Fruit Thinning Improves Fruit Size, Yield and Return Flowering in ‘Washington Navel’ Orange (Citrus sinensis L. Osbeck)

Abstract

ABSTRACT “Washington Navel” orange is the principal mid-season maturing cultivar grown in south-eastern Australia. The cultivar has strong export potential and earns high returns for citrus growers. However, trees bear a large number of small-sized fruit in an on-year followed by a small number of large-sized fruit the next year if crop load is not properly managed. Thinning can be used as a crop management strategy to adjust crop load. A thinning experiment was conducted, and crop loads were adjusted on a whole tree basis to 2, 4, 6 or 8 fruit/0.125 m3 canopy volume (CV), removing 46%, 30%, 20% and 8% of the crop respectively, and compared to non-thinned control trees. Fruit diameter was measured at regular intervals from February to July. Fruit yield and number were recorded for two consecutive seasons. Return bloom after thinning was also recorded. Thinning treatments 2, 4 and 6 fruit/0.125 m3 CV resulted in higher fruit growth rates of 24%, 9% and 14% compared to the control. Thinning to a crop load of 6 fruit/0.125 m3 CV produced the best results for fruit size and yield during the treatment year. The following year return bloom was increased in the 2, 4 and 6 fruit/0.125 m3 CV compared to the control. The 6 fruit/0.125 m3 CV treatment resulted in a higher cumulative yield of 169 kg/tree over two cropping seasons and 42% fruit in the size class > 75 mm diameter compared to the control. Economic analysis revealed that 6 fruit/0.125 m3 CV could generate an extra $4,240/ha of income for citrus growers. It was concluded that a fruit load of 6 fruit/0.125 m3 CV was required to produce high yields and large fruit size over two cropping seasons.