Effect of Chemical Thinning On Sunburn Incidence of ‘Honeycrisp' Apple

Chemical thinning through crop load has a direct effect on leaf-to-fruit ratio which strongly correlates with fruit exposure to the direct sunlight. Therefore chemical thinning is expected to affect sunburn incidence of fruit as well. In this trial, the incidence of three types of sunburn (sunburn necrosis, sunburn browning and photooxidative sunburn) on ‘Honeycrisp’/B.9 was evaluated in various chemical thinning programs. The following spraying programs were applied; spraying at full bloom (liquid lime sulfur + dam oil) or petal fall (6-BA + carbaryl) or post-bloom at 10-12 mm king fruitlet size (6-BA + carbaryl) or at any combinations of above. In the control, 9.1% total sunburn damage was observed, and the most common sunburn type found was sunburn browning (7.3%) followed by sunburn necrosis (1.1%). The symptoms of photooxidative sunburn were found very rarely (0.7%). Chemical thinner applications reduced crop load and increased leaf-to-fruit ratio resulting in lower levels of sunburn damage. The share of the different types of sunburn was changed and the relative ratio of sunburn browning decreased and of sunburn necrosis increased. The share of photooxidative sunburn was not affected by thinner application. Multiple applications of thinners reduced crop load more and resulted in less sunburn damage compared to single chemical applications. Although thinner application altered cluster composition increasing the share of singles and decreasing multiple fruited clusters, strong correlation between fruit number per cluster and sunburn incidence were found. Comparative fruit quality parameters of sunburned fruit are also reported.