Apple blossom thinning for 2013 apple crop

A potential heavy set warrants a try for apple blossom thinning in 2013.

The 2013 apple crop will be heavy. Michigan State University Extension says growers should consider thinning early to reduce the crop. Usually the first thinning begins at petal fall with standard post-bloom thinners. However, this season blossom thinning is an additional time to start reducing the crop load. I am not very impressed with the blossom thinning results from past trials in Michigan, but blossom thinning does work. It does thin fruit numbers on average 24 percent; however, fruit size is larger but less than expected, perhaps due to the thinners leaf injury or other factors.

There are two blossom thinning treatments that can be used:

  1. LSO (lime sulfur and oil)
  2. ATS (ammonium thiosulfate) fertilizer

The LSO is applied at the rate of 2 gallons of lime sulfur and 2.5 oil per 100 gallons and at 100 gallons solution per acre. The ATS is applied at 2 gallons per 100 gallons at 100 gallons solution per acre. LSO thins by three effects: it burns the stigma and pistils; it prevents pollen germination and fertilization; and it depresses photosynthesis for several days. ATS only burns the stigma and pistils.

The LSO blossom thinners are timed for full bloom the day after king bloom has opened and the king flowers are assumed to be pollinated. A second, third and fourth application can be made every seven days if needed. These second, third and fourth applications will each thin more fruits off and it is difficult to know when to stop. This year with a heavy crop predicted, at least three sprays will be needed, but your personal experience with your orchard is the best guide. Organic growers need to use lime sulfur and fish oil. ATS is timed the same; the next day after the king flowers are assumed to be pollinated. ATS is only applied one time.

These blossom thinners will thin between 10 and 30 percent of the crop load. The fruits will be larger, but my experience is that fruit size does not increase as much as expected from the thinning effect. These blossom thinning treatments can be followed by standard chemical thinning treatments at petal fall and 10 mm. If LSO is used at blossom time, then I would expect standard thinner treatments to be a bit more effective at 10 mm (perhaps 5 percent more effective). Weather conditions such as high temperatures and high humidity affect the caustic burning effect of these blossom thinners. Use caution when conditions are hot and humid.