Don’t Be Defeated by These Common Diseases
Managing Summer Blueberry Diseases
With Florida’s hot and humid summer upon us, it’s time to start scouting for summer blueberry diseases, and properly managing them when they are present. Multiple diseases are observed during the summer months, including algal, fungal, and bacterial. The most common ones are discussed below.
Algal Stem Blotch
Algal stem blotch, caused by the parasitic green alga Cephaleuros virescens Kunze, has become a significant disease on southern highbush blueberries (SHB) in Florida, and has also begun to be an issue in Georgia. The alga is thought to enter the plant through natural wounds and openings, pruning cuts, or by direct penetration of the cuticle. Once inside the plant, the alga forms colonies beneath the stem cuticle which are visible as red blotchy raised lesions. During hot, humid, and wet conditions, orange and fuzzy reproductive structures emerge and produce spores that are dispersed by wind and water splash. Symptom development is slow, and can take up to one year to produce symptoms. Plants that are stressed by abiotic or biotic factors are more susceptible to infection and subsequent disease development.
Early symptoms of algal stem blotch include small red blotches or lesions on young green stems, expanding to form irregular cankers that can encircle canes. Leaves on symptomatic canes bleach white to pale yellow (less uniform and blotchier than nutritional deficiency symptoms) (Figure 1), and growth of the entire plant can be severely stunted as the disease advances. In hot, humid summer months, bright-orange felt-like mats or tufts of algal growth appear from the blotchy lesions on young stems and older cane surfaces (Figure 2). Eventually infected canes and stems crack, and this damaged tissue can lead to Botryosphaeria stem blight susceptibility and plant death.
Figure 1. Algal stem blotch leaf symptoms
Credits: D. Phillips, UF/IFAS
Figure 2. C. virescens reproductive structures.
Credits: D. Phillips, UF/IFAS
Limited information exists about management practices for algal stem blotch. No systemic pesticide products have been found to date that will kill the algae living underneath the plant epidermis. Spray applications of copper-containing fungicides can help to reduce algal sporulation. These products kill the algal reproductive structures present on the plant, and protect healthy canes from infection for a few days after application. However, these applications do not impact existing symptoms or eradicate the disease. Copper product sprays should begin after harvest and continue through September. Applications must be done on a regular schedule prior to infection in order to be effective, and spray equipment must achieve good cane coverage. Overhead irrigation should be avoided if possible when disease is present because it promotes the spread of algal reproductive structures. In addition, removing and destroying infected canes and eliminating weeds improves air circulation in the canopy and can help to slow disease development. Disinfection of pruning equipment where symptoms are present may also help minimize the spread of disease.
For more information on algal stem blotch see Algal Stem Blotch in Southern Highbush Blueberry in Florida (https://edis.ifas.ufl.edu/publication/PP344).
Fungal Leaf Diseases
The most common fungal leaf diseases observed during summer and early fall include anthracnose, target spot, and Phyllosticta.
Anthracnose leaf spot is caused by the pathogen Colletotrichum gloeosporioides, and can cause premature defoliation, poor floral bud development, and subsequent loss of yield. Symptoms are usually circular to irregularly shaped lesions, typically at the edges of leaves, that expand from 1/4 inch to greater than 3/4 inch in diameter. The lesion centers range from brown to dark brown, with distinct concentric circles sometimes visible (Figure 3). Anthracnose leaf disease is common after harvest in Florida and persists throughout the summer months.
Figure 2. Anthracnose leaf spot symptoms.
Credits: P. Harmon, UF/IFAS
Many registered fungicides are labeled for anthracnose on blueberry in Florida, including DMI fungicides (FRAC 3) such as IndarTM, OrbitTM, QuashTM, Quilt XcelTM, and ProlineTM, which can be used in rotation or in tank mixtures with compatible products from another group to help prevent fungicide resistance, such as Luna TranquilityTM (FRAC 7 & 9), AboundTM (FRAC 11), PristineTM (FRAC 11 & 7), SwitchTM (FRAC 9 & 12), and captan (FRAC M4). Single applications of BravoTM (FRAC M5) are also recommended after harvest for foliar disease management and to reduce the chances of seeing fungicide resistance since Chlorothalonil is a multisite contact fungicide. Anthracnose resistance to AboundTM, PristineTM, and other FRAC group 11 fungicides has been confirmed in Florida, so these should be used in a premix product with two active ingredients or tank-mixed with another fungicide like captan to ensure efficacy.
Target Spot
Target spot is caused by Corynespora cassiicola. Florida blueberry growers have observed significant defoliation on many SHB cultivars from this disease. Symptoms are usually observed as 1/3- to 3/8-inch angular to irregular reddish-brown lesions, with a “target” or bull’s-eye pattern as the lesions expand (Figure 4). 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, whereas anthracnose leaf spots can increase to larger than 1/2 inch in diameter. Fewer target spot lesions are required before defoliation occurs compared to anthracnose. In the field, spores can be spread by wind or water splash (rain or irrigation).
Figure 4. Target spot symptoms
Credits: D. Phillips, UF/IFAS
Growers have reported difficulty managing target spot once symptoms become present and significant. Preventive fungicide applications where the disease is known to occur or careful scouting for the first disease symptoms are encouraged. Limiting periods of leaf wetness and high humidity within the blueberry canopy also may help reduce disease severity, and can be achieved by avoiding overhead irrigation, maintenance pruning to open canopies, and weed management in beds and row middles to increase air flow. No fungicide resistance is known at this time, and most fungicides that are used to manage anthracnose should be effective against target spot. Growers should ensure good, even coverage with spray equipment to increase the efficacy of the fungicides applied.
Phyllosticta
Phyllosticta leaf spot is caused by Phyllosticta vaccinii. This disease is more common later in the summer (August–September) than anthracnose. Symptoms are brown leaf spots with irregular borders (Figure 5). Lesions range from small (less than 1/4 inch) to larger than one inch prior to causing defoliation. Lesions are typically 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 structures) 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. This disease is common in Florida, but it is considered to be of minor importance.
Figure 5. Phyllosticta symptoms
Credits: P. Harmon, UF/IFAS
There are no published fungicide recommendations for Phyllosticta leaf spot management on blueberries in Florida; however, in other crops (cranberry, maple), related diseases are managed with applications of the contact fungicide BravoTM (FRAC M5). General maintenance applications of contact fungicides like BravoTM or captan are recommended after harvest as needed or approximately every two weeks (for up to 6 weeks).
For more information on fungal leaf diseases see Florida Blueberry Leaf Disease Guide (https://edis.ifas.ufl.edu/publication/PP348).
Phytophthora Root Rot
The most common and destructive root rot disease of SHB in Florida is Phytophthora root rot (PRR) caused by the oomycete pathogen Phytophthora cinnamomi. Unchecked development can lead to plant stunting, early fall discoloration, dieback, and death. PRR is a persistent problem that is currently managed through careful site selection, preparation, and routine Phytophthora-specific fungicide applications.
PRR develops when a susceptible plant host makes contact with the pathogen during a period of favorable environment for infection and disease development. Water-saturated root zones lack oxygen that blueberry roots need, leading to root stress and damage, and provide an ideal environment for the pathogen to produce spores that infect blueberry roots. The more frequent and increasing duration of root zone saturation, the more likely infection will occur, and the more severe the disease symptoms will become.
The first symptoms of PRR occur on roots, underground, where they are rarely observed. These symptoms include root discoloration (dark brown to black color, instead of the normal cinnamon brown) and rot. As the disease progresses and roots are unable to do their jobs, plants suffer from a lack of nutrients and water. This induces aboveground symptoms, including stunting, yellowing, early fall leaf reddening, wilting (rarely), and eventually plant dieback. Plant death and dieback incidence due to PRR may spike during periods of drought conditions—not because drought favors infection, but because plants with compromised root systems cannot keep up with water demands. Fungicides at this point in the disease cycle are not effective—will not save diseased plants.
One indicator of possible PRR in plants that show stunting or decline is excessive movement of the crown in the bed when a plant is shaken. PRR tends to affect many plants spanning down and across the rows, resulting in circular to oblong “patches” of stunted and declining bushes. These patches tend to occur first in the lowest, most poorly drained areas of a field, where PRR symptoms also tend to be most severe. Symptoms are frequently caused by more than one disease and/or stress, and the physiological drought stress caused by PRR can make plants more susceptible to these other diseases, particularly Botryosphaeria stem blight.
The key to PRR management is prevention, which starts with site selection and field preparation. Where water tables are high or where soils tend to hold water after rain events, growers should use ground contouring and ditching in conjunction with raised bed construction to prevent ponding of surface water. Maintaining drainage at the ends of rows is an ongoing process as soil tends to collect or ruts in end roads disrupt surface water flow and result in ponding.
Two fungicide groups with PRR efficacy are used on blueberry in Florida. The first contains fungicides in the phenylamide group (FRAC group 4) with the active ingredient mefenoxam. Ridomil Gold® SL is one example. Label instructions should be followed carefully to achieve the best results (apply to beds or through irrigation, not as a foliar spray). Two applications per season are allowed, with the first recommended in winter before plants bloom. This timing will help prevent root rot through freeze protection events that can leave soils saturated. The second application of Ridomil should be made after harvest, when regular summer rains return, or approximately 6 months after the first application (June to August).
The second group of fungicides labeled for PRR management in blueberry is the phosphonates (FRAC Group P07), sometimes referred to as “phites” (e.g., ProPhyt®). These products are systemic and, in contrast to mefenoxam, are recommended as summer foliar sprays to help combat PRR and fungal leaf diseases. The phites are also recommended as soil drenches where bacterial wilt has been confirmed. Soil drenches may also provide some level of PRR protection (according to recent UGA research) but this application method may reduce efficacy of the phites, particularly for fungal leaf spots (additional research needed).
A new fungicide (within the FRAC group 49) is available for use on blueberry. Oxathiapiprolin is available in the product Orondis® Gold 200, or as premix with mefenoxam in the product Orondis® Gold (Premix). These products have not been evaluated on SHB in Florida; thus, look for additional information on these products as data become available.
For more information on Phytophthora see Phytophthora Root Rot on Southern Highbush Blueberry in Florida (https://edis.ifas.ufl.edu/publication/PP374).
Ralstonia and Bacterial Wilt
Plants with bacterial wilt caused by Ralstoniasolanacearum show signs of water stress such as wilting and marginal leaf burn (Figure 6). They may also be prone to developing severe symptoms of other stress diseases, such as stem blight, and thus may show symptoms of both diseases. The crowns of blueberry plants with bacterial wilt have a mottled discoloration or light brown to silvery purple blotches with ill-defined borders. This discoloration is different from that which occurs with stem blight disease, which typically has well-defined margins between healthy and diseased wood with pie piece-shaped and pecan brown color. Additionally, wood chips floated in water from the crowns of plants with bacterial wilt will stream bacterial ooze.
Figure 6. Bacterial wilt symptoms
Credits: D. Phillips, UF/IFAS
Ralstonia can be spread easily in water, soil, or through infected plant material. It can survive for years in soil, slowly spreading down and across rows of blueberry, leaving large circular patches of dead and dying plants. These symptoms are similar in appearance to Phytophthora root rot-affected areas, but do not necessarily occur only in low-lying and poorly drained soils. Once introduced to a farm or nursery facility, the pathogen is spread most efficiently in recycled irrigation water and on pruning and other equipment. Ralstonia has been found most frequently in the cultivar ‘Arcadia’, although other varieties may also be susceptible.
To manage the disease, it is important to keep from introducing the bacterium onto your farm by limiting the movement of soil or infected plant materials between farms on equipment and workers' tools, including tractors, pickups, pruning shears, etc. Mud transferred from farm to farm on vehicles during freeze protection is another possible transfer mechanism. Purchase and use only healthy plants that are free of disease.
Where Ralstonia is detected, remove and burn or deep-bury all infected plants. Then, prior to replanting use soil drenches of products with phosphorous acids or salts (phites) to help protect surrounding plants and replants from infection. K-Phite is an example product that is labeled for blueberry and for soil drench or chemigation application for Ralstonia control. Similar alternative products are available from other manufacturers. Injection or soil applications are currently recommended for the variety “Arcadia” wherever planted and for all varieties on farms where the disease has been confirmed.
For more information on Ralstonia see Bacterial Wilt of Southern Highbush Blueberry Caused by Ralstonia Solanacearum (https://edis.ifas.ufl.edu/publication/PP332).
In summary, summer disease management is about keeping plants alive and with sufficient healthy foliage to set flower buds in the fall. To do that, all farms that have seen Phytophthora root rot should consider the summer, post-hedge, mefenoxam application as a top priority. Farms in central and south Florida growing the variety ‘Arcadia’ should add regular drenches or through drip apps of phites to that priority list as well. Where algal leaf spot is noted, regular copper applications should form the backbone of a summer spray schedule to help with that disease, and they will give some control of fungal leaf spots. Zero leaf disease is not an attainable or sustainable goal; the objective is to prevent significant summer and early fall defoliation. Scout for disease and augment the summer program with those BravoTM applications where target spot, cercospora leaf spot, or anthracnose start to get out of hand. Look for rust to return in early fall, when ProlineTM and other DMI’s will become important tools, particularly for evergreen farms. When in doubt, sending samples to our UF Plant Diagnostic Center can help nail down what diseases you are dealing with, and the UF IFAS Blueberry Team is available to help.
DR. PHILIP HARMON, Professor, Plant Pathology, UF/IFAS
& DOUG PHILLIPS, Blueberry Extension Coordinator, UF/IFAS
