Mechanical Crop Load Management (CLM) Improves Fruit Quality and Reduces Fruit Drop and Alternate Bearing in European Plum (Prunus domestica L.)

Abstract

(1) Background: With ca. 10 million tons of annual production worldwide, the plum (Prunus ssp.) ranks as a major fruit crop and can suffer from small fruit size, premature fruit drop and alternate bearing, which are addressed in this paper using a range of crop load management (CLM) tools. (2) Methods: Sixty 10-year-old European plum cv. “Ortenauer” trees on dwarfing St. Julien INRA GF 655/2 rootstock (slender spindle; 4.25 × 2.80 m) in a commercial orchard near Bonn (50°N), Germany, were thinned in 2 years and flower intensity assessed in the following year. Thinning was performed either mechanically (type Bonn/Baum) or chemically, with ATS (ammonium thiosulfate) or ethephon (Flordimex), or by a combination of mechanical and chemical methods, to improve fruit quality and the proportion of Class 1 fruit. Adjacent un-thinned trees served as controls. (3) Results: Natural fruit drop in June was reduced from 290 fruits per tree in the un-thinned controls to 265 fruits after ATS blossom treatment, and to 148 fruits after mechanical thinning at 380 rpm at a 5 km/h tractor speed at full bloom. The un-thinned control trees developed a large number of small, undersized fruits. The yield of Class 1 fruits increased per tree from 47% in the un-thinned controls, up to 69% after crop load management. Sugar content and fruit firmness were unaffected. (4) Conclusions: The study has shown that fruit quality (i.e., fruit size) and financial returns could be improved by either mechanical (380 rpm at 5 km/h) or chemical thinning, or a combination of both.