A long-term study on native variation of flowering and fruiting in spur-type 'Delicious' apple

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.