Most apple varieties do not set a full crop of fruit without a compatible pollinator, particularly if flowering is sparce or weather conditions are adverse. Successful pollination and the formation of many healthy seeds within the fruit contribute to the eventual size and quality of that fruit. Consequently, it is unwise to consider planting large blocks of a single apple variety.
Successful pollination depends on several factors.
The two apple varieties must produce compatible pollen. Compatability often favours one variety as a parent but disadvantages the donor. When planning new plantings, look critically at the weakest cross in the combinations planted. While two varieties may be compatible, if the flowering period for one is too early or too late for the other, it is unwise to select either as a pollinator. The environmental conditions must be favourable.