Written by Tory Schmidt, Project Manager, Washington Tree Fruit Research Commission. 14 March 2022.
Crop load management is a very consequential topic for tree fruit growers. With all the factors that can affect cropping, it can be overwhelming for new producers to figure out the big picture of how to produce consistently high yields of top-quality fruit. Even for those of us with gray hair and battle scars from decades of successes and failures in our pruning and thinning programs, it can be helpful to occasionally step back from the minutia of our daily farming routines and remind ourselves of the myriad of tools available to help us achieve our production goals.
As such, we offer this brief overview of your opportunities as growers to set yourself up for success in crop load management. While it is by no means a comprehensive list of every factor that affects your cropping, hopefully it stimulates some thought about some of your orchard management decisions.
It is worth reminding ourselves that at all times, we are farming two crops of fruit on our trees: the crop that will flower and bear fruit this year, and the juvenile buds that hopefully will develop into next year’s crop. Those juvenile buds will either become vegetative and produce a new shoot, or reproductive, and produce a flower and potentially a fruit. Our success in crop load management depends in large part upon the choices we make which can shift the balance of vegetative vs. reproductive growth in our trees. Generally speaking, anything we do to increase carbohydrate reserves, flatten branch angles, increase light penetration, and/or reduce tree vigor should promote flowering and fruiting in the following year. Conversely, if we grow trees with steep branch angles, dense canopies which produce shade, and lots of vigor from over-irrigation or irrational exuberance in our application of nitrogen, we run the risk of discouraging floral initiation and falling into a pattern of biennial bearing.
The first two major factors in crop load management, genetics and orchard system, involve choices that growers typically only have one chance to get right (namely when the orchard is planted), so it is particularly vital to put considerable thought into these decisions because you’ll have to live with the consequences for the life of that orchard block.
Selection of good genetics is important for both the scion and rootstock. Growers should consider not only the market prospects for various cultivars, but their growth habits (i.e. tip-bearing vs. spur-bearing, branch angles, natural canopy architecture, etc.) and how easily they can be managed in the grower’s preferred orchard training system. Rootstocks have profound effects on tree vigor, precocity, branch angle, fruit size, etc.; the choices of rootstocks can be overwhelming but helpful information can be found from many sources, including this excellent summary of apple rootstocks by my former colleague Tom Auvil: https://www.goodfruit.com/wp-content/uploads/appleRootstock-Feb12016WebPullout.pdf
There are many factors to consider when selecting the appropriate tree spacing, tree architecture, and trellis design for your particular operation, but the most productive and sustainable orchard systems are those which optimize the efficient capture and distribution of light throughout the canopy while allowing easy access to all portions of the tree by humans and machines, including sprayers. Whether on a vertical or angled (i.e. V-trellis) configuration, the most efficient systems are typically two-dimensional fruiting walls trained to multi-wired trellises. https://www.goodfruit.com/systems-trial-asks-2d-or-not-2d-video/
Training & Pruning
Pruning represents the most powerful tool for fruit growers to manage their crop loads; by removing potential flowers and fruits before they form, pruning significantly reduces competition for carbohydrates and nutrients, as well as eliminating the source of plant hormones (i.e. gibberellins) which discourage flowering for the next season; the ultimate result is increased fruit size and improved return bloom.
With more formal training systems where fruiting branches are secured to regularly spaced trellis wires, it becomes far easier for orchard workers to calculate how many fruit should be set on each branch to achieve target yields. For example, if a manager can instruct pruning or hand thinning crews that they want 5 apples set on each lateral branch and 2 apples set on the central leader in between each trellis wire, those crews are much more likely to produce results that are consistently on target than crews trying to prune more chaotic free standing trees where it is unclear how many fruit or buds are present on a given branch, let alone how many should be there to achieve target yields. Tools like the Equilifruit disk (https://treefruit.wsu.edu/article/green-fruit-thinning-with-the-equilifruit-disc/) may be helpful in suggesting appropriate targets for pruning or hand thinning in these situations, but workers may still be challenged to make accurate counts of buds or fruit in these more complex branching structures. Growers may soon have help from robots to find, count, and monitor buds in their orchards to help inform these pruning decisions, whether those cuts are ultimately made by humans or machines. (https://www.goodfruit.com/evolutions-in-imaging/) There are several research programs and ag tech companies currently working in this area with some already offering services to map every bud or fruit in your orchard, which could be very helpful information in planning pruning and thinning strategies.
Pollination & Fruit Set
It is difficult to imagine any process in agriculture that is more crucial to economic success yet so fraught with peril as setting a target crop load in a fruit tree. Despite our best efforts to set our trees up for success, our yields can be compromised by limitations due to poor bee flight, frost, poor pollen availability, pollen incompatibility, “flash” blooms, poor stigmatic receptivity, slow pollen tube growth, short ovule longevity, carbohydrate stress in the tree, etc. Growers can’t expect to control those physiological functions which are fundamentally driven by weather conditions, but they can help themselves by ensuring that they have the appropriate source of pollen available (https://www.goodfruit.com/pollenizer-research-reveals-patterns/), setting their bees up for success (https://treefruit.wsu.edu/article/honey-bee-management-in-tree-fruit-orchards/), and monitoring the foraging activity of their bees (https://treefruit.wsu.edu/article/new-decision-aid-systems-das-models-launched/).
While hand blossom thinning has proven to be consistently effective in industry practice, the ever-increasing cost of labor combined with relatively soft returns for most varieties have rendered this practice cost-prohibitive for most operations in recent years. Despite all of its inconsistencies and risks, the most cost-effective thinning strategy for most apple operations is an aggressive chemical thinning program. Just as with pruning, it is true that the earlier a grower can reduce a tree’s crop load, the better the results will be in terms of harvest fruit size and return bloom. As such, aggressive use of proven chemical bloom thinners like oil + lime sulfur (https://treefruitresearch.org/report/crop-load-and-canopy-management-of-apple-4/) can give growers a good head start on thinning their crops. The efficacy of these programs can be enhanced with the use of the pollen tube growth model found on WSU’s AgWeatherNet system (http://weather.wsu.edu/) to help determine the best timings for bloom thinning sprays (https://www.goodfruit.com/take-the-guesswork-out-of-thinning/).
Table 1. Proven chemical bloom thinners of apple.*
|ATS||15/60 (25%)||10/63 (16%)||4/55 (7%)|
|NC99||15/32 (47%)||7/34 (21%)||2/28 (7%)|
|Lime sulfur||26/58 (45%)||12/52 (23%)||9/52 (17%)|
|CFO + LS||62/115 (54%)||27/106 (25%)||22/105 (21%)|
|JMS + LS||14/24 (58%)||8/23 (35%)||4/22 (18%)|
|WES + LS||15/32 (47%)||5/31 (16%)||4/30 (13%)|
|ThinRite||7/22 (32%)||0/23 (0%)||0/12 (0%)|
|*Incidence of results significantly superior to untreated control. WTFRC apple chemical bloom thinning trials 1999-2019.|
Postbloom applications are often necessary to fine tune the crop load after bloom thinning sprays have been applied. There are a number of effective programs available to growers featuring combinations of carbaryl, NAA, NAD, 6-BA, and ethephon, with at least one promising new material potentially coming to market in the next few years (https://treefruitresearch.org/report/crop-load-and-canopy-management-of-apple-4/). There are also some new postbloom chemical thinning options in the form of ABA, which can be used in organic operations, and ACC, which offers the potential to thin fruitlets as large as 20 mm (https://www.goodfruit.com/ifta-day-3-pennsylvania-by-conference-room/).
Table 2. Proven chemical postbloom thinners of apple.*
|BA||7/29 (24%)||0/30 (0%)||0/26 (0%)|
|Card +BA||33/91 (36%)||10/89 (11%)||13/86 (15%)|
|Carb + NAA||29/78 (37%)||20/76 (26%)||13/71 (18%)|
|BA + NAA||19/41 (46%)||9/40 (23%)||7/36 (19%)|
|Metamitron||18/29 (62%)||13/28 (46%)||6/22 (27%)|
|*Incidence of results significantly superior to untreated control. WTFRC apple chemical bloom thinning trials 1999-2019.|
Plant Growth Regulators
Strategic application of plant growth regulators can help several aspects of crop load management. Application of gibberellins after a spring frost has been reported to reduce fruitlet abortion. Cytokinins like 6-BA are known to promote cell division and ultimately fruit size when applied in warm conditions after petal fall. Several bioregulators including 1-MCP (Harvista), AVG (ReTain), and NAA are frequently used to inhibit abscission of fruit near maturity, essentially delaying the need for harvest.
Some of these same plant growth regulators can manipulate return bloom by either promoting (NAA, ethephon) or inhibiting (Gibberellic Acid or “GA”) floral initiation in juvenile buds. Even though summer NAA and ethephon programs intended to boost return bloom after large crops remain popular in some sectors of the apple industry, they have performed very poorly in WA research trials (https://treefruitresearch.org/report/crop-load-and-canopy-management-of-apple-2/). Several years of research has demonstrated the effective mitigation of biennial bearing patterns by reducing return bloom with application of various GA formulations in the “off” year of an alternate bearing pattern (https://treefruitresearch.org/report/crop-load-and-canopy-management-of-apple-4/). Fortunately for apple growers, a new GA product (Arrange) has been labeled specifically to inhibit flowering and has been approved by OMRI and could be a great tool for both organic and conventional apple growers to help them manage crop load.
Much like corn farmers, backyard gardeners, and foresters, our success as tree fruit growers depends largely on our ability to efficiently capture light from the sun and encourage our plants to convert that energy to our desired end product, namely fruit. We significantly influence light distribution by our choice of orchard system but once the block is in the ground, we don’t have many tools available to substantially change the light dynamics within the canopy. Even with aggressive pruning, improvements in light distribution in the lower portions of the canopy are usually marginal and temporary.
One tool that has been proven to consistently improve yields of target fruit are white reflective groundcovers like Extenday. These products essentially bounce light reaching the orchard floor back up into the canopy where it can be used to power photosynthesis. Over time, this increased light environment and surplus of carbohydrates encourages the formation of more flower buds in previously shaded portions of the canopy, reduces fruitlet abortion associated with carbohydrate stress, and provides extra nutrients for fruit growth. The net result in numerous research studies in apples, pears, cherries, peaches, and nectarines has been increased yields of large, well-colored fruit (https://www.goodfruit.com/tools-growers-should-be-using/).
Even though crop load management is usually a topic of great interest at this time of year, we would help ourselves by thinking of it more as a year-round management task. In fact, the decisions we make even before planting a new block will have a major impact on how those trees produce fruit for the life of the orchard. Crop load management is a very demanding balancing act, but our focus on it is invariably time well spent because few other aspects of fruit production have such potential to impact our financial bottom line.
Washington Tree Fruit Research Commission
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