By Megan Dewdney
Citrus black spot is one of the newer diseases to worry Florida citrus growers. The fungal disease was first detected in Southwest Florida in 2010. The vast majority of finds have been in the southwestern citrus-producing counties of Collier, Hardee, Lee, Charlotte and Glades, but the disease has slowly moved northward. Black spot is still considered a quarantine disease, and groves that fall 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.
If your grove is near or in a quarantine area, the first step is to determine whether black spot is in your grove and to what extent. In many cases, the disease is at a low level and may not be readily apparent.
The most easily identifiable and diagnostic symptom is hard spot, which occurs 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 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 on declining trees where the sun is the most intense but will eventually be found on the whole tree. Late-hanging cultivars, like Valencia, are the most susceptible. The disease is found in isolated groups of trees, called aggregates, so you will need to look at five or six areas within a 10-acre block to have a reasonable idea of the extent of the disease.
If it is determined black spot is present, a treatment regimen should be planned. If there are many diseased trees, 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. We showed that the compost accelerator Soil Set (1.3 fluid ounces per acre) applied via herbicide boom at 50 gallons per acre on a 10-foot herbicide strip reduced the amount of disease on fruit and the number of fruit becoming diseased. 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, 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 and program details, consult the Florida Citrus Production Guide black spot chapter.
Megan Dewdney is an associate professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.