Photo: Tara Baugher, Penn State
Several sites reported full bloom Sunday through Wednesday. While Thursday was somewhat warm with highs generally in the mid-70s, afternoon showers are predicted. The weekend does not look very promising for thinning, with the chance of showers on all three days. High temperatures will only be in the 50s to 60s across the state for Saturday and Sunday. Warm temperatures will rebound beginning next Wednesday, approaching the 80s by Friday.
In advance of making a chemical thinning application you should visit the NEWA web site and become familiar with the Apple Carbohydrate Thinning Model. The link at the NEWA site is on their drop down menu under Crop Management. Choose a site nearest your orchard location and enter your Green Tip and Full Bloom Date. Then hit the green Calculate button.
As Jim Schupp mentioned last year, the Cornell model does a good job of evaluating the effects of the weather environment on fruit susceptibility for chemical thinning, but other factors also need to be evaluated. The environment interacts with the application methodology, affecting its absorption and drying time. Other factors to consider include the cultivar, tree age, vigor, and cropping history. Finally, the response to certain chemical thinners, e.g., NAA or 6-BA growth regulators, can be affected by temperature.
Apple Carbohydrate Thinning Model
The Cornell apple carbohydrate thinning model was developed by Alan Lakso at Cornell University and adapted for practical use by Terence Robinson at Cornell University. The model, using fundamental tree physiology, estimates over a season from bud break the carbohydrate production by an apple tree with a set description and the total demands for growth for both the crop and the tree growth. It calculates a balance of supply to demand each day of the season using the weather data that is entered from the chosen weather station.
It should be noted that the model does not attempt to model any specific variety or training system. The general carbohydrate balance it calculates has been found to correlate well with tree sensitivity to natural drop and with sensitivity to chemical thinners. Cool sunny periods of good carbohydrate supply lead to reduced natural drop and less response to thinners. Cloudy hot periods give carbohydrate deficits and lead to stronger natural drop and stronger response to thinners. The four-day running average is used since studies and observations have shown that the apple tree does not respond to just one or two days of good or bad weather, but has a capacity to respond more slowly to changes in weather.
The thinning recommendations given for each date are based on the forecasted carbohydrate balance over the next 4 days in the future. Thus if a spray is applied today, the weather over the next 4 days is taken into account when giving a thinning recommendation. The thinning recommendations are based on the best research available and are given as a guide to help define the proper rate of hormone type thinning chemicals (NAA, 6-BA and Carbaryl) when sprayed with a given carbohydrate status. However, since they are based on forecasted weather and biological systems which we don't completely understand they are inherently imprecise. Users of the carbohydrate model should use the recommendations at their own risk. Cornell University is not responsible for the actual outcome of thinning sprays which are influenced by many additional variables including individual spraying methods.
You should be aware that variability of chemical fruit thinning can also be caused by the chemical thinner concentration, fruit size at time of application and sprayer water volume. The mechanism of chemical absorption is also impacted by temperature, humidity and drying conditions. The final factor is the sensitivity of the tree which includes initial fruit set, blossom density, tree vigor and previous flower damage.
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.
Table 1. Decision rules for using the output of the Carbohydrate model to adjust chemical thinning rate
|4-day Av. Carb. Balance||Thinning Recommendation||Level Rating|
|> 0g/day||Increase Chemical Thinner Rate by 30%||None|
|0g/day to -20g/day||Apply Standard Chemical Thinner Rate||Normal|
|-20g/day to -40g/day||Decrease Chemical Thinner Rate by 15%||Low deficit|
|-40g/day to -60 g/day||Decrease Chemical Thinner Rate by 30%||Moderate deficit|
|-60g/day to -80 g/day||Decrease Chemical Thinner Rate by 50%||High deficit|
|< than -80g/day||Do not thin (many fruits will fall off naturally)||Very high deficit|