Improved management of avocado diseases (AV04001)

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Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.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 avocado industry continued to search for methods to improve the way they manage diseases in their orchards. This project provided some clear guidelines to manage several common avocado diseases.

The research conducted had shown that clonal rootstocks were superior to seedling rootstocks in their tolerance to Phytophthora. The researcher made substantial progress in evaluating a range of new and traditional avocado rootstock varieties for their tolerance to Phytophthora root rot, already showing that clonal rootstocks tolerated root rot far better than seedling rootstocks. Considering their uniform growth, which allowed for easier orchard management, this made clonal rootstocks very attractive. Avocado growers were keenly anticipating these results. Ultimately, this program provided data to help a grower decide on the best type of rootstocks to use when planting a new orchard.

The devastating pathogen, Phytophthora cinnamomi, continued to cause root rot which reduced tree health and eventually killed  the tree. If an avocado grower had soil infested with this pathogen, it was essential that they used an integrated approach to controlling the disease. Although these approaches could be effective, it was still possible to reduce the need to rely on chemicals.

The researcher showed that a single injection of phosphonate in late autumn provided adequate levels of phosphonate in the roots of the avocado tree to maintain tree health in the presence of P. cinnamomi in the soil. This was great news for growers, who often found injecting twice yearly time consuming, costly and, ultimately, damaging to the trunks of their trees. Another positive outcome of our phosphonate studies was the prospect of being able to spray phosphonate onto the trunks of trees with the assistance of a bark translocating agent. In the researcher's studies, this method still achieved adequate levels of phosphonate in the roots.

There were also ongoing problems with fruit quality in the 'Hass' variety due to anthracnose, pepper spot and stem-end rot. These diseases were caused by microbial pathogens, the most important of which was the fungus, Colletotrichum gloeosporioides. This pathogen infected the fruit on the tree throughout the season from fruit set to harvest. However, the sunken, black lesions which rendered the fruit unmarketable only appeared once the fruit started to ripen on the shelf. This led to fruit being wasted in the marketplace and had the pontential to make consumers think twice before buying an avocado.

It appeared that a new fungicide treatment, Cabrio®, had the potential to be added to the line-up of control measures against the insidious disease, anthracnose. With some chemicals falling out of favour and the increased desire for less chemical use, adding a more effective treatment was desirable. Applying silicon, as was commonly done to boost disease defences, gave inconsistent results in the researcher's trials. The relationship between the fungus and the avocado tree was extremely complex and so silicon applications could not be recommended.