GROWTH RETARDATION OF APPLE PLUM AND APRICOT TREES BY PACLOBUTRAZOL IN A MEDITERRANEAN CLIMATE

The use of Paclobutrazol (PP 333) as two foliar sprays in summer, or as a soil drench in autumn, restricted vegetative growth during one season. Dose/response curves depend on species, clone and rootstock, tree phase and size. Big trees, with a large trunk or long shoots, necessitated application of a larger amount of the chemical.

Granny Smith and Starking Delicious apples on vegetative rootstocks required less than on seedlings. At higher rates leaves and fruits were also restricted in growth (decreased fruit length), while fruit number increased significantly in the subsequent year, mainly in apples, and in "off" seasons. The optimal dose of the active ingredient was between 0.5 and 1.5 g per maiden or semi-dwarf tree, and between 2 and 4 g per standard adult tree.

Among the "Japanese" plums (P. salicina L.), growth retardation by Paclobutrazol was better than among apples; strong plum varieties like Methley and Golden King, and even more so with apricots cv. Canino. On treated trees the percentage of flower buds and fruit set increased markedly. Shoot elongation was restricted, and internode length was 20–80% shorter than on the vegetative control trees. No phytotoxic symptoms were observed; fruit size could be related to the higher fruit number, and the stone fruits sometimes matured earlier, sometimes, or grew bigger, than the controls. The most effective treatment was a post harvest soil application via the drip irrigation system in early autumn.

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