The Effects of Crop Load and Water Stress on Yield and Fruit Quality of Sweet Cherry Cultivars

Snow pack water supply in western North America is increasingly unreliable but improvements in drought forecasting could allow management using planned water deficits. Crop reduction is also a potential tool to manage the effects of water stress for sweet cherry cultivars. The effects of water supply and crop load management were assessed in two experiments. Experiment 1—‘Skeena’ and ‘Cristalina’ on Gisela 6 received 100% ET replacement atmometer-scheduled drip irrigation either 4 times/day or every second day in 2009–11. In 2011, crop load was either 100% or blossom cluster-thinned to 50%. Experiment 2—‘Lapins’ on Gisela 5 received daily, atmometer-scheduled, small-radius micro-sprinkler irrigation: 1) 100% ET replacement;  2) 100% ET replacement reduced to 57% ET replacement from 21 days preharvest; and 3) 100% ET replacement reduced to 57% ET replacement postharvest. In 2011, crop load was either 100% or blossom cluster-thinned to 50%. In 2012, crop load [fruit/cm² trunk cross-sectional area (TCSA)] effects were assessed using tree natural variability.  High frequency irrigation increased yield in ‘Skeena’ and ‘Cristalina’  in 2010–11 and fruit size in 2010.  Cluster-thinning in 2011 did not significantly affect yield, fruit size, or fruit quality for either cultivar.  Preharvest water deficits applied to ‘Lapins’ on Gisela 5 were mitigated by cool weather in 2011 and wet weather in 2012.  Midday stem potential of –1.5 to –1.6 MPa in preharvest deficit trees had no effect on yield or fruit quality in either year. Midday stem water potential reached –2MPa by the end of the season in deficit trees.  Cluster-thinning in Spring 2011 reduced yield but did not affect fruit size or fruit quality.  Cluster-thinning and preharvest water deficits in 2011 reduced crop load and increased fruit size in 2012.  In 2011 and 2012, crop load in all trees was compared to fruit quality. In both years there were strong inverse relationships between crop load and fruit size (R²=0.85 and 0.78) for unstressed trees, respectively. Soluble solids content, titratable acidity and stem pull force were also reduced as crop load increased. In 2012, preharvest deficits, resulted in significantly smaller fruit when adjusted for crop load as a covariate. Crop loads of 12 fruit/cm² TCSA and 22 fruit/cm² TCSA in unstressed trees and 10 fruit/cm² TCSA and 18 fruit/cm² TCSA in stressed trees resulted in average fruit sizes of 12 g and 10 g, respectively, indicating that crop load adjustment and planned deficits could be useful tools for coping with drought.