The role of rootstocks and nutrition in the quality of Hass avocado (AV00013)

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Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.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 Australian avocado industry had a strong commitment to continually improving the quality of avocado fruit offered to the consumer. Research in the past these had largely concentrated on chemicals to control fruit rots, which was one of the major factors affecting quality. However, there was growing evidence that there were a number of non-chemical means of improving quality. These were associated with improving the “vitality” of the fruit so that it was better able to prevent rots and other disorders. In particular, fruit with more calcium (Ca) and less potassium (K), often had less rots and disorders. By finding ways of improving fruit nutrition, the researcher may have been able to improve fruit quality offered to the consumer and at the same time reduce the need for chemicals.

Fruit trees were often grafted onto different rootstocks which allowed the tree to produce more and better fruit. the researcher tested whether some to these avocado rootstocks could take up more Ca, which could then improve Ca concentrations in the fruit. the researcher found that the Velvick rootstock could have more Ca, and less K in the leaves than other avocado cultivars. This may have explained why Hass avocado fruit had less rots when it was grown on Velvick rootstocks. the researcher also noticed that the graft (the point where the Velvick rootstock joins to the Hass scion) in Hass on Velvick trees did not seem to restrict Ca from moving from the roots to the leaves, but there was a restriction in movement with some other rootstocks. Therefore, careful selection of the rootstock could improve fruit health and quality without the need for additional chemicals. The avocado industry was funding another project to find rootstocks that would produce better fruit quality, and the above results would help identify these better rootstocks more quickly.

The researcher knew from other research that the balance of nutrients in the soil could affect the uptake of Ca into the plant. the researcher confirmed that high applications of K could reduce Ca uptake into the branch sap and the leaves. Therefore, re-evaluating the amount of K required by the tree, and how this was applied, may have increased fruit Ca uptake.

The researcher also knew that fruit from higher yielding trees often had better fruit quality. This project showed that about 60 leaves per fruit gave good fruit size and fruit retention on the tree. The fruit Ca concentration was higher with less leaves per fruit, but the reduction in fruit size and the larger fruit loss with less leaves per fruit could reduce overall profitability.

The researcher also looked at the accumulation of minerals into the different fruit tissues during fruit growth. It was thought that this may have indicated ways of increasing the Ca concentration in the flesh and the skin, where rots developed. However, there was little evidence that accumulation in the different tissues could be manipulated to increase concentrations in the skin and flesh.

In summary, the researcher now had a better understanding of some of mechanisms by which rootstocks and soil treatments may have improved fruit nutrition and quality, and this information could be used to develop better rootstocks. The researcher also had a better idea of the number of leaves per fruit that gives a good balance between fruit yield and quality. These results helped the avocado industry produce better quality fruit and reduce its dependence on chemicals to improve fruit quality.