A Challenging Year for Chemical Thinning

Posted: May 17, 2011

Fruit growers report that this has been a challenging spring in terms of applying cover sprays, and now chemical thinners. Fortunately, a number of varieties were thinned last week when the temperature range was favorable. However, fruit size of some varieties was too small last week and these may still need to be thinned.

Growers report seeing good fruit set on most varieties with a range of fruit sizes on each tree. This is probably the result of several separate windows of pollination during the long protracted bloom period the region experienced in 2011. However, bloom was poor on some varieties and there is not an excessive crop of fruit in some blocks. So, this season we are observing a broad range of fruit set and fruit size.

Gauging initial fruit set is the first key to proper chemical thinning, and the range of fruit sizes has made it a little more difficult this year. This can be done by counting fruits on several limbs in a block. As an example, a lightly pruned limb that is one inch in diameter should have about 35 fruits after final set. A range of fruit sizes can actually be helpful when planning thinner sprays if there is enough of the largest class to set a crop. It gets trickier if you must thin to retain some of the smaller fruits in order to make a crop. Repeated measurement of fruit diameters over several days is useful to determine if the smaller fruits are setting. This year it is important to remember that growing fruit are setting fruit, regardless of their relative size.

Developing fruits are most susceptible to NAA or 6BA thinners at about 12mm, while beyond 17mm the thinning response to NAA and 6BA dwindles. If some of the smaller class of fruits is desired to set a full crop, then a lower strength thinner is advised. When daytime highs are in the mid-80s, even larger fruits are susceptible to thinner chemistries, including “mild” thinners such as carbaryl. Still, for most blocks the question is not whether to thin apples, but what to use when weather conditions improve at the end of the week and beyond.

The forecast (as of May 17) calls for clearing skies and a return to warm temperatures by this weekend. By Sunday the highs are forecasted to be in the 80s, with lows in the 60s for several days. Given the forecast for elevated temperatures and in some cases, apple varieties with lower initial fruit set, should you consider reducing the strength of chemical thinners?

If the updated forecast continues to call for several days with daytime highs in the 80s, then growers may consider reducing the potency of thinner tank-mix in response. In the case of NAA, this is done primarily by reducing rates. For example, varieties that ordinarily would have received 4 or 6 oz Fruitone per 100 gallons may now receive 2 to 3 oz per 100 gallons.

Thinning products containing 6BA are typically tank-mixed with carbaryl and applied at 2 qt per 100 gallons. If a milder mixture of 6BA is desired, it may be applied at 1.5 qt plus carbaryl. The application of 2 quarts of 6BA alone is also a mild thinner.

With thundershowers in the extended forecast, this may also be a year to limit the rate of carbaryl to 1 pt per 100 gallons to reduce the possibility of additional thinning from re-wetting of unabsorbed carbaryl on the leaf surface. Spray oil or surfactants should be reduced or dropped from the tank mix when there is uncertainty about the strength of fruit set, a need to retain some of the small fruits that are setting, or when there are concerns about high temperatures during the four to five days after a chemical thinner application.

Contact Information

James Schupp
  • Professor of Pomology
Phone: 717-677-6116