Nerida Donovan and Steven Falivene, NSW Department of Primary Industries
Copper sprays are an important tool in the battle against a number of fungal diseases that attack citrus fruit and foliage. Examples include citrus scab in high rainfall areas, Septoria spot in drier inland regions and Phytophthora brown rot, which can strike whenever conditions are favourable. Copper can also be used to manage anthracnose. The anthracnose fungus is very common and can be a secondary invader of fruit damaged by other factors in the field, particularly in wet seasons. Anthracnose symptoms can also be seen postharvest on fruit that are over mature or held too long in storage.
Copper sprays should be applied before infection occurs as it is a protectant fungicide, which only protects where it lands. As the fruit and foliage grow, the new tissue is not protected, and wind and rain erode the copper over time. Timing re-application depends on the diseases you want to manage, the copper formulation used and weather conditions. More frequent applications are needed during wet seasons, in orchards with overhead irrigation, or in higher rainfall regions.
For low rainfall regions such as the Riverina and Sunraysia, one copper spray in autumn is generally sufficient. In recent years, an increase in fruit blemish has been seen in the Riverina where weakened rind tissue is invaded by the anthracnose fungus. Wet seasons and foggy mornings have exacerbated these issues. Varieties that hang on the tree for longer (i.e. late navels and Valencia) have a greater chance of exposure to stress events such as frost. Follow up sprays might be needed to protect the fruit.
Copper sprays should be used in conjunction with other management practices to reduce disease problems in the orchard. Fungal spores linger in the tree canopy in dead wood so pruning to remove the dead wood reduces the number of fungal spores in the canopy, thereby reducing disease incidence when conditions are favourable for disease development.
For more information on copper sprays, please refer to the NSW DPI Primefact no. 757 Using copper sprays to control diseases in citrus.