Full Bloom and Petal Fall Thinning Options

At this point most orchards in southern Wisconsin have reached petal fall with fruits in the 5 to 10 mm, but orchards in northern locations, such as the PARS research station, are entering king bloom. I wanted to provide some recommendations for growers regarding thinning options for this period.
Bloom thinning options
Bloom thinning is an excellent option to reduce the number of potential fruits early on and ensure good return bloom 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.
- You can achieve a substantial amount of the thinning.
- Required several applications.
- Is registered as a fertilizer.
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)
- Mild safe thinner @ bloom
- Best return bloom
- 10 ppm (4 oz/100 gal)
5) Naphthalenacetamide NAD (Amid-Thin)
- Mild thinner @ bloom
- Positive effect on return bloom.
- 8-10 oz/100 gal
Additional recommendations for bloom thinning from previous articles and webinars can be found here:
Using the Pollen Tue Model for bloom thinning
Petal Fall thinning options
It might be a bit early in central and northern Wisconsin for petal fall spray, so I would recommend waiting until fruits are 5-6 mm (this is usually when we reach about 100-125 growing degree days from bloom; you can calculate this using the NEWA carbohydrate model), we might reach this stage after this weekend with the warmer temperatures that are forecasted. However, in southern Wisconsin we should be right on time to get a fall petal spray on this weekend. Use the carbohydrate model to adjust your rate and do not spray if carbohydrate deficits are less than -60. In general, at petal fall all thinners have a mild effect and there is very little risk of over-thinning. Here’s a list of alternatives for petal fall thinning sprays:
Fruits at 5-6 mm
1) Sevin (1pt/100 gallons) very mild thinner.
2) Maxcel (64 oz/100) plus Sevin (1pt/100 gallons), this is a great option for Gala
3) NAA (3-4 oz) DTRV plus Maxcel (64 oz/100)
4) NAA + Sevin
Fruits at 11-13 mm
1) NAA (3 oz/100) plus Sevin (1pt/100 gallons)
2) Maxcel (64 oz/100) + Sevin (1pt/100 gallons)
3) Maxcel + NAA (Good option if you don’t want to use Sevin; not for Fuji and Red Delicious)
Consideration for petal fall sprays:
Are bees present? Caution if you are using Sevin.
You need a period of at least 8 hrs with no rain after the thinning spray.
Finally, how do you know how much thinning you have achieved with bloom and petal fall spray? You need to use the Fruit Growth Rate model.
This article was posted in Apples and tagged Amaya Atucha, Apples, bloom thinning, full bloom, petal fall thinning, pteal fall, thinning.