Ever since Europeans began colonizing North America about 400 years ago, apple has been an important crop, used fresh or cooked and as cider and farm animal feed. In 1905, S.A. Beach, of the New York Agricultural Experiment Station at Geneva, published The Apples of New York, in two volumes. This book exhaustively catalogues and describes (with many color illustrations) the hundreds of apple varieties grown in North America at that time. Most of them had been developed from native North American stock or European seed brought with them by emigrants, but some of them were stable European varieties that, in Beach’s time, were already several hundred years old. Many of the varieties that lacked desirable characteristics have disappeared from cultivation, but a number that Beach rated favorably are familiar to us today and are exact genetic copies of the trees he described.
Plants are reproduced by two general means: sexual (by seed) propagation and asexual (vegetative) propagation, in which the plant is replicated by division, cuttings, layering, buds, scions, and so forth. During seed formation, rearrangement of the genes contributed by both parents occurs and the progeny can exhibit infinite variations in characteristics, including size; form; disease susceptibility or resistance; foliage, bark and buds; growth habit; time of flowering and fruit ripening; and fruit color, flavor, size and shape. There are exceptions to this, but in general, progeny produced by seed are different from their parents. In annual plants, to produce seed that maintains the desired characteristics (the parental variety), careful breeding is required for each growing season. This is not practical or economical for woody plants that may take many years to mature, so some form of vegetative propagation is utilized. For most woody horticultural plants and fruit- or nut-producing plants, grafting is the obvious and best choice.
In grafting, a “scion” (a section of stem with leaf buds or a bud alone) taken from the plant you wish to propagate is inserted into a wound made in another plant that has an established and healthy root system. This plant is generally closely related to the scion. In the case of maintaining a specific variety, the scion is usually grafted to the stem of the host plant a few inches from the ground. When the scion has begun to grow, the majority of the host plant is removed, leaving only the stump—or “rootstock”. If all goes well, scion and rootstock will become a single, actively growing plant composed of two genetically distinct parts.
Grafting is an ancient technique. How ancient is not known. But there are references to grafting cultivated olive to wild olive trees in the New Testament (Romans 11:17–24), and Alexander of Macedon (the Great) is reported to have brought back apple varieties on rootstock from Asia Minor in 300 BC. So grafting as a horticultural practice goes back that far at least. The initial purpose for grafting would have been to maintain varietal integrity, but it would have soon become clear to the nurserymen of the time that there were other benefits (and problems) associated with particular rootstocks.
Alexander’s apples are supposed to have been on size-controlling or “dwarfing” rootstock. Not too many home gardens can accommodate a full-size fruit tree, so fruit trees are commonly sold on dwarfing rootstock at retail nurseries. As a result, this type of rootstock is most familiar to most people. But there is not a single dwarfing rootstock that has a single effect on its scion. There are lots of dwarfing rootstocks and more are always being developed. There are rootstocks that dwarf to varying degrees. In apple, for instance, there are 10 size classes of trees determined by rootstocks, in which “1” is a dwarf just 3 feet high (with pruning) and “10” is a standard-sized tree with no dwarfing.
Tree size is an important characteristic, but it is only one of a multitude of traits that rootstocks impart to their scions. Rootstocks are selected for their rooting ability (anchoring); resistance to diseases, nematodes, insects and environmental stresses such as drought; nutrient acquisition ability; and tolerance to particular soil types—heavy, nondraining clays, for example. They are selected also for qualities that are positively expressed mostly by the scion: vigor, hardiness, yield, precocity and fruit characteristics. For the commercial grower of anything from woody horticultural plants to various nuts to pome fruit, olive, citrus, avocado and grape, there are an abundance of choices of rootstock that can, in essence, be tailored to the requirements of the grower.
Choices should be considered well. A case in point is that of grape phylloxera, a serious pest of commercial grape worldwide. Phylloxera is a minute insect native to eastern North America, where it infests common grapes but without much effect; its galls can be seen on the undersides of grape leaves in fall. On susceptible grape varieties, though, it feeds on roots and leaves, stunting and killing the vines. In the mid-19th century, phylloxera was brought inadvertently to Europe on samples of North American grape. In the 1880s, it nearly destroyed the European wine industry. The epidemic was curbed only when vintners adopted the novel practice of grafting their varieties onto recently developed phylloxera-resistant rootstock.
About eighty years later, much of California’s wine-grape crop (75% in Napa and Sonoma counties) was grown on a popular and adaptable rootstock (AxR1) that was a cross between a phylloxera-susceptible French variety and a resistant American one. The rootstock had been developed in France in the early 20th century, but its phylloxera resistance had soon broken down in Europe and Australia. Despite this and the availability of other sources of resistance, AxR1 was recommended to California growers, and when resistance failed in California, the vineyards growing grapes on it were devastated. Many of these vineyards are still being replanted. There are now many more choices of phylloxera-resistant rootstock, which is the only effective means of controlling phylloxera in severely infested areas; these rootstocks are based on sources of resistance found in North American grape varieties.
Were we to resurrect Beach, he might not be surprised to see many of the old apple varieties he worked with 100 years ago, but he would be astonished by the gains we’ve made in rootstock development and the plethora of traits that are available in them. Retail consumers have fewer choices in rootstocks and must rely on what mail order and local nurseries offer. If you’re planning to plant fruit trees this spring, almost certainly what you buy will be on dwarfing rootstock. But for interest look up the particular rootstock and see what other properties it possesses.