Apple thinning may be difficult this year

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included. 

Apple thinning this year will be difficult in some areas of the state. Frost damage has eliminated kings on some varieties and, in more severe situations, has killed significant parts of the side bloom. There will be differences within orchards and even within trees.

Last year’s crop was a moderate crop for most growers. This means that this year’s bloom and fruit set should be strong. One new complicating factor is the warm winter we experienced. There have been reports around the world that when warm winters occur, fruit set is fair and thinning is easier. This year most varieties have an abundance of leaves present during the bloom. This should serve to strengthen the bloom.

Where bloom is not damaged by frost, aggressive thinning will be needed. Bloom density seems to be quite high for most varieties, due to the moderate crop last year. Plan to apply combinations of Sevin plus BA or Sevin plus NAA at the 10 mm stage. Where fruit set looks especially heavy, an across the board Sevin application at petal fall will help start the thinning process. Sevin is very effective at petal fall if weather conditions are favorable for application. Make sure bees are out before applying Sevin at petal fall. Use BA on the small fruited variety and Red Delicious and Fuji because those varieties are sensitive to NAA. Where king fruits are missing, it is going to be difficult-to-impossible to get a perfect thinning job. Be cautious and perhaps wait until 12 mm to make the first application. This way you will be better able to evaluate fruit set. At 12 mm to 15 mm you will still need combinations to achieve thinning; however, use only moderate rates. Where frost damage occurs, often the top half of the tree is probably undamaged and will need full thinning to achieve large quality fruit. Thin only the tops. As you drive up and down hills, in and out of frost pockets, gear up through the low areas and slow back down through the tops of hills to get a better thinning job.

All thinners work best when temperatures are warm, above 65 degrees, but we prefer to have temperatures after the application to be in the 80’s. If you are not getting warm temperatures, consider delaying until a warmer forecast of temperatures occurs. The temperature at the time of application is not important, the temperatures 2 to 3 days following the application are critical for effective thinning. This year, with the considerable frost damage, will make it more difficult to achieve perfect thinning. Where frost is not a factor aggressive thinning will most likely be needed.

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