What was it all about?

Alternaria leaf blotch and fruit spot diseases cause significant fruit losses in Queensland and New South Wales and occasional, but noteworthy, severe leaf losses have been observed in Western Australia and South Australia. These problems have been ongoing for several seasons, and while fungicide applications have lessened effects in some areas, a lack of understanding of the identity of the pathogen, timing of infection and disease cycle has hindered implementation of reliable disease management strategies.

This project set out to determine why these diseases of apple leaves and fruit are not being effectively controlled by fungicide applications.

Researchers used molecular techniques to reveal that multiple species are associated with leaf blotch and fruit spot on apple. Four species of Alternaria were identified: A. arborescens, A. tenuissima, A. alternata and A. longipes were identified.

Although the distribution of these four different Alternaria species varies among apple growing regions in Australia, A. arborescens was the most prevalent species in all regions, mostly on leaf blotch. The occurrence of multiple species in all regions suggests that an Alternaria species complex is responsible for leaf blotch and fruit spot of apple in Australia.

Irrespective of the origin of each isolate, whether from leaf or fruit symptoms, isolates of all four species were pathogenic to both apple leaf and fruit. This indicates that in regions where only leaf blotch but not fruit spot currently occur, the risk of fruit infection is high when conditions conducive to the disease prevail. This means that all apple growing regions, rather than only Qld and NSW, are at risk of severe yield losses.

The researchers carried out a series of field trials in commercial orchards which resulted in a better understanding of the lifecycle of the pathogen, the sources of inoculum and timing of infection for both diseases.

This information informed the development of a disease management strategy for the apple industry.

When implemented, it is estimated to save the Queensland apple industry over $2.25 million in lost production annually. In the short term it will save the Australian apple industry between 15-25 per cent in lost production of the high value varieties.

The long-term benefit of the research outcomes will lead to reduced reliance on chemicals for controlling the diseases. These findings of our research have been published in scientific journals and apple industry magazines and communicated to growers at several meetings. A PhD student was successfully trained in this project as well, boosting Australia’s biosecurity capacity in this area.