Effects of calcium, boron and dwarfing inter-stock on fruit quality of custard apple (Annona spp. Hybrid) cv. African Pride

The quality of custard apple (Annona spp. hybrid) fruit is a major factor limiting expansion of this industry in Australia. Several internal disorders occur in custard apple; especially common is ‘woodiness’ which is characterised by the presence of woody seed pockets and gritty lumps in the flesh and `brown pulp’, which is a discolouration of the pulp. Surveys and experiments were conducted to evaluate the effects of soil-applied Ca and B on fruit quality of the custard apple cv. African Pride in subtropical Australia. Dwarfing inter-stocks and higher fruit Ca concentrations were associated with a 2- to 3-fold reduction in internal disorders. Fruit Ca concentrations may be improved through repeated soil applications of Ca but uptake and response may take several years to achieve. Fruit Ca concentrations need to be maintained at > 0.15% dry weight and leaf Ca > 1.6% dry weight. The current leaf nutrient standards of adequacy for Ca appear to be set too low. Our data indicate that in the early stages of development, fruit compete poorly against leaves and shoots for available Ca, especially when Ca concentrations in the xylem are low and transpiration high. Leaves that readily transpire accumulate more Ca. In the long term, dwarfing inter-stocks of sugar apple (Annona squamosa) or dwarfing rootstocks of cherimoya (Annona cherimola) may be the most efficient method of controlling ‘woodiness’ and improving fruit quality.