Post-Bloom Thinning Update: 4.28.19

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Apple bloom image

Post-Bloom Thinning Update

The majority of post-bloom chemical thinning decisions will likely occur this week. The forecast calls for relatively warm temperatures (70’s to low 80’s) to coincide with the next several days of the post-bloom thinning window (~8-12 mm fruit size).

All post-bloom chemical thinners have good thinning activity with warm temperatures (>68 F) and all chemistries and all have potential to over-thin at excessively warm temperatures (mid-high 80’s). Additionally, warm temperatures can increase the susceptibility of trees to chemical thinners.

In 2019, several factors will further complicate chemical thinning decisions. Specifically:

Poor return bloom. In most blocks/varieties, return bloom has been satisfactory. However, despite aggressive/effective chemical thinning and use of return bloom programs in 2018, growers have reported poor/inconsistent return bloom in some varieties/blocks. I’ve heard multiple reports with Golden Delicious in particular.

Cold Damage. On multiple varieties, we’ve observed some bud kill/damage (Fig. 1). Presumably, this occurred during one of the cold snaps observed after bud break. In varieties affected, several fruitlets have already abscised or failed to develop during bloom. I’ve seen damage on most varieties that we grow in the southeast, but damage to Red Delicious has been the most significant. Where adequate return bloom was observed; however, there is generally a need for some chemical thinning.

flower bud

Fig. 1. Flower bud mortality has been observed on multiple varieties and should be considered with chemical thinner applications.

In some cases, spur leaf damage is also present (Fig. 2). Spur leaves appear to be distorted (abnormal shape and wavy leaf margins), and there can be some visible browning/yellowing on the underside of the leaves. While this injury may seem minor, spur leaves play a very important role in fruit set and are the primary source of carbohydrates for fruitlets at this time. In addition, absorption of thinning chemicals by damaged leaves is likely greater than when compared to undamaged leaves. In short, I would exercise caution when thinning trees that exhibit injury to spur leaf tissue.

damaged leaves

Fig. 2. Visible damage to spur leaves. Damage to these tissues can increase thinner uptake and subsequent thinning response.

Pollinator activity was somewhat limited during the bloom period due to cool weather and high winds, but there were a few nice windows for pollinator activity during the bloom period. Fruit set appears to be normal, in general.

I plan to provide carbon balance model updates in a post (4/30/19) across multiple locations.

Changes/Improvements to the Carbon Balance Model on NEWA

Based on a 17-year thinning study done by researchers at Cornell University, there will be some changes/improvements to the carbon balance model. To be clear: those of you that are using the carbon balance to make thinning decisions can continue to use the same model with no problems. This is still available on the NEWA website. For more information, see the following post from NEWA.

Crop removal on young trees

Complete crop removal on new/young plantings is an important step in orchard establishment. If fruit size is ~8-12 mm, the anticipated warm temperatures could provide an excellent window for de-fruiting young orchards using chemical thinners. There are several ways that this can be accomplished:

1) Post-bloom thinning. The current recommendation in the Integrated Orchard Management Guide for Commercial Apples in the Southeast is as follows:

2 oz. NAA (Fruitone L, Pomaxa, Refine 3.5 WSG) +1 lb carbaryl + 2 pint ethephon  per 100 gallons. Applied at ~ 8 to 10 mm fruit diameter. Caution: Application can suppress vegetative growth for 3 to 6 weeks.

While the above combination is an effective method in crop removal, a potential disadvantage of this treatment is that it can stunt vegetative growth.

2) Post-bloom thinning. In other apple-producing regions, de-fruiting recommendations have shifted to use of materials that do not have negative effects on vegetative growth and development. If you are interested in trying an alternative method of crop load removal on young trees this year, please consider the following:

150 ppm 6-BA (MaxCel; Exilis Plus; Exilis 9.5 SC) + 1 quart carbaryl + 1 quart non-ionic surfactant per 100 gallons.

Written By

Tom Kon, N.C. Cooperative ExtensionDr. Tom KonAssistant Professor: Southeastern Apple Research Specialist Call Dr. Tom Email Dr. Tom Horticultural Science
NC State Extension, NC State University
Updated on Apr 30, 2019
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