Progress in the development of partial rootzone drying of apple trees

Four, multi-year studies evaluating Partial Rootzone Drying (PRD) on apples (Malus domestica) with varieties 'Gala' and 'Fuji' were initiated in the spring of 2001 in the semi-arid climates of Colorado and Washington State. Irrigation systems included microsprinklers, surface and sub-surface drip lines. At each site, a PRD50 regime was compared to a well-watered Control and a season-long deficit regime (DI50). Both, PRD50 and DI50 were irrigated at 50% of the Control's volume. In contrast to PRD50 where only half the rootzone was watered the DI50 and Control treatments were irrigated over the full rootzone. At the Colorado sites, two further PRD treatments received 25% or 75% of the Control's volume on 25% or 75% of the surface area (PRD25 and PRD75, respectively). Here we present the main results achieved with PRD from four sites over two years. Seasonal potential evapotranspiration exceeded 1,000 mm and precipitation was less than 130 mm in both years at all sites. Seasonal irrigation inputs ranged from a low of 205 mm for PRD25 with drip irrigated `Gala' to a high of 765 mm for the Control with microsprinklers on 'Fuji'. There were no significant treatment effects on 'Gala' yield, fruit size and fruit quality at harvest and after cold storage in either year. With `Fuji', no significant effects on yield and fruit quality were found in the first year, but mean fruit size was reduced by PRD50 compared to Control at one site. In the second season, the DI50 treatment tended to reduce fruit size at both sites, and yield was significantly lower at one site. At one site, PRD50 and D150 fruit were firmer and had higher concentration of soluble solids at harvest and after being kept at room temperature for seven days. Fruit disorders (sunburn, russet, watercore) have not been affected by treatments. Our results indicate that PRD and DI50 allow large water savings with minor or no impacts on fruit size and yield while potentially improving fruit quality. Seasonal crop coefficients for apple might be reduced to 0.3 or less with new or modified irrigation techniques.