Effect of Different Types of Thinning on Source: Sink Relationship and Fruit Quality of Plum | Effect of Different Types of Thinning on Source: Sink Relationship and Fruit Quality of Plum

Chemical thinning agents for stone fruits are scarce. Hence, the objective of the present work was to improve fruit quality,viz size in plum, using mechanical and chemical thinning or combinations thereof; untreated, i.e., un-thinned plum trees of the same rows served as control.Plum trees of the medium-sized cv. ‘Ortenauer’ on dwarfing St. Julien GF 655/2 rootstock, with maximum flower intensity in 2009, were grown near Bonn, Germany. Trees were mechanically thinned in April 2009 with the blossom thinner developed by the University of Bonn with 300, 400 or 500 rpm at a tractor speed of 5 km/h. Half of the trees were additionally chemically thinned with both ammonium thiosulphate (ATS) (15 l/ha) at full bloom and an ethylene releasing compound (0.375 l/ha) 35 days after full bloom.The objective of 1/3 flower i.e. fruitlet removal was successfully achieved even with the slowest rotor speed of 300 rpm. The number of fruit per metre fruiting spur was reduced from 46 to 18–27, equivalent to a (source: sink) leaf: fruit ratio of 5:1. Mechanical thinning significantly enlarged fruit mass from 28 g in the un-thinned control to 30–32 g with rotor speeds of 400 or 500 rpm. Additional chemical thinning with ATS and an ethylene-releasing compound resulted in no further increase in fruit mass. Inner fruit quality (sugar) of the plums appeared unaffected by either mechanical or chemical thinning, except for fruit firmness. Plums thinned with an ethylene releasing compound were softer and ripened earlier than respective control fruit, possibly due to the ethylene release.The lesser fruit density per tree after thinning reduced the potential for fungal infections such asMonilia due to faster drying of the fruits after precipitation.Mechanical thinning reduced thinning by manual labour from 31 min. per tree in the un-thinned control to 24 min. by ca 25%, i.e., by 7 min. per tree; this is equivalent to a net financial gain of 400–500 <euro>/ha, after expenditure (120 <euro>/ha; 1.5 h/ha), for thinning.Overall, both efficacy of blossom removal and fruit mass enlargement scored best after mechanical thinning around 400 rpm, indicating that mechanical blossom thinning provides a suitable alternative for chemical and/or manual thinning or can be combined with either of those options. An additional advantage of mechanical blossom thinning is to overcome or avoid alternate/biennial bearing due to its early application at bloom time; a similar effect was observed with the ethylene-releasing compound applied 35 days after full bloom.