Thinning apples at petal fall for 2011

Follow these tips for thinning what looks like a large 2011 apple crop.

This year there are a number of orchards that had light crops in 2010. These blocks have returned this 2011 year with a heavy return bloom and abundance of stored energy that will promote the set of a heavy crop. On top of that is the late development of bloom across the state and little or no frost events to date, which will encourage additional successful set of apple fruits. Thinning a heavy, strong fruitset will require an aggressive plan commencing right after bloom. This may require multiple thinning applications.

Apples set during a period from bloom to 40 days after full bloom (DAFB). Fruitlets are responsive to stresses that occur during this time from both environmental or chemical influences. The natural sensitivity to stress changes during this important fruitset (thinning) window. At petal fall (5 DAFB), fruitlets are not very responsive to stress, but as they approach the 10-12 mm stage (15 to 25 DAFB) the sensitivity increases to a maximum level. This period of maximum sensitivity is when traditional thinning activities are performed and best thinning responses occur.  The sensitivity to thinning then gradually declines to no response at around 40 DAFB.

When a heavy crop is predicted as there is for 2011, multiple thinning applications can help successfully reduce cropload to a desired level. Growers should consider applying a first thinning treatment at petal fall (5 to 10 DAFB). This timing, in most years, is a very low risk period to get some thinning started but, in most years, will not over-thin. Usually only 10 to 20 percent thinning can be accomplished during the petal fall timing, because fruitlets are not inherently stressed or responsive to stress at this time from either natural or chemical thinning activities. This petal fall timing is a low risk time to perform general overall crop reduction. Occasionally, thinning at petal fall will thin the cropload to a perfect level and no additional thinning is needed. This occurs 1 out of 10 years in Michigan. Most years little to no thinning occurs during the petal fall thinning time period. However, it is the first time to have the chance to reduce cropload with the benefit of being able to evaluate the results 8 days later and make a second thinning application during the main traditional thinning window (15 to 25 DAFB). Second thinning applications are almost always needed. An added benefit of early petal fall thinning is the increased fruit size and the greater chance of increasing return bloom.

All thinning chemicals and rates can be used at petal fall, however there are some cautions to consider. First, Sevin is toxic to bees and should not be used if beehives are still in the orchard. Second, NAA is not recommended on varieties (Red Delicious, Fuji, etc.) that may have an adverse response to NAA (setting pygmy fruits). Third, NAA and BA are not compatible on some varieties and thus if thinning at the traditional timing 10 to 12mm stage is planned, it would be wise to use the same chemistry at petal fall and at 10 to 12 mm to avoid any negative interaction by applying BA after NAA or vise-a-versa.

The 2011 apple crop has the makings of being large and will most likely need thinning. Consider starting your thinning program with a first treatment at petal fall and follow with touch up thinning later. The benefits of this approach will be worth it and the risk of over thinning is low. Remember, you are the best judge on what thinning is needed in your blocks and what materials or rates to use and what timing to apply the thinners. Leave a few check trees to help you evaluate your thinning programs.

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