HOW TO GROW BIG PEACHES

Produce buyers have been demanding larger and redder peaches. Fifteen years ago it was possible to sell peaches that were 2 1/8” in diameter, now the minimum diameter is 2 1⁄2”. There is a negative relationship between yield and fruit size, so the challenge now is to produce fruit of marketable size while obtaining yields that are high enough to make a profit. There are a number of factors that influence fruit size and this paper will provide a discussion of those factors. Peach fruit growth. Peach fruits have three fairly discrete stages of growth (Figure 1). The first stage (Stage I) lasts from full bloom until about 50 days after bloom. During this time the fruits grow fairly rapidly and growth is primarily due to cell division. Most of the cell division probably occurs during the first 30 days after bloom, but the length of stage I may be influenced by temperature. There is an increase in both fruit size and fruit dry weight. During stage I, shoot growth begins but there is too little foliage on the tree to support the growth of the fruit and shoots. Therefore much of the carbohydrates for early fruit and shoot growth come from reserves stored in the tree during the previous season. The period of cell division may be prolonged during cool weather. Stage two begins with pit hardening, which is the lignification of the endocarp. During this stage the fruit increases little in size, but there is an increase in fruit dry weight. Rapid shoot extension occurs and adequate leaf surface develops to support fruit growth. During this stage carbohydrate is preferentially partitioned into vegetative parts of the tree rather than into the fruit. The duration of stage II depends on the variety, and it may last only a few days for early-season varieties or it may last 6 to 8 weeks in late-season varieties. The final phase of fruit growth, Stage III, is often called the “final sweet” because the fruits growth very rapidly during the final 6 weeks before harvest. Fruit growth during this stage is due primarily to cell expansion as the fruit flesh accumulates water and the nearly fully developed canopy supplies fruits with sugars. Because the number of cells in a fruit and the size of those cells influence fruit size, there are really two ways we can influence fruit size. During stage I we can try to provide conditions for maximum cell division and during Stage III we can try to provide conditions for maximum cell size. Genetics All commercial peach producers know that some varieties are bigger than others. Johnson and Handley (1989) thinned several varieties to different number so fruit per tree and then estimated average fruit weight at harvest. To simplify variety comparisons, I have taken just part of their data set to compare average fruit weight at one crop load. When there were 800 fruit per tree, average fruit weight was 90 (about 2.2”), 140 (about 2.6”), and 210 (about 2.8”), respectively for ‘May Crest, ‘June Lady’, and ‘Elegant Lady’. The other aspect of this is that we can use these data to estimate the yield for each variety when the average fruit diameter is about 2 1⁄2” (130 g). ‘May Crest’ can support 190 fruit per tree, which gives a yield of 88 lbs/tree, whereas ‘June Lady can support 1000 fruit per tree, which is 130 lbs/tree. From this study we can see that ‘May Crest’ produces smaller fruit at all crop loads than ‘June Lady’, and to obtain a high percentage of saleable fruit, we will only get about 30% of the yield with ‘May Crest’ that we can obtain with ‘June Lady’. About 15 years ago Dr. Ralph Scorza, a peach breeder with the USDA, asked me why some varieties were small: was it due to low cell numbers or to small cell size? I didn’t know the answer and he could find anyone who did, so he and his team (Scorza et al., 1991) sampled fruit from two large varieties and two small varieties throughout the season to measure fruit numbers and cell size. They found that the large varieties (‘Loring’ and ‘Suncrest’) had more cells than the small varieties (‘Boone’ and ‘Bailey’) (Table 1), but cell size was not consistently large for large varieties. In addition, the differences in cell size were apparent during the fall before bloom. There are several take-home lessons from these studies. 1.Some varieties have greater potential to produce large fruit because they have more cells. 2.We can maximize fruit size by maximizing cell division during Stage I of fruit growth. 3. Although most early-season varieties produce small fruit, growers should plant the varieties within a season that produce the largest fruit. Water Relations.