Reduced irrigation in high rainfall years and winter application of nitrogen reduce granulation in Imperial mandarin (Citrus reticulata cv. Imperial)

Abstract

Context: Preharvest granulation of Imperial mandarins is a significant problem for the Australian market. Causes of, and solutions for, this physiological disorder are poorly understood despite decades of research worldwide. Aims: This research aimed to find management practices for growers to reduce granulation. Methods: A 5-year on-farm trial in central Queensland, Australia, compared standard versus deficit irrigation and five rates of winter nitrogen application. Key results: Reducing water (irrigation plus rainfall) in the 16-18 weeks following flowering reduced granulation in 3 of 5 years. Granulation increases with ratio of total water received to evapotranspiration, particularly in low crop load years. Higher nitrogen applications reduced granulation in 4 of 5 years, although treatment means were only significantly different at α = 0.05 in 1 year. Granulation increased with stronger early spring flush growth in a low crop load year and with later spring flush growth in one of two high crop load years. The deficit irrigation treatment had less spring flush growth and higher fruit set than the control in all years. Higher nitrogen treatments had more flush growth in high crop load years and less in low crop load years. Our data suggests competition between flush growth and fruit development for mineral resources and/or carbohydrates is a factor in the variability of granulation from fruit to fruit, but crop load is more important. Conclusions: The three key strategies to minimise granulation are to maintain high crop loads, reduce irrigation after flowering, and apply sufficient nitrogen in winter. Implications: This research will improve fruit quality for the consumer and financial returns to growers.