2024 Champlain Valley Petal Fall Thinning Recommendations

Dr. Terence Robinson and Mike Basedow

Below is a review of recommendations from our Petal Fall Meeting that we held virtually on May 20th. The recording of which can also be viewed below:

Assess each block for flower damage.

We found variable damage throughout Clinton county when evaluating orchards on Monday May 20th. While there is much variability, and your mileage will vary, Honeycrisp tended to have less damage than NY-1 and Gala, and Mac types like Mac, Cortland, and Macoun had the most. Honeycrisp blocks were generally missing a few kings, NY-1 and Gala were missing a few more kings, and the Mac types were sometimes missing many kings and some laterals. While we think there are still many laterals and thinning to be done at petal fall in many varieties, the best way to know is to assess your own blocks carefully, and adjust your thinning accordingly:

If most king flowers are present, use full rates and follow the carbohydrate model.
If king flowers are damaged, then thin with lower rates.
If kings and laterals are damaged then don’t thin until 10-12mm.

Wait until king fruits are 5-6mm before applying your petal fall thinning spray. You can also use the NEWA carbohydrate model degree day calculator to target the best time for thinning. Spray your petal fall thinners when DD after full bloom are between 100-125 DD.

In Peru, we generally expect the petal fall thinning window will likely be Monday May 20-Wednesday May 22 based on the model. However, king fruitlet sizes were still generally in the 4-5 range on Monday, suggesting we should wait another day or two until they reach 5-6mm. Locations north and south of Peru will need to adjust their date accordingly, and can do so using the apple carbohydrate thinning model on NEWA.

At petal fall, all thinners have a moderate effect and are thus very safe. (There is little risk of over-thinning). The caveat would be to not spray when carbohydrate deficits are -60 or less. With the weather forecast we currently have, the carbohydrate model indicates there will be a significant but not overly severe deficit this week to help achieve good thinning.

Use full rates where set is good and damage is minimal. Reduce rates where king damage has occurred, and save thinning for 12mm where king and lateral damage has occurred.

7.5ppm (3oz) NAA + 1 pt/100 of Sevin on Honeycrisp, NY-1, NY-2, and Macoun
7.5ppm (3oz) NAA with no Sevin for Cortland
64 oz Maxcel/100 + 1pt Sevin/100 for Gala (can also be used for NY-1)
5ppm (2oz) NAA + 1pt/100 of Sevin on McIntosh and Evercrisp

Surfactants like Regulaid can increase the response of NAA.

Do not use a surfactant this year, since we have a strong deficit.

Suggested rates are dilute TRV rates. Calculate TRV for each orchard and then a concentration factor:

TRV/Volume of spray per acre=Concentration factor
Example: TRV=200 and spray volume is 100 then concentration factor=2

Multiply suggested rates by concentration factor to get rate/acre.

Example 3oz NAA X Conc factor of 2=6oz NAA/100 gal X 5 for a 500 gallon sprayer=30 oz/sprayer tank

If you use concentration factor for adjusting rate of Sevin then rains will increase the effect of the Sevin. Do not use concentration factor adjustment for surfactants.

Nozzle the sprayer for the petal fall spray with 1/3 of output to the bottom half of tree and 2/3 of the output to the top half of the tree.

Questions? Please feel free to reach out to Mike at mrb254@cornell.edu or 518 410 6823.