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Effects of applying less water by partial root zone drying versus conventional irrigation on navel orange yield

Faber, B. A., Lovatt, C. J.
Acta horticulturae 2014 no.1038 pp. 523-530
Phytophthora, drying, fruit growing, fruit trees, fruit yield, grapes, income, irrigation rates, irrigation water, markets, nutrient use efficiency, raw fruit, root rot, root zone drying, soil water, vines, California
California produces "picture perfect" navel orange fruit for the fresh fruit market on 50,339 irrigated ha. Irrigation water is a major expense. Partial root zone drying (PRD), the practice of alternately irrigating and drying the root zone on each side of the tree, was developed for year-round use to reduce water use in vine and tree fruit production. Prior California research suggested that PRD increased nutrient-use efficiency and reduced Phytophthora root-rot. For grape, benefits obtained with reduced water use were the same for PRD and conventional irrigation (CI), raising the question of whether PRD was critical to achieving water savings and yield benefits in citrus production. Our research objective was to compare effects on total yield, fruit size and quality of reducing annual water use in a commercial navel orange orchard using PRD versus CI at 25% and 40% less water (CI-RR) than the well-watered control. In Year 1, control trees were irrigated when soil moisture reached -30 cb at 30 cm; PRD and CI-RR trees were watered when soil moisture reached -60 cb. All reduced irrigation treatments reduced yield and fruit size. In Year 2, the lowest irrigation rate was 50% less water than the well-watered control, but all trees were irrigated when control tree soil moisture reached -30 cb at 30 cm. All PRD and CI-RR treatments reduced total fruit number and number of commercially valuable fruit (diameter 6.9-8.8 cm) per tree, except CI-RR-25% trees, which produced more marketable small fruit than control trees. Grower income was markedly reduced by the loss of valuable large fruit when irrigation was reduced and was not offset by the value of water saved.