Written by Tory Schmidt, WTFRC, 4/21/22
Decisions about chemical thinning and crop load management are complicated in the best of conditions, but in unprecedented weather patterns, they become even more vexing. After the very wet, cold conditions we experienced for the last month and the very late severe frost that wiped out a lot of bloom on Good Friday, most of us are left with more questions than answers about what to expect for our crops this year. This is going to be a learning experience for all of us as the mid-and long-term effects of this weather pattern continue to reveal themselves in the coming weeks, months, and perhaps years. In the meantime, here are some things to keep in mind and resources that might help us make some better crop load management decisions given our current challenges:
Effects are variable and localized
Even though you may still be hearing widespread reports of frost damage or poor pollination conditions, it doesn’t necessarily mean that your trees suffered the same fate. You are likely to see significant differences in the localized effects of bloom survival on your trees based on variety, rootstock, topography, tree vigor, etc. As with virtually all aspects of tree fruit production, it is crucial that you get out and assess your own trees before making management decisions rather than relying on reports from your neighbors and colleagues and assuming that you’re in the same boat.
Loss of king bloom
For decades, the conventional wisdom has been that our best quality, largest apples are produced by the king bloom in the center of a flower cluster. While that is generally true, many research studies have now indicated that early-opening lateral flowers can produce fruit of virtually identical size and quality to the king bloom, suggesting that there may not be anything inherently special about the king bloom other than it might have a slightly longer growing season because it presumably opened a few days before weaker side bloom. This may be cold comfort (pardon the pun) for you because frost may have taken out both kings and early side bloom in many of your orchards, but rest assured that you can still produce a very good crop of apples that were set on late-developing bloom.
Cropping high in the tree/sprayer targeting
In most cases of frost events in tree fruit, the worst damage tends to be lower in the tree where the coldest air accumulated. We also know that in any year, fruit trees with complex, vigorous architectures tend not to produce as many flower buds or fruit in the shaded, lower portions of their canopies. If you are certain that you have significant frost damage in the bottom sections of your trees, you might be well advised to shut off some nozzles on your sprayer and target your chemical thinning applications toward the tops of your trees.
Which fruit are set?
One of the most nerve-wracking decisions apple growers have to make is how aggressive to be with their chemical thinning programs when they’re not sure how many of their flowers have set fruit. That decision matrix has been complicated for many WA growers this year by uncertainty regarding potential frost damage and poor pollination conditions. Despite those additional concerns, a model developed by Duane Greene (University of Massachusetts) can predict your fruit set with reasonable accuracy. It takes some effort but can be time very well spent if it gives you more confidence in your chemical thinning decisions. Here is a summary of the model and how to use it: https://www.goodfruit.com/precision-decisions/
Late chemical thinning
Given concerns about how many fruit you may have set in your blocks, it may be preferable to delay some of your chemical thinning applications in order to give yourself more time to assess your crop and the potential efficacy of early thinning sprays. The good news on that front is that we have a new thinning product available this year from Valent called AccedeTM. Its active ingredient is ACC, a metabolic precursor of ethylene, which is known to induce fruitlet abortion in apples. We don’t have much firsthand experience with this product yet but are currently evaluating its performance in research trials in our local conditions. Nonetheless, ACC has shown good thinning efficacy in multiple studies in other regions at timings as late as 20mm fruitlet size. This product could provide a good option for growers who figure out they still need a bit more thinning after the traditional 10-12mm thinning application timing has passed.
Biennial bearing concerns
An apple block with significant frost damage not only suffers due to lost yields in the year of the frost event but potentially for several years to come if the trees are thrown into a biennial bearing cycle. Fortunately, there is a growth regulator available from Fine Americas called ArrangeTM that can help reduce the severity of that effect. ArrangeTM is formulated with gibberellic acid (GA) which is known to inhibit floral initiation in developing apple buds. By applying GA to trees with significant frost damage in 2022, those trees should produce fewer flowers in 2023 and subsequently more flowers in 2024, helping even out alternation in the cropping cycle. Proper timings for those sprays are between petal fall and 15mm fruitlet size, so growers interested in trying to stave off biennial bearing patterns in frost-damaged blocks should be making applications soon.
Even though our apple trees have endured some unique challenges this spring, we need to remember that nearly all of them still have at least 20x more viable flowers than we would want them to set, so the vast majority of growers still need to be aggressive with their thinning programs. As a wise veteran of decades of crop load management battles likes to tell me in his tongue-in-cheek fashion, “Don’t worry if growers aren’t aggressive enough with their thinning programs – they’ll have 23 months to figure out how to get it right next time.”
Fruit Matters articles may only be republished with prior author permission © Washington State University. Reprint articles with permission must include: Originally published by Washington State Tree Fruit Extension Fruit Matters at treefruit.wsu.edu and a link to the original article.
Use pesticides with care. Apply them only to plants, animals, or sites listed on the labels. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.
YOU ARE REQUIRED BY LAW TO FOLLOW THE LABEL. It is a legal document. Always read the label before using any pesticide. You, the grower, are responsible for safe pesticide use. Trade (brand) names are provided for your reference only. No discrimination is intended, and other pesticides with the same active ingredient may be suitable. No endorsement is implied.