This week, temperatures remain cooler than average for this time of year, with daytime highs ranging from the low 50s to mid-60s°F and nighttime lows in the 40s°F. A rain system is expected mid-week, especially in southern Wisconsin, bringing 1–2 inches of precipitation and gusty winds. However, a gradual warming trend is forecast heading into the weekend, creating more favorable conditions for thinning applications.

Current weather conditions make thinning more difficult than usual. The lack of significant carbohydrate deficits in trees—due to cool temperatures—means that trees are less sensitive to thinning agents. Under these conditions, the effect of thinners is reduced, and achieving adequate fruit drop becomes more challenging. For this reason, growers should plan on multiple thinning applications this season to reach their target crop load.

A good strategy, especially for cultivars that are difficult to thin, is to make an early application at 5–6 mm, followed by a second spray at 12 mm, when fruitlets are at their peak sensitivity to chemical thinner. This staged approach increases the likelihood of effective thinning while reducing the risk of having to rely heavily on the last thinning window (18–20 mm), which carries greater risk and less margin for error.

Recommendations for Chemical Thinning at petal fall (5-6 mm) and 10-13mm fruit size

Maxcel (64 oz/100 gal) + Sevin (1 pt/100 gal): Highly effective for Gala and other moderate-to-difficult cultivars.

NAA (3–4 oz/100 gal) + Maxcel (64 oz/100 gal): Stronger activity for high-set trees or vigorous cultivars.

NAA (3–4 oz/100 gal) + Sevin (1 pt/100 gal): Also effective, with increased thinning potential over Sevin alone.

The use of a surfactant like Regulaid, will increase the effectiveness of the chemical thinner. Because we have a carbohydrate surplus, it might be a good idea to include a surfactant to increase the thinning response.

Tools for Decision-Making

NEWA Carbohydrate Model: Use this tool to adjust application rates.

Fruit Growth Rate Model: Track fruitlet diameter over several days to assess thinning response from bloom and petal fall sprays.

Final Reminders

Allow at least 8 hours without rain following application.

Check for bee activity before spraying, especially when using Sevin.

With this season’s cool weather, a single thinning application will likely not be enough. Spacing sprays—starting at 5–6 mm and following up at 12 mm—is a sound approach that spreads risk and increases your chances of achieving the ideal crop load. Don’t wait until the 18–20 mm window to do all your thinning—by then, options are limited, and the risk of poor results is much higher.

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