Developments in the chemical thinning of apple and pear

A literature review is presented on past and present experiences with chemi cal flower and fruit thinning of apple and pear, amplified with some data o f recent trials with known and new flower thinners (mainly ethephon and amm onium thiosulphate) and fruit thinners (mainly 1-napthylacetic acid (NAA), its amide (NAAm), carbaryl, ethephon, conjugates of NAA and NAAm and benzyl adenine). Chemical-thinning practices give quite unpredictable results. Thi s inconsistency is at least partly caused by weather factors, such as tempe rature 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 developme nt of new fruit thinning compounds.