Greeting!
Below are the outputs of the carbohydrate thinning model for Winchester area. I used Gala’s green tip and full-bloom dates to run the model, but this should be working for the majority of apple cultivars. The fruit size of most cultivars we have in our research farm ranges between 3-5 mm. We usually recommend a fruit size of 8-15 mm, to start post-bloom thinning applications. So, the purpose of today’s post is to remind you of the basics of this model and how to read and interpret the outputs. Basically, we use temperature and solar radiation data from the nearest weather station to our orchard in order to calculate how much carbohydrate a mature apple tree would make a day (g/ day) starting from the green tip date; and how much carbohydrate a tree would demand. If the balance between the carbohydrate supply to demand is negative, the sensitivity to chemical thinners will be high, but if this balance is positive, the response to chemical thinners will be low. Cloudy days, warm nights and high temperatures for a number of days negatively affect carbohydrate production and result in carbohydrate deficit; whereas sunny and cool days promote photosynthesis, leading to carbohydrate surplus. In the chart below, the blue line shows the daily carbon balance, whereas the red line shows the average of four days (the present day and three days in advance). We consider the four-day average (the red line) when deciding for thinning time and rate.

Beside the temperature and solar radiation, the percentage of the source to sink organs also determines the tree carbohydrate balance, and subsequently the response to chemical thinners. Therefore, in the chart above, you will notice a carbohydrate deficit in the last five days despite the sunny weather. This is mainly due to two reasons: a) temperatures were high (> 88 oF) which encourages photorespiration, not photosynthesis; b) the demand of sink organs (i.e. flowers, young fruit, young leaves, buds) to carbohydrate is higher than what source organs (i.e. mature leaves) can provide at this stage (see the figure below). This balance between sink to source organs will change as leaves mature, and the competition among sink organs decreases.

Note from the model developer (March 22, 2018):
“The apple carbohydrate model simulates the response to weather of trees that are healthy with normal vigor and bloom, no significant water, nutrient or winter or spring freeze stress, and no significant carry-over stress from a previous year that will change tree responses. We are less confident in the model if temperatures are extremely cold or hot. Each orchard is unique, so use this tool, as any other, in the context of your own experience”