- Author: Ben Faber
Alternate bearing is a condition not unique to the avocado but many other crops as well. Citrus, pears, apples and apricots are just a few other crops which struggle with this problem. The question of what causes alternate bearing and how to minimize it are the subject of ongoing research which will hopefully give us better answers in the future.
Why should we care?
If the avocado markets paid the same price for all sizes from one year to the next and the total pounds produced over an on year and off year cycle were the same, it probably wouldn't matter much. The problem is that such a world does not exist and significantly higher financial rewards are made in years where fruit is scarce than in years where fruit is plentiful. If you're growing avocados to make a profit, you should be concerned about reducing or eliminating alternate bearing.
Why does alternate bearing occur?
There are a number of factors recognized by researchers as contributing to alternate bearing including plant hormones, carbohydrate reserves and competition within the plant for resources. Environmental conditions can also play a major role which, for the most part, are outside of the growers ability to control. Understanding how these factors influence alternate bearing and what we can do about it are key toward reducing or eliminating it in our groves.
Hormones
Gibberellins are considered to be one of the primary hormones produced naturally within the avocado which can influence alternate bearing. They are produced in the embryos of the developing fruit which has led some researchers to suggest fruit thinning after a particularly heavy fruit set to minimize alternate bearing. While labor intensive (there are no EPA approved chemical thinning alternatives for avocados), this step also decreases the carbohydrate sink that retained fruit makes on the trees resources further reducing alternate bearing.
Carbohydrates
Carbohydrates are produced through photosynthesis and provide the energy needed by the avocado to produce fruit, leaves, stems and roots. Depending on the status of the tree and other factors, these available carbohydrates are distributed to areas of the tree where they are needed. When a tree sets a heavy crop, a disproportionate amount of these carbohydrates are often used to develop that crop resulting in lower development of the leaves, stems and roots. While it's the fruit that we're after as commercial growers these other components of the tree are vital to sustain ongoing production in future years and maintain the overall health of the tree. Without maintaining a healthy and robust canopy, roots and limbs, the tree will decline and production will suffer. When an extremely heavy crop is set, the tree may not even have enough carbohydrates available to support full fruit development resulting in small fruit throughout the tree. If the grower does not thin the fruit when these conditions are present, special care should be given to insure that the tree is supplied with ample amounts of nutrients and water to support the existing crop and hopefully allow the tree to produce sufficient carbohydrates for the other needs of the tree. Nature has provided the tree with a defense mechanism to deal with excess fruit set called branch breaking. Many growers overcome this natural defense mechanism by propping up their branches with boards or other devices to prevent branch breaking but special care should be given to those trees where this is done as noted above.
Competition
As noted above, the different parts of the tree are all competing for the plants resources which are typically stored as carbohydrates. By reducing the demand for these resources through fruit removal, pruning or other practices the results of this competition can be minimized resulting in a reduction of alternate bearing tendencies.
Environment
For the most part there's not much a grower can do to alter the groves environment but this factor clearly plays a significant role in alternate bearing. In years where there is excessive heat or cold during the bloom period pollination may be minimal resulting in an off year condition. Alternatively when conditions are ideal, a very heavy crop may set resulting in an on year. By engaging in cultural practices that reduce alternate bearing these environmental factors on alternate bearing may be minimized but the environment is the wild card that all growers must deal with in our groves.
Strategies
Strategies are dependent on whether or not the grove is in an “on” year condition or “off” year condition. On years are when there is a heavy fruit set on the trees and off years are when there is a light fruit set on the trees. In any given grove you may find trees that are in both conditions so the practical approach toward managing you grove is to go with whatever the majority of the trees are showing (either an “on” year or “off” year condition). Below are some suggestions depending on the condition of your grove.
On years
1 Apply higher amounts of fertilizer on your grove than you would normally to support the heavy crop load.
2 Consider thinning the fruit.
3 Heavier pruning will help reduce the crop load and encourage new fruiting wood for next year's fruit set.
Off years
1 Apply lower amounts of fertilizer than in an on year.
2 Prune less in an off year.
3 Consider girdling in October to promote fruit set the following year.
Summary
Alternate bearing is a condition that can be controlled through cultural practices but not eliminated. Even the best groves experience some degree of alternate bearing but, by minimizing the effects, enjoy higher financial returns on their groves.
- Author: Sonia Rios
Avocado trees can appear completely covered with flowers. A typical full grown healthy avocado tree in California can produce up to a million flowers a year, but, on the average, fewer than 200 flowers per tree will set fruit that will hold and develop to maturity and harvest (about 10,000 lbs/acre, or less). Upon occasion, we have seen some groves with trees setting an average of 500 flowers per tree (25,000 lbs/acre), but this is rare. More commonly, only 100 flowers (or less) per tree will set and hold fruit to maturity (5,000 lbs/acre or less), much to the distress of growers (Bender 2013). Flowering is typically spread over six to eight weeks.
The avocado flower has both functional male and female organs. The male floral organ, which produces pollen, is comprised of the anthers and stamens. The female floral organ is comprised of the stigma (which receives the pollen), style and the ovary. The flowers are small (10 mm diameter). The flowers are usually only open for 2–6 hours, then close and open again for 2–6 hours on the second day. When a flower opens for a second time, it produces pollen and the stigma is not receptive. Flowers then close and remain closed. Flowers will usually be one sex in the morning, closed in the middle of the day and the opposite sex in the afternoon. The timing of the sex phases can be quite defined, but environmental conditions will affect the timing and duration of the male and female phases. The mature tree can produce in excess of a million flowers during the flowering period.
Avocado cultivars are classified in two groups (A or B) based upon their flowering behavior. In the type ‘A' cultivars, the female organs are receptive to pollen in the morning and the pollen is released in the afternoon. Unfortunately, most of the currently available "B" varieties are classed as "greenskins", which return less to growers. In the type B'cultivars, the pollen is released in the morning, while the female organs are receptive in the afternoon. Type A cultivars include: Hass, Gwen, Lamb Hass, Pinkerton, Reed, Gem, and Harvest. Type B cultivars include: Bacon, Ettinger, Fuerte, Sharwill, Sir Prize, Walter Hole, Zutano, Marvel and Nobel. Both type. It is believed that the interplanting of complementary flower types can boost fruit set and therefore yield by making pollen available. The separation in time of the male and female phases has led most observers to believe that a vector or "pollinator" is needed to move pollen from one flower to another.
Steps to take to Improve Pollination (Bender 2014):
1. Bring beehives into the grove. The University of California farm advisors have usually recommended 1 – 2 strong hives per acre, but Ish-Am (2000) suggests that 1 hive is rarely sufficient, and in many cases 4 hives are required. California growers usually have to rent hives (in 2002 hive rentals averaged $42 per hive), but sometimes beekeepers will drop a load of 80 hives for free if the grower has good bee forage nearby. Bees should have water available; floating boards on ponds or reservoirs enables them to land and drink without drowning.
2. Add pollinizers to the grove. Ish-Am recommends a pollinizer tree row be located at least every fourth row. Some growers in California use pollinizers as wind-breaks around the grove, and some replace thinned-out trees with pollinizers.
3. Keep the orchard open. Direct sunlight should reach the lower branches of each tree in order for the trees to produce a “wall” of flowers down the ground. In avocado production, this can only be accomplished by pruning the upper branches on a yearly basis. Keeping open channels through the grove encourages the flight of bees.
4. Other types of bees? Bumblebees have been reported to increase yield in avocados in Israel where honeybee populations were low. New World Carniolan bees have been used in an experiment in San Diego County for pollination: results were inconclusive as to whether they increased yield compared to Italian honeybees, but it was found that they gather more nectar from avocado (Fetscher et al. 2000). Work with these bees, and other wild bees, may eventually reveal a more efficient pollinator for avocado.
- Author: Ben Faber
Every year growers get together to learn what is being done in the citrus research world that could affect their operations. This June, University of California and the Citrus Research Board are bringing some good talks to three different growing areas. All growers are invited, but RSVPs are appreciated.
- Author: Elizabeth Fichnter and Carol Lovatt
Advances in mitigation of alternate bearing of olive: vegetative growth response to plant growth regulators
Elizabeth Fichtner, UCCE Farm Advisor, Tulare County, and Carol Lovatt, Professor of Plant Physiology, Botany and Plant Sciences, UC-Riverside.
Alternate bearing (AB) is a phenomenon in olive 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. The large swings in biennial olive production impact the overall industry, from growers to harvesters, to processors. In olive, the vegetative growth in one year produces the nodes bearing potential floral buds in the spring of the second year. Fruit suppress vegetative shoot growth resulting in fewer nodes available to bear fruit the following year. Our phenological studies have helped characterize the relationship between fruit load and vegetative growth on ‘Manzanillo’ olives in Tulare County, California.
Investigation of vegetative growth response to plant growth regulators
One strategy proposed to mitigate AB is to stimulate summer vegetative shoot growth to increase the number of nodes with the potential to produce floral buds. To address this strategy, our research team designed and implemented a proof-of-concept study in which plant growth regulator (PGR) treatments were injected into individual scaffold branches on opposing sides of ’ON‘ and ’OFF‘ trees. Plant growth regulators utilized in the study included two cytokinins, 6-benzyladenine (6BA) and a proprietary cytokinin (PCK), as well as two auxin-transport inhibitors, tri-iodobenzoic acid (TIBA) and a natural auxin transport inhibitor (NATI). Eight PGR treatments were included, with each PGR tested alone, and each cytokinin tested in combination with each auxin-transport inhibitor. PGR treatments were implemented in Summer (July 2012), and Summer + Spring (July 2012 and February 2013). Vegetative shoot growth was recorded monthly throughout the year to determine the influence of PGR treatments and timings on node production. The study was completed at the Lindcove Research and Extension Center (Exeter, CA).
Node production in response to plant growth regulator treatments
Scaffold injection with numerous PGR treatments resulted in significant increase in vegetative shoot growth. For example, nonbearing shoots on ‘ON’ control trees, produced an average of one node between July 2012 and February 2013, whereas nonbearing shoots on PGR-treated scaffold branches exhibited almost 4 times the new growth of the control trees (Table 1, shaded). Importantly, the new growth in some cases was statistically equal to and numerically greater than the new vegetative shoot growth of nonbearing shoots on ‘OFF’ control trees. The PGR treatments also had a positive effect in increasing vegetative shoot growth on bearing shoots of ‘ON’ crop trees. Bearing shoots on ‘ON’ control trees produced an average of 0.8 nodes between July 2012 and February 2013, whereas bearing shoots on PGR-treated scaffold branches of ‘ON’ trees produced over three-fold more nodes during this period. Some PGR treatments increased the number of new nodes on bearing shoots on ‘ON” trees to values equal to those of nonbearing shoots of ‘OFF’ crop control trees (Table 1, asterisk). Identify the better treatments. On average two additional nodes of growth were added to shoots in all treatments from February through April. Thus, in April shoots treated with some PGRS (Table 1, shaded) remained longer than bearing or nonbearing shoots on ‘ON’ crop control trees and equal to nonbearing shoots on ‘OFF’ crop control trees. This result suggests that with regard to increasing vegetative shoot growth there was no advantage derived from supplementing the Summer PGR treatment with the second Spring PGR treatment. However, the effect of the Spring PGR treatments on floral bud break and return bloom and fruit set remains to be determined.
|
Table 1. The effect of scaffold branch injected plant growth regulator treatments on vegetative shoot growth, as number of new nodes produced. |
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|
|
|
New Nodes |
|
|
Treatment |
Branch Status |
July-February |
July-April |
|
ON Control |
Fruit |
0.8 l |
3.3 jkl |
|
TIBA+6BA SUMMER |
Fruit |
2.3 hijk* |
4.6 cdefghij |
|
TIBA+PCK SUMMER |
Fruit |
2.5 ghij* |
4.9 bcdefghij |
|
NATI+6BA SUMMER |
Fruit |
2.7 fghij* |
4.2 fghijkl |
|
NATI+PCK SUMMER |
Fruit |
2.2 hijk* |
3.9 hijkl |
|
TIBA SUMMER |
Fruit |
2.4 hij* |
4.9 bcdefghij |
|
NATI SUMMER |
Fruit |
2.5 fghij* |
4.3 efghijkl |
|
6BA SUMMER |
Fruit |
2.2 ijkl |
4.2 fghijkl |
|
PCK SUMMER |
Fruit |
2.6 fghij* |
4.7 cdefghij |
|
TIBA+6BA SUMMER+SPRING |
Fruit |
2.4 hij* |
4.5 defghijk |
|
TIBA+PCK SUMMER+SPRING |
Fruit |
3.0 efghi* |
4.5 defghij |
|
NATI+6BA SUMMER+SPRING |
Fruit |
2.6 fghij* |
5.0 abcdefghij |
|
NATI+PCK SUMMER+SPRING |
Fruit |
2.2 ijk* |
3.7 ijkl |
|
TIBA SUMMER+SPRING |
Fruit |
2.0 ijkl |
4.1 ghijkl |
|
NATI SUMMER+SPRING |
Fruit |
3.1 defghi* |
5.5 abcdefghi |
|
6BA SUMMER+SPRING |
Fruit |
2.7 fghi* |
4.9 bcdefghij |
|
PCK SUMMER+SPRING |
Fruit |
1.3 jkl |
2.5 l |
|
OFF Control |
No Fruit |
3.6 abcdefgh |
5.0 bcdefghij |
|
ON Control |
No Fruit |
1.0 kl |
2.7 kl |
|
TIBA+6BA SUMMER |
No Fruit |
3.8 abcdefg |
4.7 cdefghij |
|
TIBA+PCK SUMMER |
No Fruit |
4.7 ab |
5.9 abcdefg |
|
NATI+6BA SUMMER |
No Fruit |
4.8 a |
6.3 abcd |
|
NATI+PCK SUMMER |
No Fruit |
4.5 abc |
6.0 abcde |
|
TIBA SUMMER |
No Fruit |
4.4 abcd |
6.0 abcdef |
|
NATI SUMMER |
No Fruit |
4.2 abcde |
4.9 bcdefghij |
|
6BA SUMMER |
No Fruit |
3.4 bcdefghi |
4.0 hijkl |
|
PCK SUMMER |
No Fruit |
4.3 abcde |
5.5 abcdefghi |
|
TIBA+6BA SUMMER+SPRING |
No Fruit |
4.2 abcde |
5.2 abcdefghi |
|
TIBA+PCK SUMMER+SPRING |
No Fruit |
4.8 a |
5.7 abcdefgh |
|
NATI+6BA SUMMER+SPRING |
No Fruit |
3.9 abcdef |
5.1 abcdefghi |
|
NATI+PCK SUMMER+SPRING |
No Fruit |
3.2 cdefghi |
4.2 efghijkl |
|
TIBA SUMMER+SPRING |
No Fruit |
4.8 a |
6.5 ab |
|
NATI SUMMER+SPRING |
No Fruit |
4.5 abc |
6.8 a* |
|
6BA SUMMER+SPRING |
No Fruit |
4.8 a |
6.4 abc |
|
PCK SUMMER+SPRING |
No Fruit |
3.6 abcdefgh |
4.7 cdefghij |
|
P-value |
|
<0.0001 |
<0.0003 |
|
Note: shading denotes treatments significantly different than ON Control + Fruit treatment. Asterisk denotes treatments significantly different than OFF Control (- Fruit). |
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Summary
These preliminary data demonstrate that PGRs increase shoot growth, which might result in more nodes with the potential to produce inflorescences the following spring. Future studies are anticipated to address the use of promising treatments in foliar applications. Naturally-occurring compounds, such as PCK and NATI, may be easier and less costly to register than PGRs, which are classified as pesticides. Therefore, significant growth response to the natural compounds tested may have commercial benefit even if proven less efficacious than the synthetic PGRs.
Acknowledgements: The financial support of the California Olive Committee and the technical support of the Lindcove Research and Extension Center were integral to the success of this project.
- Author: Carol J Lovatt
- Author: Elizabeth J Fichtner
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.
For a fuller discussion of this topic, see the full article by Carol Lovatt at:
http://ceventura.ucanr.edu/newsletters/Topics_in_Subtropics45348.pdf
The Topics in Subtropics Newsletter
