Wisconsin Apple Growers Adjust Thinning After Frost Damage

By Blake Jackson

Apple growers across Wisconsin are seeing very different levels of frost damage this season, with some orchards reporting severe crop losses while others escaped with little to no injury.

According to a UW-Madison associate professor and Extension fruit crop specialist, fruit development also varies widely from orchard to orchard, making careful thinning decisions especially important this year.

Growers with minimal frost injury are advised to continue with standard thinning programs. Experts recommend beginning with blossom thinning, followed by a petal fall spray once fruitlets reach 5 to 6 millimeters.

Another thinning application may be needed at around 12 millimeters. Waiting until fruitlets reach 18 to 20 millimeters can reduce thinning effectiveness and make crop management more difficult.

When more than half of the king blossoms survive, producers can generally follow a typical thinning strategy using petal fall and 12-millimeter sprays. Even with some frost injury, healthy lateral blossoms may still support a strong crop.

However, orchards with heavier king blossom damage should delay thinning until lateral fruitlets reach 8 to 10 millimeters, followed by another application at 12 to 15 millimeters if necessary.

In blocks where both king and lateral blossoms suffered major frost losses, growers may need to depend on bloom from one-year-old wood. In those situations, experts recommend waiting until fruitlets from that wood reach about 12 millimeters before thinning.

For chemical thinning, two commonly recommended mixtures include Maxcel combined with Carbaryl and NAA paired with Carbaryl. Maxcel programs are often effective on Gala and other difficult-to-thin varieties, while NAA combinations are commonly used for Honeycrisp apples.

Specialists also encourage growers to monitor the carbohydrate model before spraying. Trees experiencing carbohydrate stress are more sensitive to thinning materials, while trees with a carbohydrate surplus may require higher thinning rates to achieve the desired results.

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