Flower or fruit thinning is the most important technique in apple growing for improving fruit quality. In organic fruit production the use of chemical-synthetic thinning agents or plant hormones for crop regulation is forbidden. In Poland the Armicarb® is registered as a fungicide. Being a caustic chemical, Armicarb® damages petals, pistils and anthers and prevents fertilization. In 2013 and 2014 potassium bicarbonate was used twice at the doses 10 and 15 kg∙ha^-1 on trees cv. ‘Braeburn Mariri Red’/M.9 T337. The first spraying was done at the beginning of flowering (BBCH 61) and the second at the full bloom (BBCH 65). Trees of cv. ‘Šampion’/M.9 T337 in 2014 were sprayed twice at BBCH 61 and 65 at the doses 10 and 15 kg∙ha^-1, but in 2015 the single application was done. In 2014 and 2015 on trees cv. of ‘Gala Must’/M.9 only one spraying with potassium bicarbonate was done at doses: 10,15 and 20 kg∙ha^-1. Thinning of apple flowers with Armicarb® caused a decrease in the yield, especially of cv. ‘Gala Must’ and ‘Šampion’. However in all cultivars the fruit size distribution was much better than in control. Each variety responded by clear increase in the fruit size after thinning with Armicarb®. Mean fruit mass, the diameter and length of apples after using Armicarb® at all doses were much bigger than in control. Skin of apples was severe russeted after application of Armicarb®. However skin russeting was also affected by the weather. In 2014, at cold temperatures and high precipitation during the period of intensively fruit growth, the skin of apples was more russeted relative to others seasons.

29-43