Effects of Irrigation Stress and Crop Load On Yield and Quality of Ambrosia/M.9 Apple Trees

Irrigation water supply in the western Cordillera of North America is increasingly unreliable, through changes in mountain snow-packs and competition for water resources.  Improved early season drought forecasting provides the opportunity to manage reduced water allocations.  Ambrosia/M.9 trees planted at 1 x 3 m spacing received daily drip irrigation through two 4-L/hr drippers per tree, automatically scheduled using an electronic atmometer. Trees were fertigated daily after full bloom for six weeks to supply 75 g/tree of N; in a single application one day after full bloom to supply 20 g/tree of P and daily from three to six weeks after full bloom to supply 20 g/tree of K. Irrigation treatments included: 1) twice daily at 100% ET replacement; 2) twice daily at 50% ET replacement; 3) twice daily at 50% ET replacement one side of the tree; 4) 50% (2007), 25% (2008), and 18% (2009) ET replacement every two days.  Crop was adjusted to a low, medium and high rate of fruit/cm² trunk cross-sectional area (TCA), with crop load increasing over time (2.5, 4.5, 9 fruit/cm² TCA in 2007; 3, 6, 12 fruit/cm² TCA in 2008; 3.75, 7.5, 15 fruit/cm² TCA in 2009).  Efficiencies associated with drip irrigation provided a 30% water savings in addition to applied deficits.  Stem water potential was significantly lower at higher crop loads in all years, and under deficit irrigation in 2008 and 2009.  Fruit growth over the season was reduced by increased crop load and severe irrigation deficits and was more limited by inter-fruit competition for resources than by water deficits or deceases in stem water potential.  Consequently crop load effects on yield and fruit size were greater than deficit irrigation effects.  In general higher crop load also advanced harvest maturity (starch index) and decreased fruit firmness, titratable acidity and soluble solids content.  Severe irrigation deficits advanced harvest maturity but with increased fruit firmness, titratable acidity and soluble solids content. Using a target average fruit size of 200g, optimum crop loads were determined for each water deficit and ranged from 8 fruit/cm ² TCA at no deficit (all years) to 4 fruit/cm² TCA for 25% ET replacement in 2008.  No optimum crop load could be determined at 18% ET replacement (2009).  Cumulative effects of three years of water deficit were evident by 2009, where at 50% ET replacement optimum crop load had declined to 5 fruit/cm² TCA from 6.5–7 fruit/cm² TCA (2007–2008).