This is a final research report from Hort Innovation’s historical archives. Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.

What was it all about?

All avocado producing regions encountered alternate bearing, with orchards producing variations in yields from one year to the next. In high cropping years prices to growers were low while in light cropping years marketers found it difficult to supply key markets. Environmental triggers such as frost or drought initiated these ON/OFF cropping cycles, while internal physiological mechanisms were responsible for their continuation. Research in Australia and New Zealand by Plant & Food Research aimed to understand better these mechanisms in order to provide solutions for growers wanting to produce consistently high yields.

Results from this study have demonstrated a close relationship between the supply of 7-carbon carbohydrates and boron to avocado flowers, with both nutrients being transported in the phloem. A shortage of 7-carbon carbohydrates led to a shortage of boron in flowers and thus potentially poor fruit set. High crop loads competed with flowers for the available carbohydrates and boron and thus made the situation worse in ON cropping years. More careful timing of boron applications to match the requirements of growing fruit and canopy management to avoid over-cropping could therefore have been important components in strategies to mitigate alternate bearing.

Hand pollination increased fruit set fivefold, indicating that natural pollination systems may limit fruit set. It was important that insects visit all flowers and deposit sufficient quantities of pollen. This did not always happen. At one orchard, only 82 per cent of flowers were visited by insects (honey bees) and of these only 32 per cent resulted in pollen transfer. In a second orchard 100 per cent of flowers had multiple visits by multiple insects but most pollen transfer occurred in the afternoon when polliniser trees were releasing pollen. These contrasting situations suggest that more hives are required at the first orchard whereas better overlap between female and male flowers is needed in the second orchard.

But even with hand pollination and a high number of pollen grains deposited on flowers this did not guarantee fruit set. Future research will continued to consider flower quality (carbohydrate and boron composition) and pollen viability as reasons for poor fruit set.