A Tool for Making Decisions on Rates and Timings of Chemical Thinners

Determining the rate and timing of chemical thinner application is the most important task a grower has to perform in the growing season. Removing too few fruit reduces overall fruit size and quality of the remaining fruit and can reduce flower formation and the crop for the next year. Unfortunately, weather conditions affect tree response. The Cornell Carbohydrate model is an attempt to factor in the interaction of the environmental conditions and the tree physiological status. The model utilizes temperature, solar radiation and day length based upon ground based measurements. These measurements are for the most part derived from weather stations located within commercial orchards. The biofixes utilized are the date of bud break and the date of full bloom. We have used dates provided by cooperating growers around the state.

With support from the Extension Committee of the State Horticultural Association of Pennsylvania we have been able to join the Network for Environment and Weather Applications (NEWA). This network is based at Cornell University and utilizes ground based weather stations at grower orchards and nearby sites.

Cornell 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, 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.

The recommendations provided do not specify a specific material(s) or rate(s). Rather they provide a guide to adjust rates based on the environmental conditions and estimated tree physiological status. Negative balances indicate a deficit of carbohydrates and the potential for over thinning or the need to reduce chemical rates. Positive balance values indicate a surplus of carbohydrates and the potential for under thinning. Table 1 shows as the balance changes how rates can be adjusted.

Table 1. Decision rules for using the output of the carbohydrate model to adjust chemical thinning rate.

4-day Av. Carb. BalanceThinning Recommendation
> 0g/day Increase Chemical Thinner Rate by 30%
0g/day to -20g/day Apply Standard Chemical Thinner Rate
-20g/day to -40g/day Decrease Chemical Thinner Rate by 15%
-40g/day to -60 g/day Decrease Chemical Thinner Rate by 30%
-60g/day to -80 g/day Decrease Chemical Thinner Rate by 50%
< than -80g/day Do not thin (many fruits will fall off naturally)