Expanding citrus market access using a systems approach to control black spot (CT03005)

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note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.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

Citrus fruit grown in certain areas of Australia (e.g. Central Burnett, Qld) could be externally blemished by a fungal disease called ‘citrus black spot’ (CBS). Whilst the effects of CBS were typically only cosmetic, the occurrence of the fungus in these areas prevented the export of fruit to certain destinations where the disease was not known to occur; such as parts of the United States and New Zealand. Whilst CBS was under successful routine management in the Central Burnett, the potential to more intensively and sustainably control the disease to standards acceptable to foreign quarantine agencies was explored by this project to facilitate market access negotiations.

The control of CBS at the time relied on the application of fungicides to developing fruit to prevent infection by the fungus. However, past researchers generated evidence for improved CBS control using cultural practices such as the application of mulch over the leaf litter from which the fungal spores were liberated, and regular tree pruning to promote tree health, improve fungicide effectiveness and reduce the favourability of the canopy microclimate to fungal growth. Based on this evidence a CBS management system was devised and trialled under commercial conditions in the Central Burnett district to demonstrate the low levels of CBS that could be achieved by integrating cultural and improved chemical practices.

The commercial scale trials conducted during this project demonstrated that more than 99.7 per cent of premium graded fruit harvested from blocks managed by a combination of improved fungicide practices, application of mulch over leaf litter, and annual selective hand pruning, were free of infective CBS lesions. In addition to improved CBS control, mulch application and annual pruning were expected to improve control of other citrus diseases and pests, soil health and water use efficiency.