Native Variation of Flowering and Fruiting In Spur-Type ‘Delicious' Apple

Native variation is a common physiological trait in polycarpic woody species.This condition in apple often limits the value of some otherwise desirable cultivars, including ‘Delicious’. Irregular cropping in ‘Delicious’ usually develops slowly as the trees mature, but may be dramatically induced by severe stresses, e.g. frost, drought, biotic and over-cropping. We followed temporal and within year variation of flower density, yield and fruit size on a highly uniform population (n=95) of Redchief ‘Delicious’/M.106 growing on a virgin site over a period of 8 years. Crop load was adjusted by hand in the first year to establish a tree population with a normally distributed fruit load. Thereafter, all trees received identical cultural practices (no further crop load adjustment) for the duration of the study. We rated bloom (1 to 10, highest) and measured yield and fruit size on a per tree basis annually. The temporal profiles for bloom and yield show four periods of bloom and yield above and below the population mean. There were two, two-year periods of insignificant biennial alternation for both measurements. The annual range in bloom density varied from 3.0 to 8.1 and in yield from 54 to 168 kg/tree, a 2.7-fold difference in bloom and 3.1-fold difference in yield. Irregular cropping was more pronounced when expressed as a percentage deviation from the population mean or percentage change from the previous season. The intensity of alternation (Hoblyn’s Intensity Index) ranged from 0.01 to 0.37 for bloom and from 0.04 to 0.37 for yield. There was no significant relationship between yield and incremental diameter growth of the trunk in the same or following year. Annual mean fruit weight was inversely related to annual yield, but percentage of small (51 mm to 63 mm) or large (70 mm to 82 mm) fruit were not consistently related to yield. The within year variation (synchrony) varied widely with the coefficient of variation (CV) ranging from ~11 % to ~66 % for bloom and ~11 % to ~42 % for yield. The highest degree of synchrony in bloom (~11 %) occurred in the tree population which had the highest CV in yield the previous season. The relationship of performance of individual trees relative to the population and significance in selection of trees for experimental studies on flowering and fruiting will be discussed.