Like most fruit trees, selected apple varieties are generally maintained as trees in field collections. Genesys lists almost 39,000 accessions of various kinds of apple, including wild relatives that breeders may find useful. About 5% of the accessions are in the form of seeds, but because apple seeds do not breed true, these are not as useful to plant breeders, although they may be of value as reservoirs of diversity. A few samples of apple tissue are cryopreserved in liquid nitrogen.
Apple trees can take several years to reach fruit-bearing maturity. Traditional breeding is costly and time-consuming -- with 15-25 years between the initial cross and the release of an improved variety. For this reason, breeders have used the diversity of genebank collections to look for associations between genetic markers and the presence of certain traits, such as aroma, storage quality and disease resistance. These markers then allow breeders to screen a population of crosses for potentially worthwhile individuals before the trees have borne fruit. This approach is known as marker assisted selection (MAS).
Although very few apple accessions are known to be from wild trees, scientists have become increasingly interested in the diversity of wild apples as a source for traits to improve commercial varieties.