Crop Load Management Update: Pink to Bloom

By Dr. Tom Kon

Recent warm temperatures accelerated reproductive bud development dramatically over the past week. Full bloom was reported at low elevation sites and high elevation sites have a range of bloom stages (pink to 75% open) pending upon cultivar. The upcoming cool weather will likely slow bud development. Since we observed a light to moderate crop in 2019, many orchards have a relatively strong return bloom in 2020. Given this high cropping potential in 2020, early thinning may be worth considering.

Blossom Thinning

When applied during favorable drying conditions, bloom thinning with liquid lime sulfur and oil (LSO) is a safe and effective strategy to reduce apple crop load early in the season. Our research program evaluated the use of liquid lime sulfur and oil as a blossom thinner in 7 trials over the past few years (in three states across three cultivars). We observed a reduction in crop load (6 of 7 trials) and an increase in fruit size (5 of 7 trials) when using the pollen tube growth model to time LSO applications. Additionally, over-thinning and fruit russet was not observed in any of our trials. For NC apple growers (LSO is not currently registered for thinning in GA and SC) interested in bloom thinning with LSO, please see our previous article for specific guidelines.

Carbon Balance Model Updates

Based on a 17-year thinning study done by researchers at Cornell University, there will be some changes/improvements to the carbon balance model. Specifically:

• The user must include an estimate of the percentage of flowering spurs (0-25%; 26-50%; 51-75%; 76-100%).

• The model uses a 7-day running average of carbon balance (2 days before application and 4 days after) instead of a 4-day running average to make thinning recommendations/suggestions.

• Growing degree-days from bloom will be calculated and included in table output.

• When GDD is 200-250 the column with GDD will turn green, indicating a “sweet spot” for tree susceptibility to chemical thinners.

• A color-coded system will be included to indicate risk associated with thinning on specific days (green = low risk of over-thinning; yellow = caution; red = danger of over-thinning).

You may wonder why all of these changes were included. In the aforementioned 17-year trial, final fruit set was related to 1) the number of flower clusters per tree, 2) degree days after bloom, and, 3) carbon balance after bloom. A natural reduction in estimated carbon balance occurred at 200 to 250 GDD after full bloom. This was identified as a key timing for making chemical thinner applications, since trees are more susceptible to chemical thinners and a greater thinning response may be observed. After May 1st, the “old” version of the carbon balance model will be removed from NEWA. Since rescue-thinning applications will likely occur in early May, I suggest that you use the new version of the model (Apple Carbohydrate Thinning v2019) and avoid changing models mid-season.