Rapid Apple Decline research updates
Griffiths, J., Ellouze, W. 2021. Rapid Apple Decline research updates. Ontario Apple Growers Annual Report.
Despite setbacks due to the COVID-19 pandemic, multiple pathogens and biotic issues associated with Sudden Apple Decline (SAD) have been identified in the final year of the project. Diaporthe eres, Diplodia seriata, Cytospora leucostoma and other less commonly isolated species were identified in diseased trees. When inoculated into healthy trees, all these fungi can produce apple decline-like symptoms including branch die off and cankers on branches and trunks. D. eres was isolated in Nova Scotia, Ontario and British Columbia apple orchards exhibiting SAD. However, D. seriata and C. leucostoma were isolated only in Ontario and British Columbia. These fungal pathogens likely contribute to tree death and decline.
In Ontario, multiple viruses have been identified in Ontario which include Apple stem pitting virus, Apple luteovirus 1, and Apple rubbery wood virus 1 and 2. Additional viruses have been identified, but none seem to be specifically associated with tree decline. A novel species of virus infecting apples has been putatively identified in BC; however, further research is being conducted. In order to understand the pathology of this relatively novel virus, an infectious clone of Apple luteovirus 1 has been developed.
Ambrosia beetle populations were monitored in 2019 and 2021. The most abundant species appears to be black stem borer, an invasive species that can attack healthy trees. The abundance and pathogenicity of all pathogens identified in this study vary with year and location.
While multiple pathogens associated with SAD have been identified, none seem to be present in all instances of the disease. These same pathogens have also been isolated from symptom free trees. Furthermore, field and greenhouse based trials conducted in ON and BC show no correlation between herbicide applications and presence of SAD symptoms. Since there are no common pathogens in all cases of SAD, it is not possible at this point in time to identify a singular causative agent. However, detailed examinations of growing conditions and tree physiology indicate that decline is often associated with drought stress conditions and/or dramatic changes in weather and climatic conditions. In summary, multiple field observations and focused plant pathology research studies have contributed to the consensus opinion that there are multiple and possibly interacting factors that contribute to tree decline problems and the contributing factors may vary with year and location. Future research should focus in identifying the combinations of factors that are closely associated with decline within a growing season.