Irrigation Calculator developed by Reuben Hofshi, Shanti Hofshi and Ben Faber.

South African plant pathologists were the first to show that root rot in avocado could be controlled by trunk injection with both phosphorous acid and the patented material Aliette®.  Aliette was briefly registered in California in the late 1980’s, but theregistrant soon lost interest in pursuing a full pesticide registration when it became apparent that other researchers believed phosphorous acid could be registered as a fertilizer - a process much less costly and simpler than a pesticide registration. The company continued to hold on to the patents for the product and the breakdown products that were useful in root rot control.  By holding onto the patent, this effectively stopped other companies from pursuing a pesticide registration for phosphorous acid.    In 1990, a publication reported that phosphite could be used as a source of phosphorus fertilizer and this became the basis for the registration of phosphite as a fertilizer.  Subsequently, when the original patent expired, at least two materials have been registered as fungicides containing phosphite – Fosphite® and Agri-fos®.  There are, however, numerous phosphite materials that have been registered as fertilizers (for some brands see Brunings et. al., 2005, http://edis.ifas.ufl.edu/HS254), and every day seems to bring more brands onto the scene each making claims of having the best efficacy.

We wanted to see if we could detect an efficacy difference between Aliette, another registered phosphite fungicide and four different materials registered as fertilizers, for a total of six materials.  In a greenhouse, three-month old ‘Topa Topa’ seedling avocados with cotyledons removed were planted into a Phytophthora cinnamomi -inoculated organic potting mix. A control was also planted without the inoculum, as well as an inoculated control.  One of six different materials was then applied as a soil drench until draining from the bottom of the liner.  The materials were applied at the equivalent phosphorous acid concentration.  There were 20 replicates for each of the controls and treatments.  The experiment was repeated twice.

At harvest, root fresh and dry weights were highest for the non-inoculated trees and lowest for the untreated, inoculated controls, in both trials.  All treatments’ associated weights intermediate between these two were statistically the same.  Even a repeat application of one of the treatment materials in trial II didn’t result in greater root weights than single application treatments.  Shoot weight, both dry and fresh, was much less affected by root rot and treatments.  There were no differences in fresh shoot weight in the second trial, not even between the inoculated and noninoculated controls.  The root and shoot weights of all the treatments in the second trial were higher than in the first trial, indicating that either the inoculum was not as effective or that the trial was not continued long enough to produce as much damage.  

Root rot studies often have dramatic effects on root weights while shoot weights may remain little affected.   It is clear from our data that phosphites reduced the severity of root rot in this study, but that there was no benefit of a single source of phosphite relative to any other source.  

Below:  Healthy and decaying avocado roots.

Growers are faced with an ever-changing list of commercial “tools”, each with the promise of providing some advantage to the farmer.  Frequently, these are new fertilizer mixes presented as proprietary cocktails promoted and dispensed with promises of a multitude of profitable (yet improbable) benefits to the buyer. With the large number of new products available, and the number of salespeople promoting them, it is often difficult for growers to distinguish between products likely to provide real benefit, and those that may actually reduce the profitability of the farm.   
 
In all situations when a company approaches the University or a commodity research board with a new product or technology for sale to California growers, these institutions act as grower advocates.  They are charged with sorting through the available information; asking the right questions; getting the necessary research done if the available information warrants this pursuit; disseminating accurate information on these new technologies and products, and doing all that can help maximize grower profits now and in the future.  When approached with a new product or technology it is obligatory to challenge claims with the following questions:
 
Is there some basic established and accepted scientific foundation on which the product claims are made?

Language that invokes some proprietary ingredients or mysterious formulations, particularly in fertilizers mixes registered in the State of California, raises red flags. A wide range of completely unrelated product benefit claims (such as water savings, pesticide savings, increased earlier yield) raises more red flags.  Product claims that fall well outside of any accepted scientific convention generally mean the product is truly a miracle, or these claims are borderline false to entirely fraudulent.
 
Has the product undergone thorough scientific testing in orchards?
 
Frequently, products are promoted based on testimonials of other growers. While testimonials may be given in good faith, they are most often not backed up by any real scientific testing where a good control was used to compare orchard returns with and without the product.
 
A “test” where a whole block was treated with a product and which has no reliable untreated control does not meet accepted standards for conducting agricultural experiments. Also, a treated orchard cannot reliably be compared to a neighboring untreated orchard; and a treated orchard cannot be compared to the same orchard that was untreated the previous crop year.  Even a test with half a block of treated trees and half untreated is not considered dependable by any known scientific standard of testing.
 
Only a well designed, statistically replicated, multi-year trial allows for direct comparison of untreated versus treated trees with statistical confidence. Verifiable data from tests that meet acceptable standards of scientific design, along with access to raw baseline (before treatment) yield data from the same trees (preferably for the two years prior) should be used to determine the validity of test results provided.
 
Are the test results from a reliable source?
 
If the testing were not done by a neutral party, such as university scientists, agency, or a reputable contract research company using standard scientific protocols, this raises red flags.  If the persons overseeing the tests have a financial interest in seeing positive results from the product, it raises red flags.
  
Does the product have beneficial effects on several unrelated farm practices?
 
A product that increases production of trees, makes fruit bigger, reduces pests, reduces water use, and reduces fertilizer costs, is more than a little suspicious. In reality, if such a product really existed, it would not need any testing at all because its benefits would be so obviously realized by the grower community that it would spread rapidly by word of mouth and embraced by the entire grower community.
 
Are other standard and proven farm products put down in the new product sales delivery?
 
If a new product vendor claims that their product is taken up 15 times faster than the one growers are currently using, or is 30 times more efficient, it probably costs 15 to 30 times more per unit of active ingredient than the standard market price.  Growers should always examine the chemical product label to see what active ingredient they are buying.  There has to be a very good reason to pay more for an ingredient where previously there had been no problem supplying the same ingredient at a cheaper price to trees in the past.
 


There are impartial sources of such information available to farmers to help corroborate information provided by product vendors.  Perhaps the most reliable and accessible impartial research and education resources for growers are their local Cooperative Extension Farm Advisors and commodity research boards.   
 
When promising products emerge, local university Farm Advisors can advise growers on how to evaluate these products and may help design a small trial to test a particular product on a few trees under local orchard conditions.  If in these pursuits a truly promising new product or technology emerges, research board funding may follow but only on the recommendation of that board’s Research Committee.

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.