Why are my custard apple (sitaphal) trees giving small fruit and how do I improve set?

Small fruit and poor set in custard apple usually come from a mix of weak pollination, moisture stress at flowering and fruit development, poor nutrition, and growing low quality seedling types instead of improved big fruited varieties. Custard apple flowers are poorly pollinated by insects, so fruit set can be low, and in serious orchards hand pollination of the flowers in the early morning markedly improves both set and fruit size. Provide some irrigation during flowering and fruit growth even on dry land, since stress at this stage shrinks the fruit. Feed the trees with balanced nutrition and organic matter as per a local schedule. If your trees are small fruited wild types, no amount of care will match an improved variety, so for a serious orchard top work or replant with a large fruited variety. Confirm a pollination, irrigation and nutrition plan with your KVK, and treat hand pollination as the biggest single lever for fruit size.