- Author: Ben A Faber
Farm Advisor UCCE Ventura County
Plants, therefore avocados, go through different growth stages, so called phenological stages, regular periods where they grow and differentiate from seed to various vegetative stages, flowering and finally seed production. Avocado has a preset pattern of phenology that occurs depending on variety and where it is grown, driven by light, temperature, water availability and often by different stresses, such as cold, heat, and heavy or light crop load.
The successive stages of avocado phenology are demonstrated in the photomontage below, from bud break to harvestable fruit:
Main phenological growth stages of ‘Hass' avocado according to the extended BBCH scale. From: http://avocadosource.com/journals/elsevier/scientiahort_2013_164_434-439.pdf
And, depending on where in California the avocado is growing, the latitude, or on what side of the slope (in the shade or full sun, on the top of the slope, etc.), the tree will go through these successive stages at different times of the year, offset by weeks or even months. A general plan for the California coastal region was developed from accumulated experiences and tests by a variety of groups and is shown below:
Research-based information that allows producers to anticipate the regionally appropriate times for major phenological evets is limited in California. From work conducted at South Coast Research and Extension Center in Irvine, the onset of flowering in early March extends into May, normally, and the main fruit set period is between mid-April and May. In later work from the same location, a mean beginning bloom date for “on” and “off” crop years differed between Julian date 66 (March 7) and 80 (March 21), respectively. From avocado research in Carpinteria, flower abscission (as a measure of bloom) during “on” and “off' crop years peaked on two different Julian dates, (May 4 and June 2). Mid-June has been reported as the middle of the commercial ‘Hass” season for the Irvine area. Harvest in Santa Barbara and San Luis Obispo Countries may just be staring in mid-June and continue through November in the most northern growing areas.
Difference in crop development impact the timing of cultural practices, such as fertilization, irrigation, phytophthora treatment, pollination and gibberellin sprays. For instance, nitrogen fertilization is often recommended by month of the year, rather than growth stage of the tree. Several recent efforts indicated that the timing of nitrogen fertilizer may have significant impacts on yield and that two times the fertilizer rates in April and November may result in substantial yield increase. Available information indicates the April timing might correspond to fruit set for the Irvine area compared to early bloom for Santa Barbara and San Luis. By November, Irvine's current year's crop may have been harvested three months previously. In San Luis Obispo, harvest may have just ended. So just going by date is insufficient for guiding many horticultural activities.
California's trees often have two crops maturing at the same time. It becomes especially pronounced the further north the production. The management of multiple crops on each tree becomes more important and more confusing as the length of time the fruit remains on the tree. Carrying two and even three loads of crop increases the potential for alternate bearing. Reduction in alternate bearing has been determined to be an important strategic requirement for California's growers. Understanding the impacts of cultural practices on alternate bearing is important. For example, the chart below demonstrates a stylized calendar of the avocado growth cycle for California. Rate and application timing of nitrogen as predicted by the Avocado Nitrogen Model proposed by Rosecrance et al. (2013 and Calculator) are noted for a 15,000 per acre yield outcome. The “on” year suggests nitrogen rate for the late fruit growth/harvest/summer and fall flush period is 15 lbs. This application coincides with the fruit set timing for 16 lbs of N for the “off” bloom crop during the later bloom/fruit set/spring shoot growth phase. Rates and timings to support these crops ought then be combined and represent the 2X fertilizer rate that Lovatt (2001) found to have significant effects on yields.
Calendar of avocado growth cycles with both “on” and “off” years represented when they overlap, and suggested nitrogen fertilization timings and amounts.
Taking into account the actual phenology, what is happening in your trees, then is important for assessing when to make N applications. In the winter and spring of 2023, it seems like everything was on a different cycle. Thrips delayed their appearance, flowering was erratic. Honeybees seemed to have found somewhere else to hang out, because they were not flying in the avocado trees. And then suddenly, we had some fruit set in Ventura in late June. This is a really clear example of the problem of following a cookbook to farming avocados. The point here is that just going by the calendar is not going to meet the needs of the tree. The phenological stages of the tree in your environment needs to be taken into consideration.
Notes:
Lovatt, C.J. 2001 Properly Timed Soilapplied Nitrogen Fertilizer Increases Yield and Fruit Size of ‘Hass' Avocado. J. Amer. Soc. Hort. Sci. 126(5): 555?559.
Rosecrance, R., Lovatt, C.J., 2013. Management tools for fertilization of the 'Hass' avocado. FREP final report.
Calculator: https://rrosecrance.yourweb.csuchico.edu/Model/AvoModel/Avo2NKModel.html
- Author: Ben Faber
Citrus response to irrigation water deficits have demonstrated that sensitivity of yield to water stress is dependent on the phenological phase in which water stress was applied. Adequate water supply is of major importance during citrus flowering and fruit set. A second critical period coincides with the period when fruit growth is rapid (fruit set to harvest). Depending on the level of water stress developed, the abscission of flowers and young fruits will be affected in the first case, as will fruit size in the second case.
For navels and mandarins it is possible to identify these critical periods in the crop and possibly allow stress when the trees are not in those critical periods. Some varieties though are complicated by having overlap of critical periods when another crop is present at the same time. Valencias can have two crops on the tree at the same time in spring and into summer harvest and coastal lemons can have fruit in all stages from fruit set to mature fruit at all times of the year. In the case of navels, reductions of applied water by 25% or more have resulted in no fruit yield reductions, if those water reductions do not occur during critical periods (Goldhamer, 2006; Domingo, 1996; Hutton et al, 2007). Water reductions during the rapid expansion period can result in significant fruit size reduction, though, and this period should be avoided if fruit size is critical to marketing (Goldhamer, 2006; Hutton et al, 2007).
In the case of coastal lemons, the stress should be avoided when the period of the most profitable crop is in rapid expansion, this is normally the summer crop. Each grower would need to identify, when the most profitable fruit size is important. Growers in areas that have more summer heat than the coast might practice a ‘Verdelli' irrigation practice, where water is withheld for a period of time, in order to force flowering that can often result in more summer fruit being harvested the following year (Maranto and Hake, 1985).
Citations
Domingo, R., Ruiz-Sanchez, M.C., Sanchez-Blanco, M. J. and Torrecillas. A.1996. Water Relations, growth and yield of ‘Fino' lemon trees under regulated deficit irrigation. Irrig. Sci.16: 115-123 http://link.springer.com/article/10.1007%2FBF02215619#page-1
Goldhamer, D. and N. O'Connell. 2006. Using Regulated Deficit Irrigation to Optimize Fruit Size in Late Harvest Navels. Citrus Research Board. http://citrusresearch.org/wp-content/uploads/2006-GOLDHAMER1.pdf
Hutton RJ, Landsberg JJ, Sutton BG. 2007. Timing irrigation to suit citrus phenology: a means of reducing water use without compromising fruit yield and quality. Australian Journal of Experimental Agriculture (47): 71–80. http://dx.doi.org/10.1071/EA05233
Maranto, J. and K. Hake. 1985. Verdelli summer lemons: a new option for California growers. California Agriculture 39(5): 4. https://ucanr.edu/repositoryfiles/ca3905p4-62870.pdf
Phenological stages of navel orange.
- Author: Elizabeth Fichtner and Carol Lovatt
The first step in researching and developing strategies for mitigating alternate bearing (AB) in 'Manzanillo' table olive is to model the tree phenology with respect to the alternating 'ON' (high yield) and 'OFF' (low yield) cycles. In olive, the vegetative growth in one year produces the nodes bearing potential floral buds in the second year. Fruit load suppresses vegetative growth and return bloom; however, the mechanism underlying this relationship is unknown.
Hypothesized mechanisms (or combinations thereof) include:
1) Fruit inhibit vegetative growth, resulting in fewer nodes with the potential to flower and bear fruit.
2) Fruit inhibit floral development and/or spring bud break, reducing the number of inflorescences at return bloom.
3) Fruit reduce the number of perfect flowers in return bloom, resulting in fewer flowers with the ability to bear fruit.
Fruit load and inhibition of vegetative growth
Relationship of fruit load to vegetative growth. Olives are borne on one-year-old shoots; consequently shoot growth will be depressed during the year of a heavy crop, resulting in lack of fruitful shoots the following year (Sibbett, 2000). Working in both commercial orchards and at the Lindcove Research and Extension Center, our research team has similarly modeled this relationship in 'Manzanillo' olives in Tulare County. We assessed the influence of fruit on vegetative growth on 'ON' trees in comparison to 'OFF' trees, where 'ON' refers to trees with a heavy crop load, and 'OFF' refers to trees with a low or negligible crop load. Additionally, within 'ON' trees, we assessed vegetative growth on shoots bearing fruit and shoots not bearing fruit. The results of our study demonstrate the inhibitory effect of fruit on vegetative growth at both a tree and shoot level (Table 1). For example, between July 2012 and September 2012, an average of 3.3 nodes per shoot were produced on 'OFF' trees, whereas, non-bearing and bearing shoots on 'ON' trees produced an average of 0.7 and 0.6 nodes per shoot, respectively (Table 1).
Table 1. Vegetative growth represented by the cumulative number of nodes per shoot produced between July 2012 and the stated month. |
|||||||
Tree Status 2012 |
Shoot Status |
# Nodes July '12 |
# Nodes July-Aug '12 |
# Nodes July-Sept '12 |
# Nodes July-Oct '12 |
# Nodes July-Feb '13 |
# Nodes July-April '13 |
OFF Control |
No Fruit |
2.2 a |
2.9 a |
3.3 a |
3.3 a |
3.6 a |
5.0 a |
ON Control |
No Fruit |
0.6 b |
0.7 b |
0.7 b |
0.7b |
1.0 b |
2.7 b |
ON Control |
Fruit |
0.2 b |
0.5 b |
0.6 b |
0.6 b |
0.8 b |
3.3 ab |
P value |
|
0.0019 |
0.0047 |
0.0058 |
0.0059 |
0.0053 |
0.0397 |
Seasonality of vegetative growth. As a precursor to developing chemical treatments (e.g. Plant growth regulator) to mitigate AB, we investigated the fluctuation of growth rate by season (Table 1). The results of our 2012 data collection indicate that vegetative shoot growth proceeds through September, but effectively ceases sometime between September and October. Minimal vegetative growth occurs during the winter months (i.e., October through February), but the vegetative growth rate accelerates in the late winter/early spring (February-April).
When does vegetative growth on 'ON' branches effectively 'catch up' to growth on 'OFF' branches? Our data suggest that vegetative growth rapidly accelerates on 'ON' shoots between February and April; by April no significant difference was observed in the number of nodes produced since the preceding July for bearing shoots on 'ON' trees and 'OFF' trees. During the late winter/early spring, the fruit are no longer present to suppress vegetative growth, and formerly 'ON' shoots will effectively 'catch up' to the 'OFF' shoots. This late winter/early spring growth; however, will not produce inflorescences in the current year because they were formed after floral bud induction and development.
Fruit may inhibit floral bud break
Fruit inhibit return bloom in 'Manzanillo' olive (Table 2); however, it is yet unknown whether fruit only inhibit vegetative shoot growth, or also inhibit the formation of floral buds, or only inhibit the spring break of floral buds. Our research has documented the extent of fruit’s suppression of return bloom, with inflorescence counts highest on 'OFF' trees, followed by non-bearing and bearing shoots on 'ON' trees. The combined whole-tree and localized shoot effect on inflorescence counts was observed on bearing shoots of 'ON' trees, as evidenced by statistically fewer inflorescences produced per shoot than non-bearing shoots on 'ON' trees (Table 2).
Table 2. Influence of tree and shoot status on inflorescence production. |
||
Tree Status 2012 |
Shoot Status |
Total Inflorescences per Shoot |
'OFF' Control |
No Fruit |
9.3 a |
'ON' Control |
No Fruit |
2.8 b |
'ON' Control |
Fruit |
0.6 c |
P value |
|
0.0001 |
Our current data suggests that, in addition to the loss of potential inflorescences due to the inhibition of summer vegetative shoot growth, at least a portion of fruit-mediated reduction of return bloom is related to reduced spring bud break. Floral buds of 'Manzanillo' olive are formed in late summer or early fall, but branch injections with the cytokinin plant growth regulators 6-benzyladenine or a proprietary cytokinin in February 2012 resulted in over 60% increase in number of inflorescences on non-bearing shoots on 'ON' trees at bloom in 2012, consistent with overcoming bud dormancy of viable floral buds. Our data, therefore, demonstrate that a portion of reduced return bloom is related to inhibition of floral bud break.
Olives are andromonoeceous, meaning they produce both perfect flowers, containing male reproductive structure (stamens) and female (pistil) structures, and staminate flowers (containing only male parts). Staminate flowers are unable to bear fruit. During floral bud development, all buds contain pistils and stamens; however, pistil abortion approximately 8-10 weeks prior to bloom results in a reduction in the proportion of perfect flowers formed. The results of our research provide evidence that the bearing status of a shoot affects the percent of perfect flowers formed (Table 3). The results suggest that failure of the pistil to develop and form a perfect flower is strongly associated with the presence of fruit set on a shoot and not due to crop load since the percentage of perfect flowers on nonbearing shoots of 'ON' trees is equal to that of nonbearing shoots on 'OFF' trees, but dramatically reduced for bearing shoots on 'ON' trees. Consequently, shoots bearing fruit in year one will have fewer perfect flowers in year two.
Table 3. The bearing status of trees and/or shoots influences the characteristics of return bloom. |
||||||
Treatment |
Shoot Status |
Total Inflorescences per Shoot |
Total Flowers per Shoot |
Flowers/ Inflorescence |
Total Pistilate Flowers per Shoot |
% Perfect Flowers |
'OFF' Control |
No Fruit |
9.2 a |
24.6 a |
3.3 a |
21.4 a |
87 |
'ON' Control |
No Fruit |
0.2 b |
1.6 b |
1.6 a |
1.4 b |
88 |
'ON' Control |
Fruit |
0.4 b |
0.0 b |
0.0 a |
0.0 b |
0 |
P-Value |
|
≤0.0009 |
≤0.0001 |
≤0.1745 |
≤0.0001 |
|
Summary
As a result of collaborative work between UC Cooperative Extension and UC Riverside, we have enhanced the understanding of the phenology of 'Manzanillo' olive with respect to alternate bearing and the cycling of 'ON' and 'OFF' crops. This phenological modeling illustrates the influence of fruit on vegetative growth and the seasonality of vegetative growth. The work additionally addresses the influence of fruit on both return bloom and the number of perfect flowers produced. Last, our work on mitigation of AB in olive provided evidence that fruit reduce floral intensity by inhibiting spring bud break and that floral buds had developed.
Further studies are underway to elucidate the timing of flower bud development. We are currently investigating whether fruit inhibit floral development on bearing shoots of 'ON' trees by examining the expression of key genes that regulate floral development. Enhanced understanding of the phenology of 'Manzanillo' olive will allow for precision timing of practices designed to mitigate AB and minimize the annual fluctuations in crop load and industry inventory.
Acknowledgements
We are grateful to H. Fox and C. Hill, Tulare Co. Olive Growers, for cooperation in our field experimentation, as well as the Lindcove Research and Extension Center for field and laboratory support. Funding for this work was graciously provided by the California Olive Committee.
Literature Cited
Sibbett, S. 2000. Alternate bearing in olive trees. California Olive Oil News. Vol. 3, Issue 12.