Solving the problem of alternate bearing in ‘Elstar’ apple with thinning agents Brevis^® (metamitron) and Agro N Fluid^® (ammonium thiosulphate)

Apple cultivar ‘Elstar’ is very susceptible to alternate bearing. The flower cluster formation is strongly affected by the number of fruits tree^‑1 in the previous year. That is, a too high crop load results in less flower clusters and consequently low crop load in the following year, which should be avoided. Besides problems with fruit size and fruit quality, this is the main reason why ‘Elstar’ is thinned in commercial orchards. Hand thinning can reduce the number of fruits tree^‑1 but is not cost effective and usually carried out too late to positively affect flower cluster formation for the following year. In this trial, early fruit thinning was tested, i.e., i) by 2 applications of ammonium thiosulphate (ATS, Agro N Fluid^®) during flowering, ii) by applying metamitron (Brevis^®) at fruit size 14 mm, iii) by a double application of metamitron at fruit size 8-10 mm and 14 mm, and iv) by a combination of ATS during flowering and metamitron at fruit size 14 mm. The thinning efficacy was comparable for either two applications of ATS or one application of metamitron. The use of ATS, however, improved return bloom compared to both the untreated control and one application of metamitron.