Grower’s Thoughts: Blueberry Summer Disease Management

Florida experiences hot, humid, and wet weather throughout summer and early fall, which is favorable for the development of certain diseases in southern highbush blueberries (SHB). These include several fungal leaf diseases, as well as a parasitic alga infection known as algal stem blotch. Being aware of these diseases, the environmental conditions that favor pathogen spread and infection, and establishing an effective management plan will help to mitigate their effect on Florida blueberry orchards.

Fungal Leaf Diseases

Common fungal leaf diseases on SHB during summer and early fall months (although they can also appear at other times during the year) include anthracnose, Phomopsis, Phyllosticta, rust, Septoria leaf spot, and target spot. Symptoms typically appear as discoloration, spotting, and necrosis on the leaves, leading to plant defoliation when severe. Some of these pathogens also may infect plant stems, occasionally resulting in dieback. However, symptoms with different causes (drought stress, nutrient deficiency or toxicity, herbicide damage, mechanical damage, etc.) can have similar appearances, and more than one disease can occur on the same leaf. Therefore, it is important to correctly diagnose the issue before deciding on a management plan. UF EDIS Publication PP348, Florida Blueberry Leaf Disease Guide ( , has a diagnostic key, along with descriptions and pictures of symptoms, discussion of the disease cycles, management recommendations, and a table showing when each disease is typically observed and potential fungicide controls for each. In addition, affected plant samples can be sent to the UF Plant Diagnostic Center for confirmation of the presence and diagnosis of disease ( 


Anthracnose           Target Spot


Septoria Leaf Spot           Phyllosticta       Rust

There are also cultural controls that can help minimize the incidence or severity of fungal leaf disease. Using drip irrigation instead of overhead irrigation can help reduce leaf wetness duration and the spread of fungal pathogens. However, many growers supplement with overhead irrigation during summer months; in those situations, timing applications for early morning (when dew may already be present) can reduce the length of leaf wetness. Periodic pruning to increase air flow through the canopy and a good weed management program can also help reduce leaf wetness. Good sanitation practices are recommended to help decrease disease pressure, including removal of diseased plant debris from the field to help reduce numbers of fungal spores. 

Algal Stem Blotch

In addition to fungal diseases, there is a parasitic green alga (Cephaleuros virescens Kunze) 

that causes algal stem blotch in SHB in Florida (see UF EDIS Publication PP344, Algal Stem Blotch in Southern Highbush Blueberry in Florida ( The alga is dispersed by wind and water, and is thought to enter the plant through wounds, including pruning cuts, or by direct penetration of the cuticle. Symptoms of this disease are very pale chlorosis of plant leaves (usually on a few canes instead of the whole plant) and red blotches or lesions on affected stems and canes. During hot, humid, and wet conditions, an orange, felt-like substance (the alga’s reproductive structure) erupts from the lesions. Infected plants also exhibit stunted growth and are more susceptible to developing Botryosphaeria stem blight, which can lead to plant death in some cases. 



There are currently no controls that will eradicate the algae in the plant or reverse existing symptoms. However, this disease can be managed and its spread minimized through regular summer applications of copper products, such as copper hydroxide. This will kill the emerging reproductive structures, minimizing the algae’s spread to uninfected canes on the same plant or adjacent plants. During the period from June through September, copper applications should be made on a regular schedule, following label rates and reapplication interval instructions. Applications may be made more frequently (in accordance with label instructions) when conditions are favorable for disease development and when this disease has been observed on the farm. Caution should be exercised when tank-mixing products containing copper with anything else. Issues with tank mixes of acidic products or acidifying adjuvants and copper can result in plant burn. Tank mixes of copper products and products with penetrants or spreader sticker type formulations can also create problems under certain conditions. The same types of cultural controls discussed above for fungal diseases may help minimize disease pressure for algal stem blotch, and good management practices (irrigation, fertilization, sanitation, and disease and pest control) will reduce plant stress and help the plants be less susceptible to disease. 

Philip Harmon, UF Professor, Plant Pathology

Doug Phillips, UF Blueberry Extension Coordinator

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