By Megan Dewdney
Spring is around the corner, and Florida’s first wave of bloom is here. But there is no time to relax and enjoy the flowers because citrus diseases wait for no grower.
Young fruitlets become susceptible to canker toward the end of March to the beginning of April. Missing that key window of protection — if the weather turns to rain — leads to significant fruit drop. The fruit lesions that form in the early spring often become very large, stimulating the early production of ethylene that leads to fruit drop. If there is not enough fruit protection, early-season canker can take off with frequent rainfall. Ninety to 100% crop loss has been observed.
Susceptible cultivars include grapefruit, Hamlin and other early sweet oranges, navels and lemons. Susceptibility in new cultivars is not well documented, but severe outbreaks have been observed on OLL-8.
Copper is still the mainstay of a canker program for fruit. Foliar applications at a 21-day interval still give the best canker protection, particularly on mature trees. The 21-day interval maintains a protective layer, replenishing the copper layer that slowly degrades from rainfall and fruit. Breakdown of protection is seen when the intervals are stretched to 28 days.
For the less susceptible Valencia fruit, three applications in the spring should be sufficient to protect the fruit from drop. Hamlin, being more sensitive, will need applications until late June to the end of July. If fresh market fruit are a concern where any blemish is a defect, consider applications throughout the rainy season, often into October. When considering canker management alone, 0.5 pound per acre is usually adequate. However, if a grower is also managing fungal pathogens with the same application, more will be needed.
Reducing leaf lesions in new plantings is important to avoid canker-induced defoliation and to minimize inoculum for future canker. Copper is not an effective treatment for leaf lesions because of the frequent new growth and rapid expansion that outgrows the copper coverage. 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 is a good time to protect the spring flush.
For more information about canker management, see the 2020-2021 Florida Citrus Production Guide.
Megan Dewdney is an associate professor of plant pathology at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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