An apple's size is determined by the cells that make up the apple. The number of cells, size of each cell, and air space between cells all can play roles in fruit size. An average-sized apple has about 50 million cells. Some apples achieve their larger size by having a larger number of cells, and some achieve their larger size by having larger-sized cells.
Most apple growers strive to produce larger-sized fruit. To achieve this goal, growers should consider the factors that influence apple fruit size and the importance of each factor. Factors that affect fruit size can be categorized as genetic, cultural, and environmental.
Genetic Factors The number one factor affecting apple fruit size is the genetics of the cultivar. Think of a crab apple. No matter what growers do, crab apple fruit always will be small. Likewise, Honeycrisp and Mutsu fruit tend to be large. Genetics can affect both cell number and cell size.
Cultural Factors The crop load (number of fruit per tree in relation to the size of the tree) is one of the most important factors that growers can influence. Reducing the crop load (also called thinning) allows the fruit that remain to become larger. Thinning increases fruit size largely by providing resources for increasing the cell number of the remaining fruit. Generally, the earlier that thinning is performed, the greater the success in increasing fruit size. Other cultural factors that can affect fruit size are pruning, pollination, nutrition, and soil management.
Environmental Factors The most important environmental factors influencing fruit size are temperature and light. Early in the growing season, the dominant form of fruit growth is cell division; later in the season, the dominant form of fruit growth is cell expansion. The critical time period for fruit growth is the first five weeks or so following bloom. Growers can expect larger fruit size in years with warm, sunny weather in early spring than in years with a lot of cool, cloudy weather. Warm temperatures increase fruit size primarily by providing conditions that increase cell size.
To achieve large fruit size, growers should try to optimize all these factors but should concentrate on adequate early fruit thinning, selecting sites that have warm early spring temperatures, and pruning to achieve good light penetration and high-quality flower buds. Of course, achieving larger fruit size is much easier when the cultivar being grown has a high genetic potential for large fruit size.
Peter Hirst, Purdue University