A New Apple

Domesticated apple ( Malus domestica Borkh) is one of the worlds most important fruit crop. According to FAOSTAT (2004), world apple production exceeded 57,000,000 metric tons (MT) in 2003. Leading producing nations include (in descending order) China , USA , France , Poland , Turkey , Italy , and Russia . While there are well over 6,000 documented apple cultivars, commercial production still relies on fewer than twenty major cultivars, with 'Delicious', 'Golden Delicious', 'Granny Smith', ' Fuji ' and 'Gala' accounting for 61% of the total production. Apples are processed into many products including juice, applesauce, slices (dried, frozen and canned), and cider (sweet and hard). Many Malus species are also important in the landscape market and in breeding for better ornamental types. In the past, the most important commercial cultivars arose as chance seedlings (Red Delicious, Golden Delicious, Granny Smith, Rome, McIntosh, etc.) from the North American gene pool, and such seedlings continue to be commercialized today, for example, 'Ginger Gold' and 'Cameo'. However, increasingly popular new cultivars such as ' Fuji ', 'Gala', 'Jonagold', ‘Empire', and 'Honeycrisp' are derived from breeding programs. One quarter of the total U.S. production of apples in 2004 was from ‘bred' varieties whereas 70% was from ‘chance seedling' apples. A downward trend for the two leading apple varieties (Red Delicious and Golden Delicious) is making space for newly bred apple varieties. To study genetic diversity, Noiton and Alspach (1996) analyzed pedigrees of 50 apple cultivars being used in breeding. 'Cox's Orange Pippin', 'Golden Delicious', 'Jonathan', and 'McIntosh' were the most frequent progenitors. Few of the cultivars sampled were substantially inbred, but continued inbreeding is a concern. Breeders currently are working with reduced genetic diversity and must attempt to expand the genetic base. Several breeding programs are tapping into wild species to harness disease resistance genes present in those species with conventional breeding and/or genetic engineering.

Fire blight disease, caused by the bacterium Erwinia amylovora , remains to this day as the most serious bacterial disease of pears, apples, quince, mountain ash, spirea, hawthorn, pyracantha, and cotoneaster, among many other Rosaceae species. Fire blight control measures are costly, laborious, and quite often remain ineffective. Breeding efforts to introduce genetic resistance to fire blight have been difficult as sources of genetic resistance to fire blight are small-fruited Malus species, and resistance is not under simple genetic control. Recently, transgenic approaches have been used to improve the genetic resistance to fire blight; e.g., the attacin E gene from the giant silk moth ( Hyalophora cecropia) was transferred into the Gala variety and the M.26 rootstock, and some attacin-transgenic lines showed increased resistance to fire blight (Ko et al., 2000). However, due to the insect origin of the transgene, fruiting cultivars expressing the attacin gene may not be acceptable to some consumers and growers. Therefore, efforts are now emphasizing over-expression of genes involved in resistance, or silencing of genes involved in disease development.

Apple Rootstocks

Apple rootstocks in the U.S. are almost exclusively propagated asexually from layering mother plants in stool beds. Production of apple rootstocks is highly centralized with 12 nurseries producing 90% of clonal rootstocks in the U.S. Dwarfing apple rootstocks account for roughly 98 % of new plantings in the U.S. , while the remaining are standard seedling rootstocks. The germplasm diveristy of dwarfing apple rootstocks in cultivation is highly restricted. Recent figures estimate that 85% percent of dwarfing apple rootstocks being planted originated from one breeding program (East Malling, UK ). Roughly 12% originated from a Russian (Budagovsky) breeding program and 2% from a U.S. breeding program ( Geneva ). The leading genotype being planted today in the U.S. is Malling 9 (M.9, Malus × domestica Borkh.), with roughly 45% of new plantings. Second is Malling 26 (M.26, Malus × domestica Borkh., 25%) and third is Budagovsky 9 (B.9, Malus × domestica Borkh., 10%). Therefore three genotypes and their clones represent 80% of the diversity of apple rootstocks in new cultivation. This lack of diversity is compounded by the fact that the three rootstocks are closely related. The same genotypes are highly susceptible to fire blight ( Erwinia amylovora ) and components of the apple replant disease. The only successful attempts to increase genetic diversity of the U.S. apple rootstock market come from the Geneva breeding program that has used interspecific crosses (involving Malus floribunda , M. prunifolia , M. robusta , and M. sieversii ) to introduce resistance to fire blight and other diseases in the cultivated germplasm. Several of these new rootstocks are beginning to be planted commercially.