Right On Target

Monitoring and Managing Target Spot in Southern Highbush Blueberry

Target spot, a fungal leaf disease, is caused by the Corynespora cassiicola pathogen. It was first reported in blueberry in the US (on “Jewel” in Central Florida) in 2014. Florida growers have reported significant defoliation due to target spot on many southern highbush blueberry (SHB) cultivars since then. 



Typical symptoms are angular or irregular reddish-brown lesions, around 1/3- to 3/8-inch in diameter. These lesions can form concentric rings of varying colors as they expand, resulting in a target or bull’s-eye type pattern (Figures 1 and 2). These symptoms can appear similar to early symptoms of anthracnose leaf spot, and both diseases can occur simultaneously on susceptible varieties. However, anthracnose leaf spot lesions can increase to a diameter exceeding 1/2 inch, while target spot lesions tend to remain smaller. Also, defoliation can occur with fewer target spot lesions compared to anthracnose. 


Figure 1. Target spot symptoms on blueberry leaf.

Credits: D. Phillips


Figure 2. Target spot symptoms on blueberry leaves.

Credits: P. Harmon


Disease Cycle

There is currently limited data on the disease cycle of this pathogen in blueberry. In other crops, C.cassiicola typically overwinters in plant debris or alternate host species. Humid weather, temperatures between 79°F – 84°F, and moderate rainfall are all favorable conditions for the rapid development of disease and pathogen reproduction. In the field, pathogen spores can be spread by wind or water splash, including both rain and overhead irrigation.



Growers have reported that after the onset of symptoms, target spot is difficult to manage. Protective fungicide applications where the disease is known to occur or frequent and careful scouting for initial disease symptoms are recommended. Good cultural practices to help reduce disease severity include limiting leaf wetness duration and high humidity within the plant canopy. This can be done by minimizing overhead irrigation, regular pruning to open plant canopies, and weed management in beds and row middles to increase airflow to the plant. 


Many of the fungicides used to manage other summer leaf spot diseases of blueberry also will control target spot if applied before the disease becomes significant. Evergreen producers who already have a fungicide rotation program for post-harvest foliage disease management may find that target spot is problematic late season. The disease can flare between scheduled fungicide applications in summer as well. Where target spot has been problematic, tighten fungicide application intervals (i.e. reapply every 2 weeks vs 4) and consider additional late-season sprays while disease is still active in September. There is no known fungicide resistance for the target spot pathogen on blueberry at this time, and most fungicides that are used to manage anthracnose and rust have efficacy against target spot. Good spray coverage with fungicides is essential.  Where good canopy penetration and coverage is not achieved, leaves within the bush can become infected and produce large amounts of spores (inoculum) to infect the rest of the bush at the first lapse of fungicide coverage during favorable conditions.  


Specific products to rotate postharvest for target spot and other foliar fungal diseases include chlorothalonil (Bravo and others). Copper-containing products applied in summer for algal stem blotch control also have some efficacy. Rotating between the copper and Bravo applications starting when regular summer rains begin (late May, June) makes for a good backbone to a fungicide application program. Supplement these applications by adding additional sprays and/or tank-mixing the Bravo with another fungicide (where permitted under label instructions) when target spot symptoms are noted between applications. An extra application early in an epidemic will pay dividends later in the summer, and those left until symptoms are common throughout the field are not likely to give satisfactory results, requiring more fungicide apps and higher label rates to get the disease back in check.  


Additional fungicide products to consider include azoxystrobin (Abound and others). Resistance can be an issue for Abound with anthracnose, so consider using this as a tank mix, either with Bravo or with Captan. Pristine has as one active ingredient pyraclostrobin, which is in the same group as Abound, but Pristine is generally recommended as a rotation partner with Switch preharvest.  


Recent registrations of fungicides within another group of fungicides include products with SDHI (FRAC group 9) active ingredients. These are used for target spot control in other crops like tomato and will be the subject of blueberry trial work in the near future. Two products to consider incorporating into your rotation from this group include Fontelis and Luna Tranquility.  A third product is now available called Miravis Prime; however, it contains one of the active ingredients from Switch and when choosing between Switch preharvest or Miravis Prime postharvest, go with Switch and use Fontelis or Luna Tranquility for leaf disease control instead.  Producers have to choose because making allowed applications of both would put them over the legal limit of the fludioxanil active ingredient for the season.


The final group to mention includes those products with DMI active ingredients (FRAC group 3).  Proline (active ingredient prothioconazole) has stood out as a good rust management tool postharvest. Where rust and target spot occur together, consider including Proline in the rotation with those above. Quash, Indar, and propiconazole (Tilt and others) are alternatives to Proline in this group with generally less rust efficacy and largely unknown target spot efficacy.    


Growers have many fungicide tools with which to address target spot, but each additional application adds production costs, including labor, fuel, equipment maintenance, as well as the cost of the product itself. With today’s market and other challenges, the question becomes can we afford to manage target spot? Additional research is needed to document the impacts of target spot defoliation on fruit production the following season, and the economic costs associated with these potential management options and benefits in terms of increased yields. 

by DR. PHIL HARMON, Dr. Phil Harmon, Professor, Plant Pathology Department, University of Florida & DOUG PHILLIPS, Blueberry Extension Coordinator, University of Florida

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