Repeating cycles of ON and OFF yields in alternate bearing olive, pistachio and citrus trees - different mechanisms, common solutions

Alternating high yield ON-crop years and low yield OFF-crop years (alternate bearing) is a common problem of significant economic consequence to the commercial production of woody perennial fruit and nut crops. Not only does alternate bearing complicate orchard management and reduce grower income, it also negatively impacts packinghouse operation, marketing, and the stability of commodity-based industries. Alternate bearing is typically initiated by an adverse climate event that results in an OFF crop, which is followed by an intense bloom that sets an ON crop, with little to no flowering the year following the ON crop. There are four mechanisms, by which crop load (fruit number tree^‑1) influences return bloom, listed here as the effect of the ON crop: i) reduced summer vegetative shoot growth, with fewer, shorter vegetative shoots resulting in fewer nodes to bear inflorescences the following spring; ii) abscission of floral buds; iii) repression of key genes required for normal floral development; and iv) inhibition of spring bud break. All four mechanisms perpetuate alternate bearing in olive (Olea europaea), three in citrus (Citrus reticulata), with only one, presently demonstrated in pistachio (Pistacia vera). For each species, the effects of the ON crop are greater for bearing shoots than non-bearing shoots, which by default are the major source of inflorescences the following spring. Thus, ON-crop trees need to be managed to create more non-bearing shoots with properly timed fruit thinning by hand, chemical treatment, or pruning or potentially by using foliar-applied plant growth regulators to overcome the effects of the ON crop and stimulate summer vegetative shoot growth, restore floral gene transcription, prevent floral bud abscission, and promote spring bud break to increase return bloom and yield.