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Alternate cropping is a common physiological trait in several important apple cultivars, including spur-type ‘Delicious’. Alternate bearing usually develops slowly in apple trees as they mature, but may be dramatically induced by environmental or biotic stresses. We describe the native temporal and within-year variation of bloom and yield of a highly uniform spur-type ‘Delicious’/M.106 population (n = 95) over a period of 9 years. Crop load was adjusted by hand in the first year to establish a defined normal fruit population distribution. Thereafter, all trees received identical practices. Bloom density (BD) was rated (1 to 10) and yield and fruit size distribution were determined annually on an individual tree basis. Temporal profiles for bloom and yield had four periods above and below the general mean (GM). There were two 3-year periods of nonbienniality. Annual variation in BD ranged from 3.1 to 8.0 and in yield from 54 to 168 kg/tree. Variation in cropping was greater when expressed as percentage deviation from the GM. Annual mean fruit weight was inversely related to yield, but percentage of small- (51 to 64 mm) or large-diameter fruit (70 to 82 mm) was not consistently related to yield. The within-year cv ranged from ≈11% to 66% for BD and from 13% to 42% for yield. The degree of synchrony (within-year variation) was lowest in the year after crop adjustment (to normal distribution), became highly synchronized in ≈4 years, and then decreased. The relationship of native variation of individual trees to the population and to flower initiation and fruiting are discussed in relation to the alternate bearing cycle and significance in selecting trees for experimentation.

Previous reports have provided evidence that measuring fruit growth rate may be a viable method to predict if a fruit will abscise or persist through the June drop period. A series of experiments were carried out over several years to develop a procedure that could be used to predict the response to a chemical thinner application within 7 to 8 days after application and before thinners exhibit their final effect. The procedure developed involves tagging 105 spurs on seven individual trees distributed appropriately in the orchard. A minimum of two measurements must be made, one 3 to 4 days after application and again 7 to 8 days after application. This model requires that fruit measurement should not start before fruit grow to a diameter of 6 mm and individual fruit within a spur should be numbered and identified. The model is based on the assumption that if fruit growth rate of a particular fruit over the measurement period is less than 50% of the growth rate of the fastest growing fruit on the tree during the same growth period, it will abscise, whereas if fruit growth rate exceeds 50% of the growth rate of the fastest growing fruit, it will persist. All data can be entered into an Excel spreadsheet and the output in the summary page gives the predicted fruit set expressed as percentage of the total number of fruit present. The strategy for crop load adjustment with chemical thinners has evolved over the years to a point where most orchardists plan and are prepared to make two or more thinner applications. The dilemma associated with this approach is to determine if additional thinner applications are necessary. Up to this point a tool designed specifically to provide this information has not been developed.