Three-year evaluation of irrigation strategies on apple yield and quality in Germany

Climate warming is leading to higher water use by fruit trees. Longer vegetation periods, higher evapotranspiration (ET) and extended droughts are the key factors. This study investigated, if supplementary irrigation of ‘Red Topaz’ ( Malus × domestica ) trees is required in South-West Germany. From 2023 to 2025 four irrigation strategies were applied: a digital sensor-based irrigation at a soil water tension threshold of 30 kPa, a web-based soil water balance irrigation model at an available water capacity threshold of 50%, a conventional interval irrigation with 15.5 mm applied twice per week, if no rainfall of this amount was predicted and a non-irrigated control. The precipitation ranged from 576 to 972 mm during the study year’s vegetation periods. Irrigation increased trunk growth by 14% for the model and interval irrigation or 9% for the sensor-based irrigation. The total mean yield of all three years increased by 5% for interval and 6% for the 30 kPa irrigation treatment. The model-based irrigation did not show clear effects due to high crop load variance in 2024. The marketable yield in 2025 increased by only 1% for the irrigation strategies due to oversized fruit. Sensor- and model-based irrigation strategies can increase water use efficiency when compared to interval irrigation. Future irrigation demand in Lake Constance orchards is likely to increase, primarily due to increasing ET driven by higher annual mean air temperatures (+1.5 °C, corresponding to +16%), longer vegetation periods (+9%), and increased growing degree days (+23%), with all changes calculated for the period from 2020 to 2050. • To 2050 increasing temperatures & vegetation periods will increase evapotranspiration. • All irrigation strategies increased trunk cross-sectional area compared to control. • All irrigation strategies increased total yield. • Non-irrigated trees produced a higher share of marketable fruit. • Model based irrigation optimized water usage efficiency.