Apples are among the most popular fruits cultivated worldwide, but successful production relies entirely on pollination. Understanding the reproductive requirements of an apple tree is fundamental to ensuring fruit set. When a tree flowers, blossoms must be fertilized with compatible pollen to initiate apple development. This determines whether a single tree can produce fruit alone or if it requires a partner.
Defining Self-Unfruitful Apple Varieties
Self-pollinating, or self-fruitful, apple varieties possess the genetic capacity to fertilize their own ovules using pollen produced by the same tree. Although these trees can bear fruit alone, they often benefit from cross-pollination with a different variety, resulting in a larger yield and better quality fruit.
The vast majority of cultivated apples, however, fall into the category of self-unfruitful varieties, meaning they are self-sterile. These trees have a natural genetic mechanism called gametophytic self-incompatibility that prevents them from accepting their own pollen. For these varieties to produce fruit, they must receive viable pollen from a genetically distinct and compatible apple tree, a process mediated primarily by insects like bees. Planting a single, self-unfruitful tree often leads to a lack of fruit.
Genetic Status of Red Delicious
The Red Delicious apple is categorized as self-unfruitful and requires cross-pollination to set fruit. Beyond merely needing a partner, the Red Delicious variety presents a specific challenge due to its unique genetic structure. It is a triploid variety, meaning its cells contain three sets of chromosomes instead of the standard two sets found in diploid apple varieties.
This triploid status significantly impacts its role in an orchard, making it a poor pollen donor. The irregular number of chromosomes interferes with the formation of viable pollen, causing the Red Delicious variety to be functionally pollen-sterile. Consequently, the Red Delicious tree cannot effectively pollinate itself or any other apple tree.
Because of this sterility, a Red Delicious tree must be pollinated by a compatible diploid variety. If the grower plants only the Red Delicious and one diploid pollinator, the diploid tree will lack a compatible pollen source to pollinate itself. Therefore, the recommendation is to plant the triploid tree near at least two different compatible diploid varieties to ensure adequate pollination for all trees.
Choosing the Right Pollination Partner
A grower must select a pollinator that is a diploid variety, meaning it has viable pollen, and that has a bloom time which overlaps with the flowering period of the Red Delicious tree. The pollinator must be genetically distinct but compatible, which excludes varieties that share an incompatibility allele. For Red Delicious, many common varieties serve this purpose well, provided they are not also triploid.
Examples of effective, compatible diploid apple varieties often recommended as partners include Gala, Fuji, and Granny Smith. Crabapple trees are also frequently used in orchards because they produce abundant, viable pollen and have extended bloom periods. By planting at least two different diploid varieties that flower simultaneously with the Red Delicious, the grower ensures the Red Delicious receives the necessary pollen and that the donors can also successfully cross-pollinate each other.