Although some orchards have experienced some degree of frost damage, the damage seems to be very site specific and not a generalized issue across the state. In general, this year’s return bloom looks very good, with many orchards will have a heavy bloom.  Bloom thinning is an excellent option to reduce the number of potential fruits early on and ensure good return bloom for next year, in particularly for cultivars with strong biennial tendency (e.g., Honeycrisp). Don’t be afraid of taking advantage of this thinning window, specially if you have a heavy bloom! We have two types of products that are available for bloom thinning, caustic products (Liquid Lime Sulfur and ATS) that burn the stigmas of flowers and hormone products (MaxCel, NAA, NAD), the latter ones are very safe to apply because they are very mild thinners at this stage. Here’s a list of available materials for bloom thinning:

1) Liquid Lime Sulfur (LLS) (2.5%; 2.5 gal/100 gal)

• Burns the stigma of pistil and pollen tubes growing in the style.
• Can cause some leaf phytotoxicity and fruit finish problems depending in the weather conditions.
• Combined oil @ 1-2% (fish, soybean, or summer spray oils)
• Most of the thinning can be achieve with LLS but it will require several applications.
• Organic approval.
• NO label in WI for thinning.

2) Ammonium Thiosulfate (ATS) (2.5%; 2.5 gal/100gal)

• Burns stigma of pistils.
• Apply at 60% tube growth (this product is less effective than LLS).
• Can causes leaf phytotoxicity, though no fruit finishing problems.
• Best results when temperatures are between 70-80 ˚F.
• Use when there is no frost damage.

3) 6-Benzyladenine, 6-BA MaxCel (8 pt/ac)

• Mild safe thinner @bloom.
• Better results when applied with warm weather (over 65 ˚F).
• Can increase fruit size (promotes cell division in fruit) good option for Gala.

4) 1-naphthaleneacetic acid NAA (Fruitone-N; Fruitone-L; Pomaxa; Refine)

• Mild safe thinner @ bloom
• Best return bloom
• 10 ppm (4 oz/100 gal)
• Apply at 100% spur bloom open (when all flowers in a spur are open)
• Works well in cooler weather conditions

5) Naphthalenacetamide NAD (Amid-Thin W)

• Mild thinner @ bloom
• Positive effect on return bloom.
• 8-10 oz/100 gal
• Apply at 100% spur bloom open (when all flowers in a spur are open)
• Works well in cooler weather conditions

Here’s my recommendation for bloom thinning in Southern Wisconsin, based on the 10-day weather forecast. It looks like we will have relatively cool weather during bloom, and some rain. Because of this cooler weather and the fact that some orchards might have some frost damage, I would recommend growers use a hormone thinner during bloom, such as NAA or NAD, because this are safer and will work better than caustic thinners such LLS or ATS with poor weather conditions.

Additional recommendations for bloom thinning from previous articles and webinars can be found here:

Bloom thinning webinar 2022

Using the Pollen Tue Model for bloom thinning

This article was posted in Apples and tagged Amaya Atucha, apple blossom thinning, apple thinning, Apples.