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Understanding the ecology of blueberry rust to improve management in evergreen blueberry production in Australia

Rosalie Daniel: NSW Department of Primary Industries

<div>More than 80% of blueberries in Australia are grown in subtropical conditions under an evergreen cropping system. New leaves, susceptible to infection by the fungus <em>Thekopsora minima, </em>the cause of blueberry rust, develop at any time of the year. Severe symptoms may develop and cause defoliation and fruit lesions. Producers currently adhere to strict management guidelines based on calendar sprays of fungicides in the absence of a complete understanding of the disease cycle. Our aim was to identify vulnerable points in the disease cycle that enable targeted, and more effective, application of fungicides. Air sampling, field and greenhouse studies were conducted to determine factors contributing to disease development. In the evergreen system, <em>T. minima</em> survives primarily in infected, symptomatic as well as asymptomatic, leaves retained on the plant. Viability of urediniospores in leaf litter declines to 25% after 2 weeks. Air sampling has shown that urediniospores are present at any time of the year and are the main source of new infections. Preliminary analyses indicate that spore numbers are greater in humid and wet conditions. The effect of leaf wetness on spore germination and infection needs to be investigated further. We conclude that because spores and new foliage can be present all year round in the evergreen system, fungicide sprays should be targeted when weather conditions are mild and moist.</div>