As a rule most apple varieties prefer a climate where there is a definite winter period with temperatures around or below freezing, and summer temperatures not exceeding the low 90s. In contrast Southern California has a "Mediterranean" climate, with long hot dry summers and mild winters. Typical of a Mediterranean climate, almost the entire annual rainfall occurs in winter.
The key challenge for growing apple trees in this climate is the high summer temperatures and long growing season, which can play havoc with the flavor and texture of varieties which are not suited to it.
Southern California's mild winters mean it is a low-chill climate, with fewer than 600 hours of winter temperatures below 40F. Although low-chill varieties may be preferred for this reason, the lack of winter chill is more of an issue for the choice of rootstock than the choice of scion variety. It seems that more vigourous rootstocks are needed to counteract the loss of vigour induced by the low-chill climate. In addition, once the tree starts fruiting in this climate it won't grow much more, another reason for choosing more vigorous rootstocks.
Fortunately there are number of good quality apple varieties which are quite happy in these conditions, which we list here. Perhaps surprisingly, the climate of the place of origin of an apple variety is not a reliable indicator of whether it will thrive in southern California (although apples from Australia do consistently well). We are grateful to Kevin Hauser of Kuffel Creek for these recommendations.