Developments in the chemical thinning of apple and pear

A literature review is presented on past and present experiences with chemical flower and fruit thinning of apple and pear, amplified with some data of recent trials with known and new flower thinners (mainly ethephon and ammonium thiosulphate) and fruit thinners (mainly 1-napthylacetic acid (NAA), its amide (NAAm), carbaryl, ethephon, conjugates of NAA and NAAm and benzyladenine). Chemical-thinning practices give quite unpredictable results. This inconsistency is at least partly caused by weather factors, such as temperature and air humidity, but tree factors are also involved. To solve this problem, climate-room and multi-site trials are proposed, together with an intergrated approach in elucidating background mechanisms and the development of new fruit thinning compounds.