As we approach another crucial season in our orchards, it’s essential to discuss the significance of chemical thinning programs in optimizing crop yield and tree health. The goals of these programs are to reduce fruit set, enhance the size and quality of remaining fruit, and promote return bloom for consistent annual cropping.

There are so many chemical options for thinning that sometimes it can be overwhelming for new, and even experienced growers. I have put together a table with a summary of products to help growers design a thinning program for this year (Table 1). Bloom thinners, applied when flowers are open and viable, work by impeding fruit set through the disruption of flower parts or induction of plant stress. On the other hand, post bloom thinners, applied after petal fall, often mimic plant hormones to trigger specific physiological responses, such as increased ethylene production leading to fruitlet abortion.

Timing is critical in achieving effective thinning results. Fertilized flowers become increasingly challenging to thin over time, highlighting the importance of early and aggressive thinning strategies. Research indicates that early thinning not only significantly reduces fruit set but also enhances fruit size, quality, and promotes return bloom. However, with the increased risk of spring frosts in an earlier than normal season as this one is shaping up to be, I would be cautious with being too aggressive with bloom thinning.

Factors such as cultivar, rootstock, tree health, and weather conditions influence thinning efficacy. Therefore, it’s essential to tailor thinning programs to individual blocks, considering factors like products selected (always review labels), timing, and application rates. Some apple cultivars can be more difficult to effectively thin (e.g., Honeycrisp, Fuji, Golden Delicious), and in this case an effective thinning program may require both bloom and post bloom thinning to achieve the desired crop load.

Weather conditions will significantly impact the effectiveness of chemical thinners. Temperatures before, during, and after applications can increase or decrease the effectiveness of these products. Especial attention should be paid when using thinning materials when temperatures exceeding 80°F and days are cloudy, since this results in carbohydrate deficits and can lead to excessive fruitlet abortion or phytotoxicity in certain instances. Thinner efficacy may diminish below 60°F, though lower temperatures can temporarily mask the indications of a significant thinning response. Growers dissatisfied with the performance of thinning sprays in cooler conditions might find it prudent to wait for a few days of warmer temperatures before considering additional thinners. The use of the carbohydrate and pollen tube models in NEWA are great tools that incorporate weather conditions prior and post application to help you make the best-informed decision regarding chemical thinner spray timing.

Table 1. Chemical thinner products available for bloom and post bloom applications.

Chemical	Commercial	Timing	Notes
Lime sulfur + fish oil or petroleum oil	–	Bloom	Option for organic growers.
NAA	PoMaxa® Fruitone L® Refine 3.5 WSG®	Post Bloom	Thinning effect of NAA increases when trees are in a carbohydrate deficit.
NAD	Amid-Thin® W	Post Bloom	Alternative option for Red Delicious.
6-BA	Maxcel® Exilis plus® Exilis® 9.5 SC	Post Bloom	Has the benefit of also increasing fruit size.
Carbaryl	Sevin® 4F Sevin® XLR Plus Carbaryl 4L	Post Bloom	Can be combined with NAA or NAD to increase thinning efficiency.
ACC	Accede	King bloom and post Bloom	Use with non-ionic surfactant increases efficiency.
Ethephon	Motivate® Ethephon II	Post Bloom	High rates of ethephon can reduce fruit size.

***** It is YOUR responsibility to review product label********

This article was posted in Apples and tagged Amaya Atucha, apple thinning, Apples.