The effects of temperature, vapour pressure deficit and soil moisture stress on growth, flowering and fruit set of custard apple (Annona cherimola x Annona squamosa) 'African Pride'

The effects of temperature, vapour pressure deficit (VPD) and soil moisture stress on growth, flowering and fruit set of custard apple (Annona cherimola x Annona squamosa) cultivar 'African Pride' were evaluated under controlled-environment conditions. Flowering and fruit set were affected not only by the environmental conditions during flowering, but also by vegetative flushing of the tree. Flowering was closely associated with vegetative flushing. The level of natural fruit set under controlled-environment conditions, irrespective of temperature or humidity, was low (<3.1%) compared to field or container-grown trees flowering under ambient conditions (11 and 28%, respectively). The increased set under ambient conditions appears to be due to the presence of insect pollinators, in particular Nitidulid beetles which were observed in the flowers. High VPD (1.2 kPa) severely reduced both flowering and fruit set. Soil moisture stress (mid-day leaf water potential -2.0 MPa) also reduced flowering and fruit set, particularly at high temperature (28°C). Hand-pollination of flowers, even under high VPD conditions, resulted in a significantly higher set (>20%). Productivity of custard apple in the field may be improved by cultural techniques such as overhead misting, windbreaks and efficient irrigation scheduling.