This model is a new tool, available through NEWA, that provides more precision during bloom thinning. This has generally been a time in which many growers prefer not to apply thinners because of the uncertainty on potential late spring frosts. However, early thinning has tremendous benefits for the current year production, but most importantly for return bloom, specially in biennial bearing cultivars as Honeycrisp and Fuji.

The pollen tube growth model (PTGM) predicts, based on ambient temperature, the time it takes for flowers of multiple cultivars to be fertilized (union of sperm and egg) after they have been pollinated. Fertilization happens in flowers when a pollen grain lands in the flower stigma and grows down through the style to be able to reach the ovules in the flower’s ovary and fertilize them to produce fruit (Figure 1). However, the time it takes for the pollen tube to reach the ovules will depend on ambient temperature, as well as the length of the style, which is cultivar dependent. The model accounts for a set number of flowers to be fertilized, based on the target crop for a specific block, and then a bloom thinner is applied to remove all the rest of the flower.

Here are the steps needed to use the model:

1) First you need to determine you target crop for the block (see article Target crop load-Supplemental issue May 11, 2018).

2) As soon as the trees begin to bloom, the grower should measure the longest style on 25 to 50 blossoms throughout the block to calculate the average style length, which is entered in the model.

3) Then count the total number of open flowers on five to ten representative trees in the block, and when there are enough blooms open on the trees to achieve the target yield (for example, 100), the model should be started. It will predict pollen tube growth (as a percentage of the average style length), based on the weather data.

4) The grower should check the model routinely, and once the pollen tube model predicts 100-110% of mean style length, apply a chemical thinner. At this point, the desired number of blooms should be fertilized, and the thinning product should prevent further fruit set.

5) After the chemical thinner is applied, the model will reset the pollen tube growth to zero, this will allow further thinner application to prevent later opening blossom to be fertilized. For this continue to run the model, but applied bloom thinning sprays when the model predicts pollen tubes are less than 75 percent of the average style length, this will ensure that those flowers will not be fertilized.

How to measure the length of style?

To measure the styles, remove the petals from the flower, followed by removing the sepals to be able to see the base of the styles. Then measure the longest style from each flower with a digital micrometer, caliper, or ruler. Unfortunately, the length of the style from the same cultivar will varied from year to year, so this measurement must be done annually if using the model.

The PTGM is available on the NEWA website. Here’s a link to one of the blog posts from NEWA that provides instruction and technical information about the PTGM.

NEWA Pollen Tube growth Model

This article was posted in Apples and tagged Apples, bloom thinning, pollen tube, pollen tube growth model, PTGM.