8mm fruit size in Erie on May 11, 2017. Photo: R. Crassweller, Penn State
Today's run was completed at 8:30 AM. Conditions continue to show that trees will be less responsive to chemical thinner applications due to low temperatures. Temperatures are expected to return to more favorable levels for thinning beginning next Tuesday for most areas of the state. While it has been cool, the fruit are still growing, albeit slowly. The figure shows average fruit size at an orchard in Erie County on the morning of May 11 to be about 8 mm in diameter. In the southern portions of the state fruit size is probably considerably larger. Orchards that have fruit that are 16 mm or larger in diameter may need to utilize ethephon products to achieve sufficient thinning for the larger fruits. However, to further complicate matters ethephon can over-thin trees under high temperatures (>85°F).
Other factors that can affect tree response include:
- spray volume - more water increases efficacy
- water pH - should be between 3 to 5 for greater stability and effectiveness
The response to ethephon is also cultivar dependent with Golden Delicious and Rome Beauty being more responsive than Delicious or Gala. The good news is that ethephon can remove larger size fruit up to about 24 mm in diameter. For greater thinning, Ethephon can be mixed with carbaryl or mixed with NAA and oil.
As mentioned in Monday's postingthere was scattered damage to fruit at Rock Springs. The damage was pretty extensive but at this time we do not have a firm feeling for what fruit will stay and which will drop and which fruit will be malformed. At the twilight meeting Wednesday night in Enon Valley, PA some growers in the area reported light damage.
Note of caution from Alan Lakso, the model developer (May 11, 2017):
The apple carbohydrate model assumes that the trees are healthy, with normal vigor, and have no significant stress (frost, drought, nutrient) and no significant carry-over effects from previous years that might change the tree response to the weather. With the severe NY drought in 2016, there may be orchards that will have a weakened state coming into 2017, which would likely make the tree abnormally sensitive to thinners. So in those cases the model may suggest a stronger thinner concentration than is appropriate. Conversely, if irrigated last year, with the warm season and lots of sun, those trees may be in better than normal condition and harder to thin.
With the current cold period, flowering, pollination, initial set and early fruit growth may not be very well modeled as we did not have much data under these very cold (or also excessively hot) conditions when the model was developed. For all these reasons, you should have less confidence in the model this year. Use your experience with your unique situation.
Apple Carbohydrate Thinning Models for Pennsylvania Counties
For more information on the carbohydrate thinning model, visit Carbohydrate Thinning Model. For more details on factors that affect plant growth regulator performance and chemical thinner rates, visit Growth Regulators in Apple and Pear Production.