By Evan Johnson
Now that bloom has arrived, it is time to start thinking about canker management. As the fruitlets develop, they will become susceptible around the end of March to the beginning of April. The spring fruit lesions are particularly concerning because they cause fruit drop. Without proper management, early season canker can cause 90 to 100% crop loss with favorable spring weather (frequent rain).
Susceptible varieties include grapefruit, Hamlin and other early sweet oranges, navels and lemons. Susceptibility in new varieties is not well documented, but severe outbreaks have been observed on OLL-8.
Foliar applications of copper provide the best management on bearing trees with the main aim of protecting fruit. Copper sprays on a 21-day interval provide the best protection currently available. The 21-day interval maintains a protective layer, replenishing the copper layer that slowly degrades from rainfall and fruit.
For Valencia fruit, three sprays in spring should be sufficient. Hamlins will need continued applications until late June or through July. For susceptible fresh market fruit where rind blemish is a concern, applications are recommended throughout the rainy period, often into October. For canker management, 0.5 pounds per acre is usually sufficient, but higher amounts will be needed when appropriate for fungal pathogens.
Reducing leaf lesions on new plantings is important to avoid canker-induced defoliation and to minimize future canker. Copper is not an effective treatment for leaf lesions because of the frequent new growth and rapid expansion. However, acibenzolar-S-methyl (Blockade) can reduce leaf lesions by activating the plants systemic acquired resistance. It needs to be applied before conditions are favorable for canker infection, so now would be a good time to protect the spring flush.
Evan Johnson is a research assistant scientist at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.