The effect of selective picking on the harvest fruit size distribution of 'Royal Gala' apples

The size distribution of fruit on an apple tree on any potential harvest da te can be estimated by measuring a sample of fruit early in the season and projecting forward in time using standard growth curves. However, 'Royal Ga la' apples grown in New Zealand are picked in multiple harvests spread over about three weeks, so the overall size distribution of harvested fruit can differ significantly from that on any single harvest date. On each havest date, fruit are selected for picking according to background colour, not si ze. By measuring the sizes at harvest of all fruit on selected limbs of eig ht 'Royal Gala' trees in Hawkes Bay, New Zealand, we show that the relation ship between the size distributions of picked and unpicked fruit can be des cribed by a single-parameter model. Although size is not used directly in f ruit selection, we find that larger fruit still tend to be picked first, wi th the odds of a fruit being selected approximately doubling for each 20 g increase in size. The mean harvest fruit size is largest for the first harv est, and decreases in later harvests, despite ongoing fruit growth. Because the smaller fruit left on the trees continue to grow, the standard deviati on of the overall fruit size distribution from multiple harvests can be exp ected to be significantly smaller than it would have been had all fruit bee n harvested on the same date. The model is tested using size and background colour measurements collected on tagged fruit at three sites in two years, including a range of shading and crop load treatments. For most combinatio ns of site, year and treatment, the model predicted the size distribution o f picked fruit well, but in a few cases a different parameter value was nee d to obtain a good fit.