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Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.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?

The “Rootstock Improvement for the Australian Avocado Industry” project was developed to address a number of key issues important to the long-term sustainability of avocado production in Australia. Namely these were to identify: 1) if clonally propagated rootstocks were superior (higher yields and improved fruit quality) to seedling rootstocks which were historically used by the Australian industry; 2) do rootstocks from different horticultural races impact differently on crop performance when grafted to either ‘Hass’ or ‘Shepard’; 3) would rootstocks perform differently across the range of environments where avocados were grown in Australia; and 4) could significant Phytophthora root rot resistance be located in rootstocks used by the Australian avocado industry. To address these issues rootstock experiments using both cloned and seedling material grafted to ‘Hass’ and ‘Shepard’ were planted in each significant production region in the country and their agronomic and postharvest fruit performance monitored for 3-6 years. Additionally, rootstocks identified with potential Phytophthora root rot resistance were cloned and evaluated in soils providing intense disease pressure.

The funding of this research had an immediate impact on the Australian avocado industry through creating grower-awareness of the importance of the selection of rootstocks when establishing a new orchard. Empowerment through knowledge led to a significant improvement in rootstock choices from the nursery’s servicing the avocado industry.

From both agronomic and postharvest perspectives cloned rootstocks in general did not improve orchard or postharvest fruit performance with seedling rootstocks largely being equal to or in some cases better than their cloned genetic pair. Regarding productivity there was no single rootstock that had superiority across all production regions. However, the highest yielding rootstocks overwhelmingly came from the Guatemalan and West Indian horticultural races, e.g. ‘A8’, ‘Nabal’, ‘Plowman’, ‘Reed’, ‘SHSR-02’, ‘V1’ and ‘Velvick’. Hybrids with Mexican and Guatemalan race genes were in the second most successful group, e.g. ‘A10’, ‘Shepard’, ‘SHSR-03’ and ‘Zutano’, whilst Mexican race rootstocks were overall the least represented group in the high performance echelon, viz. ‘Barr Duke’, ‘Parida’ and ‘Thomas’. Although the influence of rootstocks on fruit quality varied according to growing location and year of assessment, a number of trends could be identified. In some experiments there were no significant effects of rootstock on fruit quality, but in other trials there were a number of differences. Some rootstocks (e.g. ‘A10’, ‘ SHSR-03’, and ‘Velvick’) frequently had a positive influence on ‘Hass’ and ‘Shepard’ fruit quality (i.e. reduced flesh disorders after storage and the incidence of postharvest rots) compared to others (e.g. ‘Barr Duke’ and ‘Duke 7’ in ‘Hass’, or ‘Thomas’ and ‘Duke 7’ in ‘Shepard’). Fruit with the highest postharvest quality often had the lowest N and highest Ca skin concentrations. This was supported by positive correlations between fruit quality and skin nutrient ratios of N/Ca, and negative correlations between fruit quality and skin nutrient ratios of Ca+Mg/K. These may have been a useful diagnostic tool for predicting postharvest fruit performance.

The project also developed a new rootstock (‘SHSR-04’) with high Phytophthora root rot resistance, although some strategic chemical support would still be required to maintain good tree health under high disease pressure with trees in heavy crop.