Research Advances on Mitigation of Alternate Bearing in Olive

Alternate or biennial bearing is a phenomenon where fruit production alternates between large crops consisting of smaller, lower value fruit during an "ON" year and smaller crops consisting of larger, higher value fruit during an "OFF" year.  Alternate bearing is not unique to olive, but also affects other perennial California crops including (but not limited to) pecan, pistachio, apple, avocado and citrus, especially mandarins.  The large swings in biennial fruit production impact the overall industry, from growers to harvesters, to processors.  The 2009-2011 seasons exemplify the magnitude of the affect of alternate bearing on olive production and crop value in Tulare County (Table 1). 

Table 1. Tulare County Olive Production

Causes of alternate bearing in olive

In olive, the current year's fruit is borne on the prior year's vegetative growth.  The current year's fruit, and specifically the pit, inhibits the vegetative growth that supports flower buds for the following year (Sibbett 2000).  Consequently, during an ON year, fruit production directly inhibits vegetative growth.  A recent Israeli study (Dag et al 2010) demonstrates the inhibitory effect of fruit on vegetative shoot growth and return bloom in the oil cultivar 'Coratina'. Similarly, in 2011 and 2012 we investigated the relationship between fruit load and vegetative growth on 'Manzanillo' olives in Tulare County.  In our study, we assessed the influence of fruit on vegetative growth on ON trees in comparison to OFF trees.  Additionally, within ON trees, we assessed vegetative growth on branches bearing fruit and branches not bearing fruit. Our study demonstrated the inhibitory effect of fruit number (crop load) on vegetative growth (Table 2).  Vegetative shoot growth was lower for shoots that did not set fruit (-fruit) on ON trees than shoots –fruit on OFF trees indicating a whole-tree effect of crop load in alternate bearing.  Additionally, our data demonstrate that fruit-bearing branches exhibit even less vegetative growth than non-fruit-bearing branches on ON trees, providing evidence of a strong localized effect of fruit on shoot growth (Table 2). 

Our studies also demonstrated that the bearing status of a shoot influences the following year’s percent bud break of floral buds.  For example, shoots bearing fruit in 2011 exhibited over 90% fewer inflorescences than did shoots without fruit, regardless of whether non-bearing shoots were on an ON-or OFF- tree. 

Table 2. Effect of ON- and OFF-crop tree status and the presence (+fruit) or absence (-fruit) of fruit set on a shoot on shoot extension growth. (Orchard 2, Exeter, CA, 2011).

       ^z Values in a vertical column followed by different letters are significantly different at 

         specified P levels by Fisher’s LSD Test.

Alternate bearing is typically initiated by adverse climate. Once initiated, in the absence of additional environmental constraints affecting crop load, the bearing status of an orchard alternates between ON and OFF years, with ON years exhibiting less vegetative growth than OFF years.  This biennial cycle, however, can be reset by adverse environmental conditions affecting bloom and fruit set.  Adverse conditions 8-10 weeks prior to bloom may cause abortion of female flower parts, resulting in a high proportion of staminate (male) flowers that do not give rise to fruit.  Additionally, adverse weather conditions at bloom may impact pollination and subsequent fruit set.  Any conditions resulting in loss of crop during an anticipated ON year may render the season an OFF year. 

Mitigation of Alternate Bearing

Reduction of fruit load prior to the major period of vegetative shoot growth during an ON year may mitigate alternate bearing.  Chemical thinning with NAA at bloom may result in a smaller crop with larger sized fruit during an ON year, and allow for more vegetative growth to support the following year's crop. 

Current Research on Mitigation of Alternate Bearing using Plant Growth Regulators (PGRs)

During the 2012 growing season, we investigated the potential for applications of PGR treatments to mitigate alternate bearing in olive.  The specific goal of PGR treatments was to enhance spring bud break, summer vegetative shoot growth, and return bloom.  In the first phase of this project, individual scaffolds of mature ‘Manzanillo’ olives were injected with a suite of PGR treatments. PGR treatments were injected at two points during the growing season, with winter/spring (pre-bloom) treatments targeting floral bud break, and summer treatments targeting vegetative shoot growth.  Additionally, the winter/spring injections were introduced over a four month timeframe (January-April) to assess the optimal timing of injections for enhanced floral bud break. Scaffold injection treatments resulting in desired growth responses will be carried forward in future studies focused on determining compound efficacy in foliar applications.  Treatments included either of two auxin transport inhibitors (tri-iodobenzoic acid and naringenin) injected alone, or in combination with two cytokinins (6-benzyladenine, and a proprietary cytokinin).

In the 2012 growing season, PGR treatments had encouraging results.  Cytokinin treatments injected in February resulted in over 60% more floral bud break on non-bearing shoots of ON- trees, as compared to the untreated control.  Similar treatments also increased floral bud break over 6 fold on bearing shoots on ON-trees; however, due to the variability in floral bud break, there was no significant difference between treated trees and controls on bearing shoots on ON-trees.    All summer PGR treatments (either auxin transport inhibitors or cytokinins, alone or in combination)  increased vegetative shoot growth on both bearing and non-bearing branches  by over four fold; however, the influence of PGR-induced enhancement of summer vegetative growth on return bloom is not yet known.  Return bloom and fruit set will be quantified during the 2013 season to determine the efficacy of PGR treatments on mitigation of alternate bearing on olive.   

Selected Literature

Dag, A., Bustan, A., Avni, A., Tzipori, I, Lavee, S., Riov, J. 2010. Timing of fruit removal affects concurrent vegetative growth and subsequent return bloom and yield in olive (Olea europaea L.). Scientia Horticulturae 123:469-472.

Sibbett, S. 2000. Alternate bearing in olive trees. California Olive Oil News. Vol. 3, Issue 12.