Responses to Weather and Chemical Thinners

After years of study of post-bloomapple thinning with numerousfield trials, we have not been able to fully understand and adequately con-trol apple thinning. This is due to the extreme complexity of the entire system with dozens of interacting factors re-ported to affect chemical thinner re-sponse, which is so variable from year to year and orchard to orchard (Figure 1). There are probably two major aspects of chemical thinning that cause so much variability: (1) the spray application Many factors affect the response of apple trees to chemical thinners. One of the most difficult to understand is the variation in tree sensitivity related to previous yield history, bloom density, initial set, leaf area, leaf function, temperature, sunlight, tree vigor, etc. In this report, we attempt to understand the parts of variation induced by weather and carbohydrate supply and demand balances