Investigation of the distribution and incidence of Avocado sunblotch viroid in Australia (AV07001)

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note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.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

Sunblotch disease affected both the yield of avocado trees and also the quality of the fruit. Sunblotch disease was caused by the smallest pathogen known to science, Avocado sunblotch viroid (ASBVd). This pathogen was so small that it was invisible even under the most powerful microscope and could only be detected using biochemical methods. Fortunately, ASBVd was not very contagious and could easily be controlled by avocado growers using clean planting material. To address this need, the Avocado Nursery Voluntary Accreditation Scheme was initiated in 1978. Nurseries participating in ANVAS were obliged to have avocado nuclear stock tested for ASBVd on a regular basis.

Prior to the commencement of this project, there had been no official record of ASBVd in Australia since 1989. Questions therefore arose as to whether this pathogen still occurred in Australia and if so, how common was it and was it a significant constraint to production? To answer these questions, the researcher undertook broad-scale surveys in the subtropical production areas of northern NSW and south-eastern Queensland. Before these surveys could be undertaken, the researcher had to develop semi-automated testing procedures to allow large numbers of avocado leaves to be processed in a short period of time with minimum labour.

To maximize chances of finding ASBVd, older trees were targeted during the surveys. Over 3,000 trees were tested and only one infected tree discovered, a tree growing on Mt Tamborine. This tree was infected with a very mild strain of the pathogen. Despite comprehensive testing of other trees on this farm and elsewhere on Mt Tamborine, no other infections were discovered. It was likely that the tree was infected when it was first planted over twenty years ago and had remained undetected the whole time.

Finally, to allow survey data to be collated in a central location, a web-based database was created. Using the website, avocado growers were able to create confidential user accounts, fill out online diagnostic sample submission forms and have test results reported back to them as soon as they become available. There was potential to expand this web site for all diagnostic services.