Fruit Count, Fruit Weight, and Yield Relationships in ‘Delicious’ Apple Trees on Nine Rootstock

Annual yields per tree for ‘Starkspur Supreme Delicious’ apple (Malus domestica Borkh.) trees on nine size-controlling rootstock were related linearly to number of fruit per tree at harvest each year, independent of rootstock. Mean fruit weight was inversely and less closely related to number of fruit per tree when adjusted for tree size (fruit load). Annual yield-fruit count data for 9 years analyzed together showed that the number of fruit per tree was the principal factor determining yield, regardless of rootstock or tree age. A curvilinear relationship between yield and fruit count per tree during 9 years suggests that the sink strength of an apple crop is nearly, but not precisely, proportional to the number of fruit per tree. The mechanisms by which apple rootstocks influence growth and productivity in scion cultivars are understood poorly (Tubbs, 1973a, 1973b). Apple rootstock genotypes produce large differences in tree size, precocity, yield, and yield efficiency (Elfving and McKibbon, 1991; NC140 Cooperators, 1987, 1991). Factors influencing apple fruit growth include weather-related phenomena, cultivar, rootstock, fruit load, tree vigor, and carryover effects from previous years (Forshey and Elfving, 1989). Nonetheless, an apple tree’s yield is primarily a function of the number of fruit at harvest (Forshey and Elfving, 1977). Received for publication 8 Dec. 1992. Accepted for publication 22 Mar. 1993. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. lPresent address: Tree Fruit Research and Extension Center, Washington State Univ., 1100 N. Western Ave., Wenatchee, WA 98801. 2Research Scientist (Resigned). Present address: 785 Nahal Meshushim St., Makabim D.N., Modiim 71908, Israel. HORTSCIENCE, VOL. 28(8), AUGUST 1993 The relationship between fruit size and the number of fruit per tree is more variable, a result reflecting differences in tree vigor and growing conditions (Forshey and Elfving, 1977; Volz, 1988). A linear relationship between number of fruit per tree and yield has been reported in several studies with field-grown (Forshey and Elfving, 1977; Grauslund, 1976; Volz, 1988) and potted (Hansen, 1975) trees, while a curvilinear relationship also has been reported for field-grown (Landsberg et al., 1977) and potted (Hansen, 1977a, 1982) trees. Forshey and Elfving (1977), Grauslund (1976), and Volz (1988) found that a single, linear relationship fit yield–fruit count data from trees on up to three rootstock of similar vigor or on a single rootstock for up to five seasons. However, no studies have examined this relationship by comparing trees grown on size-controlling rootstock of widely different vigor. In addition, we know of no reports that evaluate yield relationships in the same trees over several years from the first year of production to tree maturity. The detailed harvest data collected annually for a rootstock trial with ‘Delicious’ apple at the Horticultural Experiment Station in Simcoe, Ont., Canada, allowed us to explore the relationships among number of fruit, fruit weight, and yield over a wide range of tree sizes during 9 years starting from the first year of production. ‘Starkspur Supreme Delicious’ apple trees on nine rootstock were planted as whips in 1980 at the-Horticultural Experiment Station. The trees were managed according to guidelines established by the NC 140 regional project protocol (NC 140 Cooperators, 1987). The following rootstock were included in this trial: M.7 East Mailing–Long Ashston (EMLA), M.26 EMLA, M.9 EMLA, M.27 EMLA, M.9, Michigan Apple Clone (MAC) 9, MAC 24, Ottawa (0) 3, and Oregon Apple Rootstock 1. The planting was spaced at 3.5 × 5.5 m on a well-drained Tavistock sandy loam consisting of 40 to 100 cm of sandy-loamy material over glaciolacustrine clay (Hohner and Presant, 1989). The trial was established with five randomized complete blocks. Trunk diameter was used as the criterion for assigning trees to blocks at planting. The trunks of all trees on M.9 EMLA, M.9, and M.27 EMLA were supported in 1983 with a stake extending 80 cm from the soil surface. The free-standing canopy of each tree was trained to the conventional central-leader system (Tehrani et al., 1988). Fertilizers, pesticides, and herbicides were applied according to commercial practices. One tree each on O 3 and M.9 EMLA was lost before the first year of production in 1983. All trees except those on MAC 24 were thinned chemically in 1985 and 1988 using naphthaleneacetic acid at 5 mg·liter-1. Crop load was not adjusted further. In 1987, a frost at bloom reduced the crop in the portion of the canopy below ≈1 m in height. Yield and mean fruit weight were determined by counting all fruit on each tree plus any fruit that dropped at harvest and weighing them. Trunk circumference was measured on each tree each year after leaf drop at a premarked location 30 cm above the soil surface. Annual growth and cropping data for all rootstock combined for each year from 1983 through 1991 were evaluated by analysis of variance on year. Means were separated using Duncan’s multiple range test for unbalanced data when the overall F test was significant (Damon and Harvey, 1987). Yield data across rootstock were analyzed further by nonradi-