Model Background
The pollen tube growth model (PTGM) was developed to better inform growers when to apply chemical bloom thinners by Virginia Tech and has proven to aid in this decision in various states across the U.S. The model is currently being validated in Ontario and was trialed at various locations across the province starting in 2020.
The PTGM uses temperature, amongst other weather-related aspects, to calculate ideal timing of thinning. The model is based on flower fertilization timing, which is why it is cultivar specific, although a “universal” PTGM is currently being developed by Dr. Greg Peck. Models for various varieties have been developed, one of those being Honeycrisp, in hopes of addressing the biannual bearing nature of the cultivar. The model is not currently accessible in Ontario.
How To Use
- Growers start by assessing their orchard to determine their target crop load
- Multiply 1.5 by your target crop load
- Once the amount of open king blooms is equal to 1.5 x target crop load, move on to step 3
- Collection and measurement of 25 to 50 king styles per block
- King style lengths are input into the modelling system
- Based on weather forecasting, the model will suggest the best time for effective bloom thinning application
- Application of chemical thinner is applied
- Grower assesses if a second or third thinner application is necessary
- Model will be restarted if needed for additional applications
Benefits and Challenges of the PTGM
Benefits | Challenges |
|---|---|
– Early bloom thinning tool – One-time field collection – Easy to use | – Field collection required – Requires weather data, including forecasting – Precise timing for thinning applications – Only available for certain varieties – Not available and currently being validated in ON – Unknown cost associated with use |
Project Overview
The model uses percent fertilization as the indicator to apple bloom thinner and it is recommended to spray once the model reaches 100%. In 2021 we had 6 days from collection to ideal bloom application timing, while in 2022 we had 3 days. This is caused by various factors, but it is something to consider if this is a tool you want to try on your orchard. The model can be restarted to apply as many bloom thinners as you wish.
PTGM: Pollen Tube Growth Model
GT: Grower Timing
LS: Lime Sulphur
ATS: Ammonium Thiosulphate
1: 1 application of bloom thinner
2: 2 applications of bloom thinners
Control: No bloom thinner applications
2020/2021 Trial
Site A Data
Treatment | Fruit Count Per Tree at Harvest | Floral Return Bud Count | Vegetative Return Bud Count | Vegetative : Floral Return Bud Count |
|---|---|---|---|---|
GT ATS | 44 | 10 | 56 | 28 : 5 |
PTGM ATS 1 | 53 | 16 | 58 | 29 : 8 |
PTGM ATS 2 | 52 | 9 | 57 | 19 : 3 |
Site B Data
Treatment | Average Fruit Count | Average Fruit Weight (grams) | Average Return Bloom Count |
|---|---|---|---|
Control | 249 | 245 | 6 |
PTGM LS | 195 | 286 | 15 |
PTGM ATS | 203 | 264 | 12 |
2020/2021 Takeaways
Site A Data Overview
- PTGM ATS 1 had the highest fruit count at harvest, while the GT ATS had the lowest
- PTGM ATS 1 had the lowest vegetative to fruit return bud ratio, while PTGM ATS 2 had the highest
Site B Data Overview
- Control had the highest fruit count at harvest, while PTGM LS had the lowest
- Control had the lowest weight, while PTGM LS had the highest
- Control had the lowest return bloom, while PTGM LS had the highest
Overall
- Bloom thinners decreased the average fruit count, but increased fruit weight and return bloom.
- Highest return bloom was seen in treatments with one bloom thinner application. This suggests that the timing of chemical bloom application is more important than multiple chemical applications.
- One GT ATS application was compared to PTGM timing, where the PTGM timing demonstrated higher return bloom. This suggests that modelling systems can aid growers in more effective bloom thinning decision making.
2022/2023 Trial
Treatment | Control | PTGM ATS 1 | PTGM ATS 2 |
|---|---|---|---|
Treatment Details | – Pruned – No Bud Thinned – No Fruitlet Thinned | – Pruned – Bud Thinned – Not Fruitlet Thinned – 60% application | – Pruned – Bud Thinned – Not Fruitlet Thinned – 60% and 100% application |
2022 Data
Treatment | Fruitlet size (mm), Julian Day 159 | Fruitlet size (mm), Julian Day 244 | Fruitlet size (mm), Change | Fruit count, Julian Day 159 | Fruit count, Julian Day 244 | Fruit count, Change | Fruit harvest, Weight (g) | Fruit harvest, Size (mm) | Fruit harvest, Count |
|---|---|---|---|---|---|---|---|---|---|
Control | 13.95 | 62.55 | 48.60 | 208 | 118 | -89 | 264.28 | 63.78 | 163 |
PTGM ATS 1 | 13.67 | 71.90 | 58.22 | 251 | 67 | -184 | 381.10 | 73.24 | 102 |
PTGM ATS 2 | 13.75 | 70.32 | 56.57 | 208 | 68 | -140 | 379.98 | 73.52 | 102 |
2022/2023 Data Comparison
Treatment | 2022 Size (mm) Julian Day 208 | 2023 Size (mm) Julian Day 207 | Change | 2022 Fruit count Julian Day 208 | 2023 Fruit count Julian Day 207 | Change |
|---|---|---|---|---|---|---|
Control | 49.34 | 61.83 | 12.49 | 184 | 8 | -176 |
PTGM ATS 1 | 55.98 | 61.95 | 5.97 | 109 | 37 | -72 |
PTGM ATS 2 | 54.44 | 60.33 | 5.89 | 108 | 23 | -85 |
2022/2023 Takeaways
2022 Data Overview
- PTGM ATS 1 had the largest increase in fruit size and largest decrease in fruit count, with comparable numbers of weight and size from PTGM ATS 2 treatment in 2022.
- The Control treatment had the least increase in size, lowest fruit count change, along with the lowest weight, and size at harvest.
- Control had the highest amount of fruit at harvest
2022/2023 Data Comparison Overview
- Fruit size in 2023 is larger across all treatments, this has also been seen across the province in most varieties, where the largest sizing is in the PTGM ATS 1 treatment
- Fruit count in 2023 is less than 2022, where the PTGM ATS 1 has the highest amount of fruit and Control has the lowest
- The Control treatment had that largest vegetative to floral bud numbers in 2022 and 2023
- The PTGM ATS 1 treatment had comparable vegetative to floral bud numbers as the PTGM ATS 2 in 2022
- PTGM ATS 1 had the best vegetative to floral bud numbers in 2023, with almost a 1:1 ratio
Overall
- Overall, both thinning applications demonstrated that without any crop load strategies fruit would be smaller and be in abundance
- PTGM ATS 1 had the most success regarding size, weight, and crop load, indicating that timing is critical for chemical blossom thinning efficacy
Project Summary
When dissecting the data, we see that this trial showed the role of bloom thinner application on ‘Honeycrisp’ trees.
- Bloom thinning timing is critical to the success of ‘Honeycrisp’ trees
- Return bloom can be increased with bloom thinner application
- Models can be developed to aid in decision making during critical times throughout the season
- More tools, means more grower ability to be successful
Grower Comments
All growers stated that the model would be worth using in the future, as it is another tool to become more efficient with chemical thinning. It is a guideline that could be invaluable when utilizing chemical thinners, where one grower stated that it requires less work than other models currently being used. A grower also stated that the PTGM could be used across the province on various orchard production styles. Most growers were interested in utilizing the PTGM specifically in ‘Honeycrisp’ apples due to the biennial nature of the variety. The biggest challenge associated with the PTGM would be the integration and access of the model for Ontario growers. One grower had never utilized bloom thinners and has since integrated ATS into their thinning program.
This past spring, I ran the model for a few growers who had not utilized it before, where they stated they had effective results in application timing. This season was a challenging thinning year, where many growers struggled to get their crop load down to their target, and from what I have heard most growers were successful in bloom thinning application.
Final Notes
If you are considering blossom thinning, I suggest you do a trial block in a small area of your orchard, read the label and do some research about the chemical, discuss with other growers in your area who have experience, and expect petal browning a few days after application. ATS and Lime Sulphur have shown to increase russeting of certain varieties.
This preliminary trial demonstrated that chemical thinners could decrease the biennial nature of ‘‘Honeycrisp’’ apples, but further research should be conducted, trialing the Pollen Tube Growth Model on different cultivars with various thinners and applications. This trial also demonstrated that the PTGM could have a place for use in Ontario, showing positive results in blossom thinning, return bloom and crop load management.
Acknowledgements
Thank you to Dr. John Cline who assisted with analysis and the growers who permitted trialing the model on their orchards.
For further information about the Pollen Tube Growth Model please refer to “Grower Trial on the Pollen Tube Growth Model” in the August 2021 ONNL and “Nibble Thin for a Quality Bin” in the April 2022 ONNL.
Erika DeBrouwer
OMAFRA Tree Fruit Specialist