A review of the most cost effective and efficient methods of thinning tree fruit crops in order to improve fruit quality and identify approaches of further development

Fruit trees regularly set excessive numbers of fruit in relation to the tree size and leaf area. This results in small low value fruit which often has a poor storage potential or is unmarketable and ends up going to waste. Fruit thinning is used to help prevent the trees from producing excessive numbers of small fruit. Thinning the trees involves the removal of the small and/or damaged fruit from the tree and allows the remaining fruit to reach a greater size and be retained by the tree until harvest.

Approaches to fruit thinning vary from orchard to orchard, year to year based on cropping history, orchard system and personal choice with combinations of mechanical, chemical, hand thinning at flower or fruitlet stages along with removal of the sites of floral bud development during winter pruning. All approaches have their advantages and disadvantages. Mechanical thinning is relatively quick and cheap but is very unselective, relies on a specific orchard system such as the fruit wall and concerns have been raised about effects on overall tree health and fruit quality by damaging blossoms. Chemical thinners are widely used, again they are a relatively quick and cheap approach, but efficacy relies heavily on timing and climatic conditions, which in the UK are not consistently achieved at the critical time. Equally, longer term effects on tree health and fruit quality are unknown. Hand thinning is the most precise and ‘gentle’ option for tree fruit thinning. However, it is time consuming and therefore expensive.

Fruit thinning makes the remaining fruit easier and cheaper to pick when it comes to harvest, as well as the fruit being easier and cheaper to grade. Recent research has even shown that fruits from thinned trees also have a higher concentration of polyphenols benefitting human health and nutrition. Understanding and optimising current approaches and finding effective inexpensive novel approaches is of vital importance for top fruit production. Understanding also how different and new approaches may affect other aspects of fruit quality (other than size and yield) such as dry matter, fruit firmness, and storability could also present very real benefits in terms of storing varieties such as Gala for longer. As the area of Gala continues to rise in the UK an extended marketing period is very desirable for cost effective marketing of an increasing tonnage of fruit.
From discussions with the industry and the literature it appears that the side effects of the different thinning approaches are less well understood and need further investigation.

There is a need to look back at basic physiological tree fruit research as well as forwards to leading overseas research on tree fruit thinning, to review approaches, understand further how these methods affect trees and fruit more generally and identify new strategies or combinations of approaches which can be trailed for UK orchards. This needs to be to reviewed and collated into one document to inform the HDC Tree Fruit Panel where to direct future research.