Little Known About New Disease Gloeocerospora Leaf Spot
When blueberry samples are submitted to the UF/IFAS Plant Diagnostic Center, it’s not uncommon for us to observe a handful of pathogens, most of which are minor and unrelated to the problem the grower is concerned about. Alternaria and Pestalotiopsis leaf spots are examples that we find on most every leaf sample but that fungicides aren’t needed to control because the damage done is not severe. A dozen or more other minor leaf spots are listed as occurring on highbush blueberry in the American Phytopathological Society’s Compendium of Blueberry Diseases that rarely— if ever — cause Florida growers problems. However, occasionally, we will see something flare up out of the ordinary. That was the case with target spot caused by Corynespora cassicola that showed up around 2015 and that has required some adjustment to fungicide programs since.
The latest new disease issue was observed in 2022 on the Sentinel variety — Gloeocercospora leaf spot. We’ve observed the disease every year since, and on many additional varieties across farms from north central to southern production within the state. The disease is listed in the compendium, but no research has been conducted on it for the past 50 years or so. Quite a bit has changed since then in terms of names of fungi, and our initial investigation into the fungus suggests it’s likely a Psuedocercospora sp. The symptoms match what NC State researchers originally described, so until convincing evidence emerges that it’s something different, we are going with that ID.
There are no fungicide recommendations for gloeocercospora leaf spot on blueberry, so knowing what is effective for related pathogens can help give a starting point. Related fungi cause leaf spots on a diverse range of ornamental and crop plants. Damage can range from very minor to severely limiting production. Resistant varieties are used where available, like with sugar beet production. Fungicide resistance has been well-documented to several modes of action on beets and other crops and is something to look out for in blueberry as well, but it has not been confirmed for blueberry to date.
The DMI fungicides are some of the most effective on related pathogens across multiple crop systems, with contact products that contain chlorothalonil and copper reported as less effective, but consistent. We use several of these for rust prevention during the periods of the year when gloeocercospora leaf spot tends to occur. Fungicides like azoxystrobin can work well for many related cercospora diseases of other crops, but resistance renders them ineffective where present. The blueberry disease has occurred on farms using a good rust fungicide program, so additional work is needed to know what additional inputs can be used to try to prevent severe disease. The economic importance of managing this disease and whether a return on fungicide investment in increased yields will be realized by attempting control is still unknown, and is an equally important question.
The symptoms are distinct and vary slightly from the most susceptible varieties like Sentinel to more resistant varieties and selections. The symptoms are visible on upper leaf surfaces and tend to be angular in shape with colors ranging from reddish brown (Fig. 1 and 2) to dark brown necrotic with black to purple borders and a general yellowing of severely affected leaves (Fig.3, 4, and 5). Some defoliation has been observed, but leaves can be held on the bush with a surprising amount of symptom. Sporulation of the fungal pathogen is typically visible on the underside of affected leaves collected directly from the field, or may be produced with incubation in a moist chamber. The sporulation looks like a dingy gray to black stain in the same angular shape as the upper leaf surface discoloration (Fig. 6-9). Some varieties show abundant sporulation on the lower leaf surface, but little to no upper leaf symptom. The fungus produces spores in open clusters that can be seen as tiny black dots within the gray discoloration on the underside of affected leaves (Fig. 10, 11). The closest lookalike symptom is caused by blueberry necrotic ring blotch virus, but lacks the black to gray sporulation and tends to be more circular than angular.
In summary, Florida blueberry growers deal with more than their fair share of disease problems, and new issues are still discovered from time to time. Some problems turn out to have minor impact on production with need for minimal adjustment in integrated disease management plans, and others require research and diligent efforts to address. I don’t think we know exactly which this new (gloeo)cercospora leaf spot will be yet. I’d encourage growers to be aware that this new disease is out there, scout your Sentinel for characteristic symptoms, and please keep us at UF/IFAS Extension informed on how severely this new problem affects your operation. We will update with new data on management strategies as they become available.
CREDITS:
PHILIP HARMON & DOUG PHILLIPS
