Alternate bearing in fruit tree crops: past, present and future

Abstract

Alternate bearing, i.e., year-to-year alternation between large and small yields of fruit tree crops worldwide, is often caused by abiotic stress such as late frost at flowering. Economic losses for apple due to alternate bearing were calculated at between ½ and 5 mil US$ year-1 depending on growing region. We recorded the number of flower clusters over three years (autumn 2009 to May 2011) and yield of 2,086 trees of alternate-susceptible 'Elstar' apple on M.9 trees with and without hail nets (for light deprivation) resulting in a data set of 16,000 points. We calculated 'biennial bearing index' (BBI), the ratio of differences in tree yields to cumulative tree yield, and graphed to data to show the tree-to-tree alternation. A Pearson correlation between yield in year x and year x+1 and flower clusters in year x, gave an intersect of 91-106 flower clusters, whereas the mean yield in year x and year x+1, resulted in a range of 72-133 flower clusters, or 9.6 kg tree-1, as a thinning target for sustainable cultivation of 'Elstar' in a pedestrian orchard at 50°N or larger yields with taller trees at a lower latitude. The interaction between the crop levels on the spur and on the whole tree showed that if the tree crop level was low, the tree provided adequate resources for flower bud development in spurs carrying 1 or 2 fruit. Conversely, to obtain good return bloom on normal-to-heavily cropping trees required having good numbers of non-fruiting (also called “resting”) spurs. A scheme for the underlying regulatory mechanisms was developed, which includes light deprivation and subsequent lack of flower initiation, polar basipetal GA7 transport, cytokinin synthesis in the root and level in the xylem and phloem. Countermeasures include early chemical or mechanical thinning of young trees before reaching full crop load and early in terms of the flowering stage.