Learn More About Fungal Leaf Diseases in Florida Blueberries
CREDIT: Doug Phillips, Norma Flor, and Phil Harmon
Image: Septoria leaf spot. Credits: P. Harmon
Southern highbush blueberry (SHB) cultivars are commercially grown throughout much of Florida, in both deciduous and evergreen production systems. Growers in deciduous production should strive to keep leaves healthy through flower bud differentiation in fall to ensure optimum yield potential. In evergreen production, it is critical to maintain the prior year’s foliage through winter months to support early fruit production the following season. In both systems, leaves can be damaged by many factors including environmental conditions, chemical applications, insects, and diseases (including fungal, viral, bacterial, and algal disease).
Several leaf diseases affect SHB in Florida and have the potential to defoliate bushes. Symptoms with different causes can have similar appearances, and more than one disease can occur on the same leaf. For fungal leaf diseases, growers have many effective chemical management options; however, proper product selection and timing of application depends on correct disease diagnosis. Because fungicides are only effective for fungal diseases, differentiating between symptoms caused by fungi and other factors can help prevent unnecessary fungicide use and costs. Growers should consult UF IFAS Extension or use a lab-based diagnostic service for accurate diagnosis.
The more common fungal leaf diseases on SHB in Florida are listed below, along with a description of symptoms. Table 1 is a calendar of blueberry fungal leaf disease activity with potential fungicide management options. For disease management recommendations, see the University of Florida 2019 Blueberry Integrated Pest Management Guide (edis.ifas.ufl.edu/hs380). Make sure to follow all label instructions (including minimum recommended rates and total applications per season) and use rotations of products with different modes of action to avoid development of fungicide resistance.
Septoria leaf spot is a common and prevalent disease in the southeastern US caused by Septoria albopunctata. Severe infections can decrease yield due to reduced levels of photosynthesis, premature defoliation, and reduced flower bud production. Septoria spots are numerous but small (around 1/8 inch) and nearly circular, with light brown to gray centers and a broad purplish margin. The lesions can grow together into larger necrotic areas prior to defoliation. Symptoms tend to be more severe on older leaves which are close to the ground. The disease typically occurs from mid to late harvest through June, and may pick back up during mild wet periods in fall.
In Florida this disease is caused by Naohidemyces vaccinii. Infected bushes can show premature defoliation, decreased floral bud differentiation, and reduced yield. Different levels of susceptibility to this disease can be found in SHB; for example, certain cultivars including ‘Jewel’ are known to be highly susceptible.
Symptoms are initially observed on the upper leaf surface and begin as small, somewhat angular yellow spots that turn reddish brown to black over time. Multiple black to red lesions can occur on the same leaf, ultimately turning the leaves yellow and red before causing defoliation. Brightly colored yellow to orange spores are produced on the underside of the leaf, opposite the lesions on the upper leaf surface. Masses of these spores are key to distinguishing this disease from other leaf spots.
Anthracnose leaf spot (also known as Gloeosporium leaf spot) is caused by Colletotrichum gloeosporioides. This disease can cause premature defoliation, poor bud development, and subsequent loss of yield. Typical symptoms are circular to irregularly-shaped lesions expanding from 1/4 inch to greater than 3/4 inch in diameter. The centers of the lesions are necrotic and range in color from brown to dark-brown, with distinct concentric circles occasionally visible (bull’s-eye patterns). These symptoms occur frequently at the edge of the leaves. Anthracnose leaf disease is common after harvest in Florida and persists through the summer. Blueberry cultivars differ in their susceptibility; ‘Jewel’ is considered to be very susceptible.
Target spot is caused by Corynespora cassiicola. The fungus has a wide host range and was first reported in blueberries in the US in 2014. Florida growers have observed severe defoliation on many SHB varieties since then. Typical symptoms are 1/3 to 3/8-inch, angular to irregular, reddish-brown lesions. As the lesions expand, color can vary in concentric rings, resulting in a “target” or bull’s-eye pattern. Symptoms can be difficult to differentiate from early symptoms of anthracnose leaf spot, and both diseases can occur on susceptible varieties at the same time. However, target spot lesions tend to remain smaller, and fewer target spot lesions are required before leaves fall from the bush compared to anthracnose.
Phyllosticta leaf spot is caused by Phyllosticta vaccinii. This disease is more common later in the summer (August-September) than anthracnose. Symptoms are mahogany brown leaf spots with irregular borders. Lesions range from small (less than 1/4 inch) to larger than one inch prior to causing defoliation. Lesions typically are surrounded by a dark brown to purple margin. A distinguishing feature of this disease is the presence of tiny black fungal pimples (pycnidia, the reproductive structure) that develop within the lesions. However, other fungi, including some that do not cause disease, can also produce small black structures on dead or decaying leaves.
General Fungal Leaf Disease Management
Horticultural inputs can help reduce the likelihood and severity of fungal leaf diseases. Disease is favored by leaf wetness and high humidity. Drip irrigation can help reduce moisture in the canopy compared to overhead. When overhead must be used, timing applications to correspond to periods when dew is present reduces additional duration of leaf wetness. Maintenance pruning of bushes to increase air flow can help dry canopies, as can a good weed management program for rows and row middles. Implementation of good sanitation practices is also recommended to help reduce disease pressure. Remove diseased plant debris from the field to help reduce numbers of fungal spores available to cause disease.