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The taxonomy of Malus has traditionally been challenging. There are few morphological characters to distinguish the species, almost all of which readily form hybrids. Cultivars of the Domestic Apple have been widely planted around the world and have occasionally hybridised with wild species. Indeed, the nomenclature of the Domestic Apple has only recently been resolved (the correct name being Malus pumila Mill.: Mabberley et al. 2001). Several taxa of horticultural interest are unknown in the wild and may represent interspecific hybrids or the products of introgression or horticultural selection. Because of this ongoing uncertainty in Malus taxonomy, the number of species recognised fluctuates, but it ranges between 25 and 47 (Robinson et al. 2001). Apples are deciduous (rarely evergreen or semi-evergreen) trees or shrubs native to Asia, Europe and North America. They are usually unarmed and have simple, alternate, stipulate leaves. The leaf margins are serrate or lobed, and venation can be craspedodromous or camptodromous. Inflorescences are corymbose to racemose. The flowers are white or pink, 5-merous, with 15–50 stamens and a bowl-shaped hypanthium. The fruit is a green, yellow or red pome with mealy flesh and a tough core (Robertson et al. 1991, Fiala 1994, Gu et al. 2003).
One of the most interesting horticobotanical stories to be elucidated in recent years is that of the Domestic Apple Malus pumila, traced to its origins in the wild ‘fruit forests’ of central Asia by Barrie Juniper and others, and the unravelling of a complex taxonomy to reveal the correct name to be applied to the plant (Mabberley et al. 2001). Perhaps the most remarkable discovery is that our culinary apples seem to be unhybridised and more or less accurately reflect the variation found in wild populations of M. pumila (Juniper & Mabberley 2006). Among variants that have not been selected for cultivation, unsurprisingly, are the occasional fiercely armed trees to be found in the same central Asian fruit forests (B. Juniper, pers. comm. 2007). Leaving aside its culinary values, the Domestic Apple is an excellent ornamental tree, producing reliable, beautiful blossom and good crops of attractive fruit, while in old age it can be picturesquely gnarled: it lacks, usually, only good autumn colour. Wild-origin material is often found in collections under the name M. sieversii (Ledeb.) M. Roem. The heavily red-pigmented apple known as M. niedwetzkyana is apparently a mutation occurring very infrequently in wild populations (B. Juniper, pers. comm. 2007).
The taxa described below are an assortment of obvious hybrids, putative hybrids and apparently ‘good’ species, probably of most interest to the serious collector. They do not stand up to a comparison with the more recent hybrid crabapple cultivars, which are frequently outstanding for both floral and fruit effect. A handful of particularly good new cultivars are mentioned here, but for further information there are several useful monographs that could be consulted, including Fr John Fiala’s Flowering Crabapples (1994) – although this is now somewhat out of date with regard to newer cultivars. Dirr (1998) also discusses a wide range, with special reference to their disease tolerances. The International Ornamental Crabapple Society is the International Cultivar Registration Authority for crabapples; new cultivars are discussed in their journal Malus, and the Society promotes trial and evaluation collections, comparing cultivars under prevailing conditions at 24 sites across the United States. A good range of cultivars can also be seen in many public gardens elsewhere, including two National Plant Collections in the United Kingdom.
Chris Sanders has kindly provided notes on the following recent cultivars, notable for having proved themselves as reliable garden trees and which should be among the first considered for a collection. The French-origin ‘Evereste’ has red buds opening to white flowers that produce heavy crops of distinctive, orange-red fruits, persisting until December. It makes a small round-headed tree with good disease resistance, and was released by the Institut National de la Recherche Agronomique (INRA) in about 1980. With the same floral coloration, and also from France, raised by Jean-Pierre Hennebelle in the early 1980s, is ‘ Comtesse de Paris’ whose globular yellow fruits persist unscathed on the tree well into the new year and outdo any other yellow-fruited crabapple. Similarly coloured flowers again are a feature of the American cultivars ‘Adirondack’, bred by Don Egolf of the US National Arboretum and introduced in 1987, and ‘Snowcloud’, from Princeton Nursery, New Jersey in 1970. ‘Adirondack’ is compact and slow-growing but with an upright habit, and is a good choice for smaller spaces. Its fruits are bronze-red. ‘Snowcloud’ has abundant semi-double flowers and produces a relatively sparse crop of yellow fruits, but has a vigorous upright habit that makes it a very useful garden tree. It is said to be susceptible to scab in the United States, but this is not a problem in the United Kingdom. Bright pink flowers opening from even darker buds are among the attractive characters of ‘Indian Magic’, introduced by Simpson Nursery, Indiana in 1975. The fruits change from glossy red to golden-orange and persist well on the tree, which also produces good autumn colour. ‘Princeton Cardinal’ (introduced by Princeton Nursery in 1990) has the brightest red flowers of this assortment of suggestions, abundantly produced and opening early in the season, combined with glossy red disease-resistant foliage on this small but broadly vase-shaped cultivar.
As extremely cold-hardy trees Malus cultivars are popular in North America, where they can withstand severe winters with impunity, but some are susceptible to a range of pests and diseases, and appropriate homework is wise before a choice is made.
A genus of some twenty-five to thirty deciduous trees, including a few shrubs, distributed over the temperate parts of the northern hemisphere. Leaves simple, occasionally lobed. Flowers usually in umbel-like clusters, white to various shades of pink or purplish. Stamens fifteen to fifty, anthers usually yellow. It differs from Pyrus, with which it was long generically united, by the styles being united at the base – free in Pyrus. The fruits in most species are crowned with the persistent calyx and vary in shape from rounded to ovoid, but are never truly pear-shaped as in Pyrus. In Pyrus, moreover, the flesh of the fruit contains stone-cells (grit-cells). In Malus these are absent except in a few anomalous species, notably M. prattii, M.yunnanensis, M. tschonoskii, M. florentina, and M. trilobata.
So far as is known there is no hybrid between Malus and Pyrus, nor with any other of the genera once included in Pyrus, and the fact that it will not readily intergraft with them further shows that Malus is more distinct from the other sections of the old genus Pyrus than was once imagined. It was the incompatibility of the apple and pear in grafting that chiefly led Philip Miller to maintain the genus Malus in defiance of Linnaeus, who had sunk it in Pyrus. 'I shall therefore beg leave,' he wrote, 'to continue the separation of the Apple from the Pear, as hath been always practised by the botanists before his time.'
Many of the crabs rank highly as ornamental trees. In all the range of flowering trees and shrubs there is nothing more beautiful and effective than the best of this group, such as M. spectabilis and M. floribunda. In regard to fruit the genus contains many valuable species and hybrids, such as M. × robusta, 'John Downie', 'Golden Hornet', 'Red Sentinel' and many others.
The species of Malus hybridise with each other, so that with many one cannot rely on seeds to reproduce the parent exactly. Some crabs can be rooted from cuttings made of leafless shoots in early winter, and put in a cold frame, but most of them are increased by grafting on the various stocks used in nurseries for garden apples.
Attention must be called to the fact that the crab apples, like all members of the section Pomoideae of the Rose family, are subject to attack by the devastating disease 'fire-blight', for which see the note in Vol. I, p. 730.
Useful notes on the species and hybrids grown in Britain will be found in the article by H. S. J.Cranein Journ. R.H.S., Vol. 86 (1961), pp. 160-167; and in Hillier's Manual of Trees and Shrubs. An assessment by H. J. Grootendorst, based on trials at Boskoop, Holland, was published in Dendroflora, Vol. 1 (1964). A valuable American work is: A. F. Den Boer, Ornamental Crab Apples, published by the American Association of Nurserymen in 1959. In Dr Donald Wyman's Trees for American Gardens (ed. 1965) there is an extensive section devoted to Malus.
For the hybrids see the special section starting on p. 714.
The report on trials in Holland published in the first issue of Dendroflora (1964) is superseded by a later report by P. Lombarts, published in No. 21 of the same periodical (1984), pp. 39-62, with a summary in English.