Effect of Growth Regulators on 'Gala' Apple Fructification

Blooming is one of the most important events during the growing season of an apple tree because it determines the next harvest. At this time, environmental and physiological factors interact to define the next steps of the fructification. Under mild winter climates, such as those experienced in the Southern Brazilian apple growing area, it is common for blooming and vegetative growth to start together. In this situation, the vegetative growth strongly competes with the flowers for carbohydrates, negatively affecting the fruit set. Growth regulators that act as cytokinins, auxins or gibberellins may increase the fruit set. The effects of prohexadione calcium (PCa) at the rates of 0, 55, 110 and 220 mg L-1, sprayed alone or combined with thidiazuron (TDZ) 10 mg L-1 or with Promalin (TM) 0.5 L ha(-1), were compared for their effects on the fructification and on the fruit characteristics of 'Royal Gala' apples. The treatments were applied between the pink bud and the full bloom stage. Applications of PCa at 55 mg L-1 plus TDZ at 10 mg L-1 increased the fruit set and the number of fruits per cluster, but did not differ from the treatment with only TDZ 10 mg L-1. When applied singly, the PCa treatments gave variable results, with a tendency to be better than the control. PCa plus Promalin (TM) reduced the fruit set. PCa 55 mg L-1 + TDZ 10 mg L-1 increased 132% the number of fruit per tree in comparison with untreated trees. The growth regulators PCa and TDZ were effective in increasing yield when there was lack of pollination.