Chemical thinning of apples – Facts for Fancy Fruit

Chemical thinning of apples

Cool, wet weather has complicated thinning decisions. Remember that thinning is all about supply and demand for carbohydrates in the tree. Natural fruit drop occurs when demand is high (many developing fruitlets needing carbohydrates to grow) and supply is limited (small leave area on the tree to supply carbohydrates through photosynthesis). Because of this shortfall of carbohydrates, some fruit don’t receive enough and drop off the tree. It’s survival of the fittest.

The post-bloom thinners we use in the Midwest act to exacerbate this shortfall, making the competition among fruitlets more intense so even more drop off. The do this in three ways:

  1. Reducing supply of carbohydrates by reducing photosynthesis (NAA)
  2. Decreasing translocation of carbohydrates from leaves to fruitlets (carbaryl)
  3. Increasing demand for carbohydrates by increasing respiration (6-BA).

This knowledge helps us understand thinning responses. For example, experienced growers know that cloudy weather soon after bloom results in increased fruit drop so they back-off a little on their chemical thinning strategy. And of course, this makes total sense – cloudy weather means less light therefore less photosynthesis and lower carbohydrates available for developing fruitlets. We also know that warm weather (over 65F) is necessary for post-bloom chemical thinners to work. As we mentioned in the last newsletter, these warm temperatures are necessary over a window of time stretching from two days before applying the thinner to the day after thinners are applied.

But, and it’s a very big BUT, before making chemical thinning decisions, assess crop load and any freeze damage (see Free Damage article in this newsletter).

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