Photo by M. Seetin
The general carbohydrate balance calculated 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 result in carbohydrate deficits and lead to stronger natural drop and stronger response to thinners.
The Cornell apple carbohydrate thinning model was developed Alan Lakso and adapted for practical use by Terence Robinson (both pomologists at Cornell University). Using fundamental tree physiology (beginning at bud break), the model estimates over a season the carbohydrate production by an apple tree (with a set description) and the total demands for growth for both the crop and tree growth. It calculates a balance of supply to demand each day of the season using the weather data entered from a 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 result in 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 that we don't completely understand they are inherently imprecise, and therefore Cornell scientists advise growers that use of the carbohydrate model recommendations is 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.
Table 1. Decision rules for using the output of the carbohydrate model to adjust chemical thinning rate.
|4-day Av. Carb. Balance||Thinning 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)|
As we have stated at various meetings the results are an estimation and do not represent any specific tree variety or system. You can use the results as an indication of how you might want to consider altering your normal thinning program based upon the carbon balance.
We encourage you to follow our postings and those on the NEWA site and make observations on how your thinning applications perform. By providing feedback to us we can further refine the process and hopefully provide you with better insight on your chemical thinning applications.