Control of fruit set in apple by ATS requires accurate timing of ATS application

The efficacy of inhibition of fruit set in apple by spraying ammonium thiosulphate (ATS) during bloom depended on the concentration and time of application after flower bud opening. Established and 2-year old nursery 'Elstar' trees planted and flowering at different times during spring were used to study the relation between weather conditions and the effect ATS on fruit set. Almost 100% inhibition of fruit set was obtained when flowers were exposed to ATS shortly before pollination. After pollination the efficacy of fruit set inhibition declined linearly with the time between pollination and ATS application. The time for ATS to affect fruit set was shorter at higher temperature and corresponded to the calculated pollen tube at different temperatures. At a calculated 50% growth of the pollen tube of the distance between the stigma and the ovary of the flower, the efficacy of inhibition of fertilization decreased by about 50%. At 80% pollen tube growth or more, ATS application no longer inhibited fruit set. The time to reach 100% pollen tube according to the pollen tube model used and the local temperature conditions during the trials varied between 95 and 49 hours at average daily temperature of 13.5 and 21.2°C during the first and last experiment between early May and the end of June, respectively. Efficacy of fruit set inhibition was similar for a 15 kg ha-1 ATS applied in 200 or 400 L ha-1. Addition of 1% propylene glycol to the ATS only increased the inhibition if severe phytotoxicity to the leaves of the tree occurred. All fruits developing after ATS application contained on average 4 to 5 seeds while the few fruits from non-pollinated control flowers contained 0.5 seeds fruit-1 only. The results of this study show that to inhibit fruit set ATS needs to be applied before pollen tube growth has passed 50% of its distance towards the ovary. The pollen tube growth model and the temperature in the orchard can be used to calculated the course of pollen tube growth and to determine the best time for the first ATS application. At normal spring temperatures in The Netherlands this requires a first application within 48 h after the time the king flowers of sufficient flowers clusters, needed to obtain the target fruit load of the trees, have opened and had the chance to be pollinated.