By Megan Dewdney
Editor’s note: This article originally stated that citrus black spot was in Hardee County. It should have said Hendry County, and has now been corrected.
Citrus growers in Florida continue to battle new pests and diseases. They have been learning to manage citrus black spot for the last 12 years. The disease is still localized mainly in Southwest Florida in Collier, Hendry, Lee, Charlotte, and Glades counties. Over this time, a slow northern movement of the disease has been observed.
In 2022, the affected areas fortunately have not greatly expanded, likely because of the dry spring and early summer conditions in 2021. Groves within the quarantine areas must follow the specific regulations concerning fruit movement and grove management. These rules and quarantine maps can be accessed via the Citrus Health Response Program website.
For groves near or in a quarantine area, the first step is to determine whether black spot is in the grove and to what extent. Often, the disease is at a low level and may not be readily seen. The most easily identifiable and diagnostic symptom is hard spot on ripe fruit. Hard spot lesions are small (3/16 to 1/8 inch), round and slightly depressed with brick red to dark chocolate brown edges. The margin color depends on lesion age, with the younger ones having the red color. The center is often tan with little pinpoint dots, which are the fungal structures. There are very few symptoms on leaves. Resources to help identify the disease are available here.
The disease is most frequently seen first on declining trees. The late-hanging cultivars, like Valencia, are the most susceptible in Florida. Initially, the disease is found in isolated groups of trees, called aggregates. Therefore, growers need to look at five to six areas within a 10-acre block to have a reasonable idea of the extent of the disease.
If black spot is present, treatment should be planned. If there are many diseased trees in an area, a leaf litter treatment should be considered. It is suspected that most of the inoculum comes from the leaf litter. Growers have reported good disease suppression using compost to cover the herbicide strip where much of the leaf litter is found.
Research showed that the compost accelerator Soil Set (1.3 fluid ounces/acre) applied via herbicide boom at 50 gallons per acre on the 10-foot herbicide strip reduced the amount of disease and the number of diseased fruits. Soil Set was applied to the leaf litter in March to April.
Leaf litter amendments will not work by themselves but should be paired with a fungicide program. Fruit is susceptible from petal fall to near maturity, but the end of March through April tends to be dry in Florida, reducing the infection risk for this period. The focus of a fungicide program for black spot should be from May through September with monthly applications.
For recommended products, program details and regulatory guidance, consult the black spot chapter in the 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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