The end of March to the beginning of April is the time when young citrus fruitlets really start to be susceptible to citrus canker; it’s made worse by a warm, rainy spring as the bacteria that causes it thrives in such weather. While drier weather is in the forecast for Florida, experts say it does not take a lot for the bacterium to spread. It’s recommended that citrus growers get their timing for canker management right, especially if they are having to rely on limited applications. Citrus canker can cause significant fruit drop before harvest, so it’s important to have some applications rather than none.
The Importance of Timing Applications
The exact timing of citrus canker applications will change every year because the timing is dependent on fruitlet size. Different types of citrus and varieties all have susceptibility at different times. Research by UF/IFAS shows that oranges are the most susceptible to citrus canker when their fruitlets measure between 0.25 to 1.25 inches; however, grapefruit are most susceptible when their fruitlets measure between 0.5 to 1.5 inches.
If a citrus grower can only commit to a limited number of citrus canker applications, then they should prioritize early-season canker applications over applications that are later in the season. Citrus lesions that are caused by canker early in the season have time to grow larger than those later in the season; these lesions cause the fruit to produce ethylene, which causes fruit to drop.
Research has also shown that early oranges are also more susceptible to canker than oranges that are late-season. Similarly, navels, lemons, and grapefruit are also susceptible, with grapefruit being extremely susceptible.
Treatment Options for Citrus Canker
Researchers at UF/IFAS maintain that the most effective and economical canker management product, including being able to reduce fruit lesions, is copper. However, copper does decay from rainfall and fruit growth, so it does need to be reapplied. Experts recommend using either AgrocClimate’s Citrus Copper Application Scheduler, found at http://agroclimate.org/tools/citrus-copper-application-scheduler/, or a 21-day interval between copper applications.
Unfortunately, a 28-day interval does not allow for an effective copper barrier between applications; this inadequate protection can increase canker infection and the severity of the infection. Leaf lesions need to be treated with acibenzolar-S-methyl (Actigard) or Aura Citrus.
In terms of treatment length, it’s recommended that Valencias get three applications at minimum in the spring to be protected. However, for early oranges that are more susceptible to canker, it is advised that copper applications last until late June or early July. Citrus that will be marketed as fresh fruit will need applications until October.