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.

In 1985, H. Schneider and J. Pehrson published an article documenting a decline of Frost Nucellar navel oranges on a number of trifoliate rootstocks (H. Schneider and J.E. Pehrson, Jr.  1985. Decline of navel orange trees with trifoliate rootstocks. California Agriculture. Sept. – Oct. 1985 p. 13-16)

http://ucce.ucdavis.edu/files/repositoryfiles/ca3909p13-62889.pdf

In this decline, which began in the 1970s, trees began demonstrating symptoms when they were 15 to 20 years-old.  Affected trees showed leaf discoloration, some defoliation, twig dieback and subnormal growth.  They describe how some declining orchards were removed and, in others, individual trees were removed and replanted.  Schneider and Pehrson concluded the following:  “disorganized phloem and cambial tissues at the budunion proliferate into a tongue like wedge that protrudes from the inner side of the bark.  Affected tissue acts as a girdle and is presumed to be responsible for the decline of the trees.”   In this article, Schneider and Pehrson provide excellent micrographs illustrating what was occurring at the budunion.  However, the actual cause of this aberrant growth pattern was not described.

If we fast-forward to 2012, citrus growers in Kern and Tulare County, and presumably in other counties of the San Joaquin Valley, are experiencing similar tree symptoms to those described and pictured by Schneider and Pearson.  The problem has been observed with blood oranges; navel oranges, including Fukumoto, Earli-Beck, Newhall, Atwood, and Powell; and on Satsuma and Page mandarins; on trifoliate and citrange rootstocks such as C-35 and Carrizo.  This decline has not been reported, to our knowledge, in California outside of the Central Valley.  There are similar reports of bud union disorders in Florida.  We are observing symptoms much earlier in orchards than did Schneider and Pehrson.  Decline is present in one two-year old blood orange orchard and in several navel orange orchards that are 7 years-old or less. This decline is not common, but can be devastating in a particular orchard, with most trees within an affected orchard showing decline or evidence of the disorder of the graft union. In some orchards only a few trees may initially demonstrate symptoms.

Normally, the scion of a navel orange tree grafted onto trifoliate or citrange rootstock will grow more slowly than the rootstock and a ‘bench’ will form at the graft union.  This bench begins to form when a tree is six or seven years old. Conversely, the growth of the scion and rootstock are more similar in affected trees when young and the scion will usually show a slight overgrowth of the rootstock. In Fukumoto, the graft union is an area of intense suckering, and the graft union can become much distorted.  As described by Schneider and Pehrson, a groove containing a light brown gum is apparent at the graft union of affected trees.  In young trees only staining may be present at the union. The groove does not always traverse the entire circumference, especially in the early stages. This groove is associated with the decline and death of trees.

The cause of the decline is not known.  No pathogen has been identified, consistently, in affected trees. If the decline is a result of incompatibility between the scion and rootstock, there must be an additional stimulus, as the decline is not common and trifoliate and citrange rootstocks are the preferred rootstocks in this citrus growing area.  In some young affected orchards, most trees show the groove at the graft union, and it seems unlikely that this uniformity was the result of tree-to-tree transmission of a pathogen. Currently, we have no suggestions on how to prevent this problem or alleviate the symptoms once found. Causes of the problem are being investigated.

Avocado Trunk Cankers

This has been a low rainfall year, and often with the low rainfall, cankers will seem to suddenly appear on the woody parts of the tree. There are a number of causes for the white exudate from cankers on the trunk and limbs of avocado.  Any wound will cause the tree sap to run and crystalize on the surface.  It is a seven carbon sugar of mannoheptulose, or its alcohol form perseitol.  It’s sweet.  So any wound that might be caused by woodpeckers or little kids climbing the trees will damage the bark, and where the damage has occurred, the sugar will form.  There are also diseases that can cause a wound that will exude the sugar.  Three of these are due to water stress of some form that allows infection to occur.  One of these is a bacterial – Bacterial Canker.  Another is caused by a fungus which in the past was called Dothiorella Canker.  We now know it as a different name and UCR plant pathologist has actually identified seven different species of fungus that invade the wood and can eventually weaken the tree so limbs can break and the tree becomes unthrifty.  In the case of very young trees, they can be killed by the fungus.  A third cause of sugary cankers is Black Streak, the cause of which was unclear until recently when Eskalen has possibly identified it coming from a similar set of fungi that cause Dothiorella Canker.  It makes sense, because in all three of these cases, they most appear after a low rainfall year, where irrigation pressures are insufficient, where emitters have clogged and where general water or salinity stress have occurred. The bacteria and fungi that cause these cankers are everywhere in most orchards and are just waiting for the stressed tree to appear.  The grower just needs to identify where this stress is occurring, correct the problem (clogging, low pressure, poor irrigation design, infrequent scheduling, inadequate leaching, etc.) and if the damage is not too extensive, often these symptoms will disappear with time.

The fourth cause of canker is caused by Phytophthora citricola, a relative of avocado root rot.  This is caused by a moist trunk, either from irrigation water hitting the trunk, or on the north side of the tree that doesn’t dry out from morning dew.  This is a much slower acting disease than root rot, although it can rapidly kill young trees.  The cankers occur at about 18 inches from the ground and gradually girdle the tree.  The first thing to do before ever seeing this disease is to make sure irrigation water isn’t hitting the trunks.  If you do have cankers appear, though, it responds to the same materials used for root rot, but should actually be sprayed right on the canker.

For the last few years citrus growers in the San Joaquin Valley have been nervously watching the establishment of Asian citrus psyllid in southern California and bracing themselves for the day of northward movement.  That day arrived in November 2012 when two psyllids (Strathmore 16 Nov. and Terra Bella 21 Nov.) were caught on yellow sticky card traps, in addition to a third capture back in January 2012.  These captures have now resulted in restrictions on the movement of citrus in the heart of California's principal citrus production region.

Asian citrus psyllid is a small insect the size of an aphid that feeds on citrus leaves and stems.  It is the vector of a deadly bacterial disease of citrus called huanglongbing, often referred to as HLB or citrus greening.  This pest and disease combo has resulted in devastating losses to the citrus industry in Florida, and has the potential to have a similar affect in California.

Prior to November 2012 Asian citrus psyllid had been reported in eight California Counties, mostly in the southern part of the state, with a combined total of approximately 26,000 square miles under quarantine.  However, the two finds in Tulare County mark the first time the psyllid has been found in the heart of California's principal citrus production region of the lower San Joaquin Valley: Kern, Tulare and Fresno counties produce over 200,000 acres of citrus at an annual value of approximately $1.7 billion.

The capture of individual psyllids on sticky traps in Strathmore and Terra Bella gives CDFA the authority to establish a quarantine of citrus within a 20-mile radius of the find in Strathmore.  Prior to doing this, however, CDFA has opted as an interim step to only regulate citrus in a 5-mile radius around each find until further trapping and delineation can determine if psyllids are truly established in the region, or if the psyllids caught were just non-breeding hitchhikers brought to the corridor along State Highway 65 from infested counties in Southern California.  If further delineation detects an established population it is anticipated that quarantines would be established.  If established, a quarantine for Asian citrus psyllid would last a period of 2 years since the most recent capture.  If additional psyllids were captured during the two-year quarantine the clock would reset itself for another two years.

Due to the fact that the psyllid only feeds on leaves and stems (and not fruit), citrus growers within quarantine zones in California have several options for harvesting and shipping fruit.  Fruit harvested within quarantine zones can be picked, transported and packed within the quarantine zone without restrictions.  Once clean fruit is packed (no leaves or stems) it can be shipped to locations outside of the quarantine.

Packing fruit from within the quarantine at packing houses outside of the quarantine is also possible under a CDFA compliance agreement that can be accessed through the County Agricultural Commissioner.  These agreements state that the grower is willing to comply with CDFA and USDA regulations regarding the movement of bulk citrus, the most important of which is that bulk citrus must be processed through trash-removal equipment (to remove all leaves and stems) before it is shipped in bulk to a packer outside of the quarantine.

The Asian citrus psyllid quarantine also affects retail nursery stock.  Currently there are compliance agreements and protocols available that allow retail nursery stock to be moved within the quarantine zone.  However, no provisions are currently available to move nursery stock from the quarantine zone to regions outside of the quarantine zone unless the plants were budded and produced within a federally-approved screenhouse facility.

Regulations regarding Asian citrus psyllid can change quickly.  For that reason citrus growers are encouraged to maintain good contact with their local Agricultural Commissioner.  Additional information on the status of quarantines and other restrictions can be found online at

http://www.cdfa.ca.gov/plant/acp/.

Photo below.  Asian Citrus Psyllid nymphs with waxy exudates from feeding.