Advances in predicting chemical thinner response of apple using a MaluSim carbon balance model

Chemical thinning remains one of the more unpredictable parts of apple production with large variations within years and from year to year. We have measured this variability each year from 2000-2009 on mature 'Royal Gala'/M.9, 'McIntosh’/M.9 and 'Ace Delicous'/M.26 trees. Our data suggest the variability in thinner efficacy is related both to stage of fruit development and carbohydrate availability to support fruit growth. The variability related to stage of fruit development is evident from low sensitivity of small slow growing fruits at petal fall which are usually about 4 mm diameter and high sensitivity to rapidly growing fruits between 10-15 mm. After fruits reach 20 mm, they are much less sensitive to chemical thinners even though growth rate is high. The physiological or genetic basis for the differing sensitivity to chemicals at differing fruitlet stages is not clear. A second source of variability is the availability of carbohydrates to support fruit development which we have estimated using a MaluSim carbohydrate prediction model. Model simulations from several years showed that there are often periods of particularly negative or positive carbon supply:demand balance which were associated with severe thinning or mild thinning respectively. Comparisons of carbohydrate availability and thinning also indicate that carbohydrate balance interacts with stage of fruit development resulting in different thinning efficacies due to both carbohydrate supply level and fruit development stage. Based on greenhouse and field studies we have generated a family of response curves which can be used to predict thinning response at various fruit sizes and under different carbohydrate balance conditions. We have used these curves, the MaluSim model and weather forecasts to predict thinning response for several regions of New York State with good but not perfect success.