Reduce Fruit Drop From Citrus Canker

By Megan Dewdney

Citrus canker can contribute significantly to the loss of fruit prior to harvest. As bloom begins in Florida, it is time to finalize management plans for foliar diseases. For many growers, very aggressive canker management programs are no longer a possibility, however, some management is important.

Citrus canker susceptibility of young fruitlets usually starts at the end of March or the beginning of April. The exact timing is dependent on fruitlet size in a particular year. Oranges are most susceptible when the fruitlets are between 0.25 to 1.25 inches in diameter. Grapefruit are most at risk when fruit are 0.5 to 1.5 inches.

Prioritize early-season canker management when planning a limited number of applications. The lesions on fruit from early-season infections tend to grow large. These lesions stimulate ethylene production which loosens the fruit, leading to drop.

It is uncertain if Florida will have the warm wet weather that canker loves when the fruit are vulnerable. But it only takes one ill-timed wind and rain event to cause significant losses. Florida is expected to have a drier than average spring but don’t ignore forecasted rain events. Be prepared, particularly if you have highly susceptible cultivars.

Early oranges are more susceptible in Florida than late-season oranges. Navels and lemons are also susceptible. Grapefruit are extremely susceptible and can serve as inoculum sources if not well managed. There is less information on some of the newer cultivars, but OLL-8 can be badly affected.

Copper is still the most effective and economical canker management product, particularly for suppressing fruit lesions. Unless you are using the Citrus Copper Application Scheduler, the 21-day interval is the best balance between enough copper to be an effective barrier and intervals between applications. Copper does not redistribute on the plant surfaces, so it needs replenishment as it decays from rainfall and fruit growth. Stretching intervals to 28 days leads to inadequate protection and increased disease incidence and severity.

Depending on the application timing, copper can help manage other foliar diseases like melanose and greasy spot. Copper rates as low as 0.5 pounds of metallic copper/acre can be used, but if planning to manage fungal diseases with the same application, higher rates will be required.

In many Florida Valencia blocks, three springtime applications should be adequate to keep the fruit from dropping. On the more susceptible early oranges, applications are recommended until late June or early July. If you plan to market fresh fruit, applications may be required until October to keep the fruit blemish-free.

Particularly in fresh fruit production, keep leaf lesions to a minimum. They provide inoculum that affects fruit. Copper is not an effective treatment for leaf lesions because of the frequent new growth and rapid expansion that outgrows the coverage. However, acibenzolar-S-methyl (Actigard) reduces leaf lesions by activating the systemic acquired resistance in plants. A second product, Aura Citrus, is also a good alternative for foliar canker.

For more information about citrus canker management, see the 2024–25 Florida Citrus Production Guide.

Megan Dewdney is an associate professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred.

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