples that is similar to birch pollen, and people affected by this protein can also develop allergies to other fruits, nuts, and vegetables. Reactions, which entail oral allergy syndrome (OAS), generally involve itching and inflammation of the mouth and throat, but in rare cases can also include life-threatening anaphylaxis. This reaction only occurs when raw fruit is consumed—the allergen is neutralized in the cooking process. The variety of apple, maturity and storage conditions can change the amount of allergen present in individual fruits. Long storage times can increase the amount of proteins that cause birch-apple syndrome.
In other areas, such as the Mediterranean, some individuals have adverse reactions to apples because of their similarity to peaches. This form of apple allergy also includes OAS, but often has more severe symptoms, such as vomiting, abdominal pain and urticaria, and can be life-threatening. Individuals with this form of allergy can also develop reactions to other fruits and nuts. Cooking does not break down the protein causing this particular reaction, so affected individuals can eat neither raw nor cooked apples. Freshly harvested, over-ripe fruits tend to have the highest levels of the protein that causes this reaction.
Toxicity of seeds
The seeds of apples contain small amounts of amygdalin, a sugar and cyanide compound known as a cyanogenic glycoside. Ingesting small amounts of apple seeds will cause no ill effects, but consumption of extremely large doses can cause adverse reactions. It may take several hours before the poison takes effect, as cyanogenic glycosides must be hydrolyzed before the cyanide ion is released. The United States National Library of Medicine's Hazardous Substances Data Bank records no cases of amygdalin poisoning from consuming apple seeds.
The proverb "An apple a day keeps the doctor away", addressing the health effects of the fruit, dates from 19th century Wales, according to Caroline Taggart, author of "An Apple a Day: Old-Fashioned Proverbs and Why They Still Work". The original phrase, Taggart said, was: "Eat an apple on going to bed, and you'll keep the doctor from earning his bread". In the 19th century and early 20th, the phrase evolved to "an apple a day, no doctor to pay" and "an apple a days sends the doctor away", while the phrasing now commonly used was first recorded in 1922.
Different kinds of apple cultivars in a wholesale food market
- Elizabeth E. Dickson (2015), "Malus pumila Miller, Gard. Dict. ed. 8. Malus no. 3. 1768", Flora of North America, 9
- Wendy L. Applequist (2014), "Report of the Nomenclature Committee for Vascular Plants: 66: (1933). To conserve Malus domestica Borkh. against M. pumila Miller, M. communis Desf., M. frutescens Medik., and Pyrus dioica Moench (Rosaceae). Proposed by G.-Z. Qian, L.-F. Liu & G.-G. Tang in Taxon 59(2): 650–652. 2010. Votes: 6–11–1 (not recommended)", Taxon, 63 (6): 1358–1371, doi:10.12705/636.20
- "Production/Crops, Apple, Area by World". FAOSTAT, UN Food & Agriculture Organization, Statistics Division. 2014. Retrieved 10 February 2017.
- "Origin, History of cultivation". University of Georgia. Archived from the original on 21 January 2008. Retrieved 22 January 2008.
- "Apple". Natural History Museum. Retrieved 5 September 2013.
- Jules Janick; James N. Cummins; Susan K. Brown; Minou Hemmat (1996). "Chapter 1: Apples". In Jules Janick; James N. Moore. Fruit Breeding, Volume I: Tree and Tropical Fruits (PDF). John Wiley & Sons, Inc. p. 9. ISBN 0-471-31014-X.
- "Natural Waxes on Fruits". Postharvest.tfrec.wsu.edu. 29 October 2010. Retrieved 14 June 2013.
- Lauri, Pierre-éric; Karen Maguylo; Catherine Trottier (2006). "Architecture and size relations: an essay on the apple (Malus x domestica, Rosaceae) tree". American Journal of Botany. Botanical Society of America, Inc. 93 (3): 357–368. doi:10.3732/ajb.93.3.357. PMID 21646196.
- Amandine Cornille; et al. (2012). Mauricio, Rodney, ed. "New Insight into the History of Domesticated Apple: Secondary Contribution of the European Wild Apple to the Genome of Cultivated Varieties". PLOS Genetics. 8 (5): e1002703. doi:10.1371/journal.pgen.1002703. PMC . PMID 22589740.
- Sam Kean (17 May 2012). "ScienceShot: The Secret History of the Domesticated Apple".
- Coart, E.; Van Glabeke, S.; De Loose, M.; Larsen, A.S.; Roldán-Ruiz, I. (2006). "Chloroplast diversity in the genus Malus: new insights into the relationship between the European wild apple (Malus sylvestris (L.) Mill.) and the domesticated apple (Malus domestica Borkh.)". Mol. Ecol. 15 (8): 2171–82. doi:10.1111/j.1365-294x.2006.02924.x. PMID 16780433.
- Clark Brian (29 August 2010). "Apple Cup Rivals Contribute to Apple Genome Sequencing". Cahnrsnews.wsu.edu. Retrieved 28 December 2012.
- Velasco, R; Zharkikh, A; Affourtit, J; et al. (October 2010). "The genome of the domesticated apple (Malus × domestica Borkh.)". Nature Genetics. Nature.com. 42: 833–839. doi:10.1038/ng.654. PMID 20802477. Retrieved 28 December 2012.
- An Italian-led international research consortium decodes the apple genome AlphaGallileo 29 August 2010. Retrieved 19 October 2011.
- The Science Behind the Human Genome Project Human Genome Project Information, US Department of Energy, 26 March 2008. Retrieved 19 October 2011.
- "An apple a day keeps the doctor away". Vegetarians in Paradise. Archived from the original on 11 February 2008. Retrieved 27 January 2008.
- Torrejón, Fernando; Cisternas, Marco; Araneda, Alberto (2004). "Efectos ambientales de la colonización española desde el río Maullín al archipiélago de Chiloé, sur de Chile" [Environmental effects of the spanish colonization from de Maullín river to the Chiloé archipelago, southern Chile]. Revista Chilena de Historia Natural (in Spanish). 77: 661–677.
- Smith, Archibald William (1997). A Gardener's Handbook of Plant Names: Their Meanings and Origins. Dover Publications. p. 39. ISBN 0-486-29715-2.
- Lawrence, James (1980). The Harrowsmith Reader, Volume II. Camden House Publishing Ltd. p. 122. ISBN 0-920656-10-2.
- James M. Van Valen (2010). History of Bergen county, New Jersey. Nabu Press. p. 744. ISBN 1-177-72589-4.
- Brox, Jane (2000). Five Thousand Days Like This One: An American Family History. Beacon Press. ISBN 978-0-8070-2107-1.
- "Controlled Atmospheric Storage (CA)". Washington Apple Commission. Archived from the original on 14 March 2012. Retrieved 3 April 2012.
- Ellis Davidson, H. R. (1965) Gods And Myths of Northern Europe, page 165 to 166. ISBN 0-14-013627-4
- Ellis Davidson, H. R. (1965) Gods And Myths of Northern Europe, page 165 to 166. Penguin Books ISBN 0-14-013627-4
- Ellis Davidson, H. R. (1998) Roles of the Northern Goddess, page 146 to 147. Routledge ISBN 0-415-13610-5
- Sauer, Jonathan D. (1993). Historical Geography of Crop Plants: A Select Roster. CRC Press. p. 109. ISBN 0-8493-8901-1.
- Wasson, R. Gordon (1968). Soma: Divine Mushroom of Immortality. Harcourt Brace Jovanovich. p. 128. ISBN 0-15-683800-1.
- Ruck, Carl; Blaise Daniel Staples (2001). The Apples of Apollo, Pagan and Christian Mysteries of the Eucharist. Durham: Carolina Academic Press. pp. 64–70. ISBN 0-89089-924-X.
- Heinrich, Clark (2002). Magic Mushrooms in Religion and Alchemy. Rochester: Park Street Press. pp. 64–70. ISBN 0-89281-997-9.
- Hyginus. "92". Fabulae. Theoi Project. Translated by Mary Grant. Retrieved 7 December 2017.
- Lucian. "The Judgement of Paris". Dialogues of the Gods. Theoi Project. Translated by H. W. & F. G. Fowler. Retrieved 7 December 2017.
- Edmonds, J.M. (1997). "Epigrams". In Cooper, John M.; Hutchinson, D.S. Plato: Complete Works. Indianapolis: Hackett Publishing Co. p. 1744. ISBN 9780872203495. Retrieved 7 December 2017.
- Macrone, Michael; Tom Lulevitch (1998). Brush up your Bible!. Tom Lulevitch. Random House Value. ISBN 0-517-20189-5. OCLC 38270894.
- Kissling, Paul J (2004). Genesis. Books.google.com. 1. College Press. p. 193. ISBN 978-0-89900875-2. Retrieved 25 August 2014.
- Hendel, Ronald (2012). The Book of Genesis: A Biography. Books.google.com. Princeton University Press. p. 114. ISBN 978-0-69114012-4. Retrieved 25 August 2014.
- Elzebroek, A.T.G.; Wind, K. (2008). Guide to Cultivated Plants. Wallingford: CAB International. p. 27. ISBN 1-84593-356-7.
- "Apple – Malus domestica". Natural England. Archived from the original on 12 May 2008. Retrieved 22 January 2008.
- "National Fruit Collection". Retrieved 2 December 2012.
- "ECPGR Malus/Pyrus Working Group Members". Ecpgr.cgiar.org. 22 July 2002. Retrieved 25 August 2014.
- Sue Tarjan (Fall 2006). "Autumn Apple Musings" (PDF). News & Notes of the UCSC Farm & Garden, Center for Agroecology & Sustainable Food Systems. pp. 1–2. Archived from the original (PDF) on 11 August 2007. Retrieved 24 January 2008.
- "World apple situation". Archived from the original on 11 February 2008. Retrieved 24 January 2008.
- Weaver, Sue (June–July 2003). "Crops & Gardening – Apples of Antiquity". Hobby Farms magazine. BowTie, Inc.
- John Lloyd and John Mitchinson (2006). QI: The Complete First Series – QI Factoids (DVD). 2 entertain.
- Ranney, Thomas G. "Polyploidy: From Evolution to Landscape Plant Improvement". Proceedings of the 11th Metropolitan Tree Improvement Alliance (METRIA) Conference. 11th Metropolitan Tree Improvement Alliance (METRIA) Conference held in Gresham, Oregon, August 23–24, 2000. METRIA (NCSU.edu). METRIA. Retrieved 7 November 2010.
- William G. Lord; Amy Ouellette (February 2010). "Dwarf Rootstocks for Apple Trees in the Home Garden" (PDF). University of New Hampshire. Retrieved 1 September 2013.
- Esmaeil Fallahi; W. Michael Colt; Bahar Fallahi; Ik-Jo Chun (January–March 2002). "The Importance of Apple Rootstocks on Tree Growth, Yield, Fruit Quality, Leaf Nutrition, and Photosynthesis with an Emphasis on 'Fuji'" (PDF). Hort Technology. 12 (1).
- ML Parker (September 1993). "Apple Rootstocks and Tree Spacing". North Carolina Cooperative Extension Service. Archived from the original on 11 September 2013. Retrieved 1 September 2013.
- Ferree, David Curtis; Ian J. Warrington (1999). Apples: Botany, Production and Uses. CABI Publishing. ISBN 0-85199-357-5. OCLC 182530169.
- Bob Polomski; Greg Reighard. "Apple". Clemson University. Archived from the original on 28 February 2008. Retrieved 22 January 2008.
- Barahona, M. (1992). "Adaptation of Apple Varieties in Ecuador". Acta Hort. 310: 139–142. doi:10.17660/ActaHortic.1992.310.17. Retrieved 17 July 2008.
- Adamson, Nancy Lee. An Assessment of Non-Apis Bees as Fruit and Vegetable Crop Pollinators in Southwest Virginia. Diss. 2011. Web. 15 October 2015.
- S. Sansavini (1 July 1986). "The chilling requirement in apple and its role in regulating Time of flowering in spring in cold-Winter Climate". Symposium on Growth Regulators in Fruit Production (International ed.). Acta Horticulturae. p. 179. ISBN 978-90-6605-182-9.
- "Apples". Washington State Apple Advertising Commission. Archived from the original on 20 December 2007. Retrieved 22 January 2008.
- Yepsen, Roger (1994). Apples. New York: W.W. Norton & Co. ISBN 0-393-03690-1.
- "Refrigerated storage of perishable foods". CSIRO. 26 February 2015. Retrieved 25 May 2007.
- Karp, David (25 October 2006). "Puff the Magic Preservative: Lasting Crunch, but Less Scent". New York Times. Retrieved 26 July 2017.
- Lowther, Granville; William Worthington. The Encyclopedia of Practical Horticulture: A Reference System of Commercial Horticulture, Covering the Practical and Scientific Phases of Horticulture, with Special Reference to Fruits and Vegetables. The Encyclopedia of horticulture corporation.
- Coli, William; et al. "Apple Pest Management Guide". University of Massachusetts Amherst. Archived from the original on 12 February 2008. Retrieved 3 March 2008.
- Bradley, Fern Marshall; Ellis, Barbara W.; Martin, Deborah L., eds. (2009). The Organic Gardener's Handbook of Natural Pest and Disease Control. Rodale, Inc. pp. 32–34. ISBN 978-1-60529-677-7.
- Erler, Fedai (2010-01-01). "Efficacy of tree trunk coating materials in the control of the apple clearwing, Synanthedon myopaeformis". Journal of Insect Science. 10 (1). doi:10.1673/031.010.6301.
- "Nutrition Facts, Apples, raw, with skin [Includes USDA commodity food A343] Nutrition Facts & Calories". Nutritiondata.com. Retrieved 3 January 2013.
- "Apple Varietals". Washington Apple Commission. Archived from the original on 2 July 2017. Retrieved 7 December 2017.
- Boyer, Jeanelle; Liu, RH (May 2004). "Apple phytochemicals and their health benefits". Nutrition journal. Cornell University, Ithaca, New York 14853-7201 USA: Department of Food Science and Institute of Comparative and Environmental Toxicology. 3 (1): 5. doi:10.1186/1475-2891-3-5. PMC . PMID 15140261.
- The Brown Apple. // The New York Times, 22 November 2010
- Ferdman, Roberto A. (19 May 2016). "A clever tweak to how apples are sold is making everyone eat more of them". The Washington Post. Retrieved 20 May 2016.
- "Organic apples". USDA Agricultural Marketing Service. February 2016. Retrieved 23 February 2017.
- "European Organic Apple Production Demonstrates the Value of Pesticides" (PDF). CropLife Foundation, Washington, DC. December 2011. Retrieved 23 February 2017.
- Ribeiro FA, Gomes de Moura CF, Aguiar O Jr, de Oliveira F, Spadari RC, Oliveira NR, Oshima CT, Ribeiro DA (September 2014). "The chemopreventive activity of apple against carcinogenesis: antioxidant activity and cell cycle control". European Journal of Cancer Prevention (Review). 23 (5): 477–80. doi:10.1097/CEJ.0000000000000005. PMID 24366437.
- Sapers, Gerald M.; Douglas, Frederic W. (1 September 1987). "Measurement of Enzymatic Browning at Cut Surfaces and in Juice of Raw Apple and Pear Fruits". Journal of Food Science. 52 (5): 1258–1285. doi:10.1111/j.1365-2621.1987.tb14057.x. ISSN 1750-3841.
- Determination of Polyphenolic Profiles of Basque Cider Apple Varieties Using Accelerated Solvent Extraction. R. M. Alonso-Salces, E. Korta, A. Barranco, L. A. Berrueta, B. Gallo and F. Vicente, J. Agric. Food Chem., 2001, volume 49, pages 3761–3767, doi:10.1021/jf010021
- Gosch, C.; Halbwirth, H.; Stich, K. (2010). "Phloridzin: biosynthesis, distribution and physiological relevance in plants". Phytochemistry. 71 (8): 838–843. doi:10.1016/j.phytochem.2010.03.003. PMID 20356611.
- Yu, Xiuzhu; Van De Voort, Frederick R.; Li, Zhixi; Yue, Tianli (2007). "Proximate Composition of the Apple Seed and Characterization of Its Oil". International Journal of Food Engineering. 3 (5). doi:10.2202/1556-3758.1283.
- Gerhauser, C (2008). "Cancer chemopreventive potential of apples, apple juice, and apple components". Planta Medica. 74 (13): 1608–24. doi:10.1055/s-0028-1088300. PMID 18855307.
- "General Information – Apple". Informall. Archived from the original on 23 July 2012. Retrieved 17 October 2011.
- Landau, Elizabeth, Oral allergy syndrome may explain mysterious reactions Archived 15 December 2011 at the Wayback Machine., 8 April 2009, CNN Health, accessed 17 October 2011
- Lewis S. Nelson; Richard D. Shih; Michael J. Balick (2007). Handbook of poisonous and injurious plants. Springer. pp. 211–212. ISBN 978-0-387-33817-0. Retrieved 13 April 2013.
- "Amygdalin". Toxnet, US Library of Medicine. Retrieved 20 April 2017.
- Pollan, Michael (2001). The Botany of Desire: a Plant's-eye View of the World. Random House. p. 22, cf. p. 9 & 50. ISBN 0375501290. Retrieved 3 January 2015.
- Apple (fruit and tree) at Encyclopædia Britannica
- Apple at the Encyclopedia of Life
- "Apple". National Center for Biotechnology Information (NCBI).
- "Malus pumila". Integrated Taxonomic Information System. Retrieved 18 February 2017.
- Malus pumila Mill. (accepted name)
- Apple Facts from the UK's Institute of Food Research
- National Fruit Collection (UK)
- Grand Valley State University digital collections- diary of Ohio fruit farmer Theodore Peticolas, 1863