What was it all about?

This investment investigated the causes, mechanisms and potential management strategies for red leaf disorder in strawberries. Red leaf disorder is characterised by reddish discolouration of plant leaves, with affected plants typically displaying reduced vigour and yield. The disorder limits the fruiting productivity and plant health of major cultivars grown in Queensland.

The research identified potential causal agents and developed a clearer understanding of the spread and economic impact red leaf disorder has on the commercial Queensland strawberry industry.

A comprehensive approach was taken to explore the possible causes of red leaf disorder. These included biological indexing, transmission electron microscopy, fungal isolations, molecular, microbiological and microscopy studies to further explore or eliminate possible causal agents. A farm survey and yield trial also examined the impacts of this disorder on the strawberry industry.

Findings from the research included:

• Eight farms (one a substrate farm) in south-east Queensland were surveyed once a month during the 2020 season for red leaf disorder. All had the disorder present, with observations of the disorder in both runners and plug plants. A low percentage of cv. Red Rhapsody plants in substrate showed symptoms only on water-stressed plants, with symptoms diminishing when water supply issues were corrected. This suggests that stress may play a major role in occurrence of the disorder.
• A yield comparative study of 45 plants of cv. Red Rhapsody was undertaken to determine the effects of red leaf disorder on fruit quality and quantity as well as overall plant health. Plants were examined weekly and rated for disorder severity, fruit weight, fruit numbers and quality. All plants showed some degree of reddening, with red leaf disorder severity increasing progressively through the season. The number, weight and quality of Class 1 (1st grade) fruit decreased as severity increased.
• Biological tests by grafting suspect plants to indicator plants (genotypes free from red leaf disorder but susceptible) were used to determine if the disorder was transmittable via vascular tissue transfer. The results showed no symptom transmission.
• Pathology testing of plants with and without red leaf disorder symptoms showed inconclusive results to support the notion that a fungal pathogen maybe the causal agent.
• Plants with symptoms of red leaf disorder and comparative asymptomatic controls were analysed, with a range of virus particles or virus-like particles observed in the strawberry leaf samples
• A comprehensive and controlled suite of molecular investigations into causal bacterial, fungal or viral agents were made to compare plants with various stages of red leaf disorder symptoms to controls without the disorder. This did not identify any clear pathogens specific to the infected plants, however several organisms, particularly Phytoplasma, were present in all samples. This may indicate these species, along with viruses they contain, contribute to red leaf disorder in the presence of other biotic or abiotic constraints.

This research study (Phase 1) has delivered data and useful information for future research to build upon existing foundations. The project updated stakeholders and delivered project results via industry channels, farm visits, industry meetings, articles, and presentations.

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Read these articles published about the project in Australian Berry Journal:

• Developing knowledge and management of strawberry red leaf disorder, Summer 2020 edition, pp77-78
• Red leaf disorder 2020 farm survey update, Winter 2021 edition, pp76-79