Don't Let Disease Derail Your Fields

Summer in Florida is wet, humid, and hot—great conditions for plant disease. Blueberries suffer from a range of summer problems caused by algal, fungal, and water mold pathogens. This article is meant to be an overview of some of the most important problems and ways to tie together management strategies over the summer that will cover the bases. Knowing what the typical symptoms of these diseases are and how to scout for them are critical first steps toward effectively managing healthy bushes through the summer months.

 

Time to Hedge and Protect Roots: Stem Blight and Phytophthora Root Rot

Summer kicks off right around the time most growers do a post-harvest pruning to rejuvenate bushes and spur on a flush of new growth that will ultimately produce the next year’s crop. The first post-harvest application of fungicide that growers may want to consider is at or just after hedging. Captan mixed with Abound is one option to help slow or prevent Botryosphaeria stem blight from occurring on the fresh pruning wounds. Some growers use hedging equipment that can also put the fungicide out at the same time and others follow the hedger with a sprayer the same day. There are many other options for fungicide products to consider as well.

 

Hedging is typically followed by the start of regular rains, and then the hurricane season follows. A banded, or through-drip irrigation application of a fungicide with mefenoxam (example: Ridomil Gold) into the root zone before all the rain can help protect from Phytophthora root rot, our most important root disease of Florida blueberries. On well-drained farms with sandy soils and elevated growing beds, root rot applications may not be needed across entire fields but can be beneficial for low or poorly drained areas that are prone to flooding, heavier soils, and particularly prior to tropical weather systems capable of producing large amounts of rainfall.

 

Follow up applications of fungicides with mono- and dipotassium salts of phosphorous acid including Agri-Fos™, K-Phite™, and ProPhyt™ (potassium phosphite) (aka “phytes”) throughout summer on a monthly interval can further help with root rot and provide some leaf disease suppression if sprayed on leaves. Where bacterial wilt has been an issue on farms, drenching the products into the bed is a better option that targets roots. Drenching is a term used to describe applications of pesticides that target the root zone; directly applied to the bed in a high delivery volume, or watered in after application. Some fungicide products can be applied through the drip irrigation system, also delivering product directly to the roots. 

 

Summer Maintenance: Algal Stem Blotch

 

Algal stem blotch is a significant disease on southern highbush blueberries (SHB) in Florida, caused by a parasitic green alga. This disease often results in mottled to very pale leaf yellowing (Figure 1), stunted growth, and increased susceptibility to Botryosphaeria stem blight. Stunting is the result of a reduction in plant vigor, and can appear as poor regrowth following summer pruning. Over time infected canes and stems may crack, which can lead to Botryosphaeria stem blight susceptibility and plant death.

 

Figure 1. Algal stem blotch leaf symptoms

Credit: D. Phillips, UF/IFAS

 

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 and red blotchy raised lesions develop. During hot, humid, and wet conditions, bright orange felt-like mats or tufts of algal growth (sporangiophores) appear from the blotchy lesions on young stems and older cane surfaces (Figure 2). These reproductive structures are dispersed by wind and water splash, with peak spore production typically occurring between May and September. Plants that are stressed by abiotic or biotic factors are more susceptible to infection and subsequent disease development.

 

Figure 2. Algal stem blotch sporulation

Credits: D. Phillips, UF/IFAS

 

There is currently limited information about effective management practices for algal stem blotch. Certain cultural practices may help reduce the spread of disease, including good horticultural inputs and practices (irrigation, fertilization, sanitation, and disease and pest control) which will help reduce plant stress and related disease susceptibility. Overhead irrigation should be avoided if possible when disease is present to help minimize 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 and the use of disease-free planting stock are also recommended.

 

No pesticide products (including fungicides, since this pathogen is an alga) have been found to date that will kill the algae living underneath the plant epidermis, so prevention is key. Spray applications of copper-containing fungicides can help to reduce algal sporulation and disease spread. However, these products only kill the algal reproductive structures present on the plant and protect healthy canes from infection for a few days after application; they do not treat existing symptoms or eliminate the disease. Copper product sprays with good coverage should begin after harvest and continue through September, on a regular schedule prior to infection. 

 

The specific spray schedule and rate will vary by label instruction of product chosen with reapplication intervals ranging from 7 to 28 days. These applications can be made 14 days after the phyte applications for root rot and bacterial wilt (above), but should not be combined because some phyte products are acidic, and when mixed with copper can burn plants. More frequent applications are recommended when weather forecasts are favorable for disease development and when the disease is known to occur on the farm. Follow all label instructions and be cautious when considering tank-mixing products that contain copper with anything else. Specific issues with tank mixes of acidic products or acidifying adjuvants and copper are well-documented and can result in plant burn. Tank mixes of copper products and products with penetrants or spreader sticker type formulations also can be problematic in some conditions (Bravo Weatherstik is an example). For specific product rates, timings, and additional details on this disease, see the 2022 Florida Blueberry Integrated Pest Management Guide (https://edis.ifas.ufl.edu/publication/HS380) and Algal Stem Blotch in Southern Highbush Blueberry in Florida (https://edis.ifas.ufl.edu/publication/pp344). 

 

Fungal Leaf Diseases

There are several fungal leaf diseases that can be problematic for Florida blueberry growers during summer and early fall. These include anthracnose, rust, Phyllosticta, Septoria, target spot, and several others. 

 

Anthracnose

Anthracnose leaf spot can cause premature defoliation, poor floral bud development, and subsequent loss of yield. Symptoms are typically circular to irregularly shaped lesions beginning at the edges of leaves, expanding from 1/4 inch to more than 3/4 inch in diameter, with brown to dark brown centers and concentric circles of damaged tissue (Figure 1). Anthracnose leaf disease is common after harvest in Florida and persists through summer. 

 

Figure 1.  Anthracnose leaf spot lesion. 
Credit: P. Harmon, UF/IFAS

 

The pathogen overwinters in infected leaves and stems from the previous season. In spring, when the weather gets warmer and more humid, the fungus produces reproductive spores, which are spread by splashing water (rain or overhead irrigation), workers, and equipment. 

 

Many registered fungicides are labeled for anthracnose on blueberry in Florida. Applications work best before symptoms become severe. On susceptible cultivars, applications should begin after post-harvest pruning, with reapplications according to label instructions through September. Demethylation inhibitor (DMI) fungicides (FRAC 3) such as Indar™, Orbit™, Quash™, Quilt Xcel™, and Proline™ are options to be used in rotation or in tank mixtures with compatible products from another group to help prevent fungicide resistance. Fungicides with different modes of action such as Luna Tranquility™ (FRAC 7 & 9), Abound™ (FRAC 11), Pristine™ (FRAC 11 & 7), Switch™ (FRAC 9 & 12), and captan (FRAC M4) are suitable for rotation with DMI fungicides. Single applications of Bravo™ (FRAC M5) are also recommended after harvest. Anthracnose resistance to Abound™ and other FRAC group 11 fungicides has been confirmed in Central 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. Fungicides with mono- and dipotassium salts of phosphorous acid including Agri-Fos™, K-Phite™, and ProPhyt™ (potassium phosphite) have shown some effectiveness against anthracnose, so when they are used as above for phytophthora root rot management, you may also see foliar disease suppression. Where the anthracnose still remains severe, consider one of the fungicides above to replace or tank mix with the foliar phyte.

 

Rust

Rust can result in premature defoliation, a decrease in floral bud differentiation, and a corresponding reduction in yield. Symptoms begin on the upper leaf surface as small, somewhat angular yellow spots that turn reddish brown to black over time. Multiple lesions can occur on the same leaf, eventually turning the leaves yellow and red (Figure 2) and causing defoliation. Brightly colored yellow/orange spores emerge on the underside of the leaf, opposite the lesions on the upper leaf surface, and are the distinguishing characteristic of this disease (Figure 3). 

 

Figure 2.  Rust lesions on the upper leaf surface. 
Credit: P. Harmon, UF/IFAS

 

Figure 3.  Rust reproductive spores on the leaf underside. 
Credit: P. Harmon, UF/IFAS

 

Rust spores are spread by wind. New leaf infections can begin in spring during or just after harvest, and disease activity typically increases again in early fall. In Central Florida, the fungus survives on evergreen Vaccinium plants, and in weeds or other plant hosts next to production fields. The disease is favored by overhead irrigation and/or frequent rains. In evergreen production in Central and South Florida, the pathogen can survive in infected leaves that remain on the plants throughout winter. Rust has recently become a significant disease concern in evergreen blueberry production. 

 

Applications of fungicides are the best method of control, although they work best as a preventative measure. Although systemic fungicides can move into the infected leaves and potentially stop some rust development, most products will only reduce or delay the amount of sporulation because fungicides do not effectively kill the fungus inside the leaf. Fungicides do a better job protecting against new infections, so making repeated applications to maintain a protective residue on the leaves is key to preventing the disease. Fungicides such as Proline™, Quilt Xcel™, Quash™ (FRAC 3) and Propulse™ (FRAC 3 and 7), have very good to excellent effectiveness against rust. Tilt™, Indar™ (FRAC 3), Bravo™ (FRAC M5), and Abound™ (FRAC 11) have good effectiveness. Applications should begin after harvest where leaf rust is found to be problematic and continue through late October on susceptible cultivars. In the evergreen system applications may need to continue through the winter months. Fungicides with different modes of action should be used in rotation or in a tank mix, and as part of an integrated post-harvest foliage management strategy. A sprayer capable of good canopy penetration and leaf coverage should be used to get maximum benefit from the fungicides used. 

 

Phyllosticta

This disease is more commonly observed later in the summer (August–September) than anthracnose. Symptoms are dark brown leaf spots with irregular borders, surrounded by a dark brown to purple margin (Figure 4). These lesions range from less than 1/4 inch to larger than one inch, and can ultimately lead to defoliation. A distinguishing feature of this disease is the presence of tiny black fungal pimples that develop within the lesions and can be seen with a hand lens. 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 4.  Phyllosticta leaf spot symptoms. 
Credit: P. Harmon, UF/IFAS

 

Although there are no published fungicide recommendations for Phyllosticta leaf spot management on blueberry in Florida, it has been managed with Bravo™ (FRAC M5) in other crops. General maintenance applications of contact fungicides like Bravo™ or captan are recommended after harvest as needed or approximately every two weeks (for up to 6 weeks).

 

Septoria

Septoria leaf spot is a common disease in the southeastern United States. Significant infections can decrease yield due to defoliation, reduced floral bud production, and lower levels of photosynthesis. Symptoms are observed as numerous small, circular leaf spots (about 1/8 inch in diameter) with light-brown to gray centers and broad purplish margins (Figure 5), usually present more often on older leaves. These lesions can coalesce into larger necrotic areas, followed by defoliation. Septoria typically occurs from mid to late harvest through June, and may return during mild wet periods during fall. Reproductive spore germination and infection are favored by mild wet weather (75°F–82°F), and are spread by water splash (rain and overhead irrigation).

 

Figure 5.  Septoria leaf spot. 
Credit: P. Harmon, UF/IFAS

 

Applications of protective and systemic fungicides with different modes of action can help to reduce Septoria leaf spot severity. Fungicides of FRAC group 3, such as Tilt™, Indar™, Quash™, Quilt Xcel™ and Proline™, and fungicides of other FRAC groups, such as Luna Tranquility™ (FRAC 7 & 9), Abound™ (FRAC 11), Switch™ (FRAC 9 & 12), Pristine™ (FRAC 11 & 7), and Bravo™ (FRAC M5), are effective against this disease. Systemic phosphite fungicides such as Agri-Fos™, ProPhyt™ and similar phosphonate products are also effective against Septoria; however, they must be applied after harvest to avoid possible fruit damage.

 

Target Spot

Target spot was first reported in blueberry in the United States in 2014. Some Florida growers have observed severe defoliation on many SHB cultivars since then. Symptoms are 1/3- to 3/8-inch angular to irregular reddish-brown lesions with color varying in concentric rings, resulting in a “target” or bull's-eye pattern (Figure 6). Symptoms can appear similar to the early symptoms of anthracnose leaf spot, and both diseases can occur simultaneously on susceptible varieties. However, target spot lesions tend to remain smaller, and fewer target spot lesions are required before defoliation occurs compared to anthracnose.

 

Figure 6.  Target spot symptoms. 
Credit: D. Phillips, UF/IFAS

 

Environmental conditions such as high humidity, temperatures between 79°F–84°F, and moderate rainfall favor abundant fungal sporulation and rapid disease development. Spores can be spread by wind or water splash (rain or irrigation).

 

Some growers have reported difficulty managing target spot after symptoms appear and become severe. 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. This can be accomplished by avoiding overhead irrigation, maintenance pruning to open canopies, and weed management in beds and row middles to increase air flow. Specific products to rotate postharvest for target spot and other foliar fungal diseases include chlorothalonil (Bravo™ and others – FRAC M5), Proline™ (FRAC 3), Abound™ (FRAC 11), and Quash™ (FRAC 3). Copper-containing products applied in summer for algal stem blotch control also have some efficacy. Rotating between the copper and other fungicide applications starting when regular summer rains begin can provide a good foundation for a fungicide application program. Supplement these applications by adding additional sprays and/or tank-mixing fungicide products (where permitted under label instructions) when target spot symptoms are noted between applications. 

 

Summary

Those are some of the most important summer diseases to consider for Florida blueberry production. Taken individually, that adds up to a lot of fungicide applications, but not all diseases occur each year on every farm, and there are many products that offer protection for multiple diseases from each application. When problems arise, send a sample to a UF IFAS diagnostic lab and reach out to local Extension resources for additional help tailoring a management plan to address your specific disease concerns. 

 

CREDIT

Dr. PHIL HARMON, Professor, UF/IFAS

 

& DOUG PHILLIPS, Blueberry Extension Coordinator, UF/IFAS

ALGAL STEM BLOTCH

Algal stem blotch is a blueberry disease caused by the parasitic green alga Cephaleuros virescens Kunze, and it has become a significant disease on southern highbush blueberries in Florida. The alga is thought to enter the plant through natural wounds and openings, through pruning cuts, or by direct penetration of the cuticle. Early symptoms include small red blotches or lesions on green juvenile stems, with lesions expanding to form irregular cankers that can encircle canes. Bright-orange felt-like mats or tufts of algal growth appear from the lesions on young stems and older cane surfaces during summer and early fall when conditions are hot and humid. Leaves on symptomatic canes bleach white to pale yellow, and growth of the entire plant can be severely stunted as the disease advances. Leaf yellowing tends to occur on a few canes of each plant and is less uniform and blotchier than symptoms of nutritional deficiency. Algal stem blotch can lead to increased susceptibility to Botryosphaeria stem blight, in some cases leading to plant death. Plants that are stressed by abiotic or biotic factors are more susceptible to infection and subsequent disease development. To date, no systemic pesticide products have been found that will kill the algae living underneath the plant epidermis. Spray applications of copper-containing fungicides can help to reduce algal sporulation and protect healthy canes from infection for a few days after application. However, these applications do not impact existing symptoms or eradicate the disease. See Table 1 and EDIS publication PP344, “Algal Stem Blotch in Southern Highbush Blueberry in Florida” (https://edis.ifas.ufl.edu/publication/PP344).

ANTHRACNOSE LEAF SPOT, RUST, SEPTORIA, AND TARGET SPOT

Anthracnose, Septoria, rust, and target spot can cause premature defoliation, resulting in poor bud development and subsequent loss of yield. Fungicide timing for leaf spots varies across the state and by specific disease. Septoria can occur prior to harvest through late spring. Anthracnose leaf spots and target spots generally start postharvest and persist through summer. Rust is a problem in all Florida production areas. On susceptible varieties, rust can prematurely defoliate plants. Where leaves are not dropped in winter, rust can carry over on the previous year's foliage and can cause rust problems in early spring as well. Bravo Weather Stik® is labeled for control of both rust and Septoria leaf spots; this chlorothalonil product makes an excellent rotation partner for the strobilurin-containing products Abound® or Pristine®. However, Bravo Weather Stik® can only be used after harvest because chlorothalonil will damage fruit. See Table 1 and EDIS publication PP348, “Florida Blueberry Leaf Disease Guide” (https://edis.ifas.ufl.edu/publication/PP348).

AZALEA CATERPILLAR

The azalea caterpillar, Datana major, can be found in Florida from late summer to early fall on blueberries. Immature caterpillars are around 1/2 inch long and reddish to brownish-black with yellow and white stripes. Mature caterpillars are about 2 inches long and black with yellow to white stripes and a reddish head and legs. If left uncontrolled, a significant infestation can defoliate much of a plant. Although these caterpillars seldom kill the plants they feed on, the stress caused by defoliation can reduce flowering or fruiting the following spring. See Table 2.

BACTERIAL SCORCH (XYLELLA)

Bacterial leaf scorch, caused by Xylella fastidiosa, was first identified on blueberry in 2006 in the southeastern United States. Symptoms begin as a marginal-irregular leaf scorch, which may appear similar to symptoms of bacterial wilt or drought stress. Symptoms are initially observed on leaves attached to individual stems or groups of stems on one side of a plant. Plant vigor is reduced, stems and twigs of some varieties such as 'Meadowlark' turn a distinctive yellow color, and the plants eventually die. Diseased plants are typically observed randomly scattered throughout a field, rather than in distinct circles or groups within a row. Infected plants should be removed and destroyed. This bacterial pathogen is vectored by insects called sharpshooters and spittle bugs, including the glassy-winged sharpshooter. Infection levels may be reduced by controlling the vector with suggested insecticides.

BACTERIAL WILT (RALSTONIA)

Symptoms of bacterial wilt are similar to those of bacterial scorch, exhibiting signs of drought stress such as wilting and marginal leaf burn. Infected plants may also be more susceptible to developing severe symptoms of other stress diseases, such as Botryosphaeria stem blight, and therefore may show symptoms of both diseases. Crowns of plants with bacterial wilt have a mottled discoloration or light-brown to silvery purple blotches with poorly defined borders (distinct from the discoloration associated with stem, which is typically pie-piece-shaped and pecan brown in color). Wood chips floated in water from the crowns of plants with bacterial wilt will stream bacterial ooze (unlike plants with bacterial scorch or stem blight). Ralstonia can be spread easily in water, in soil, or through infected plant material, and infected plants may not initially show symptoms. Ralstonia can survive for years in soil, slowly spreading down and across rows of blueberry, leaving large circular patches of dead and dying plants. Where the bacterium is detected, remove and burn or deep-bury infected plants. Then, use soil drenches of products with phosphorous acids or salts to help protect surrounding plants or any replants from infection. See Table 1 and EDIS publication PP332, “Bacterial Wilt of Southern Highbush Blueberry Caused by Ralstonia solanacearum” (https://edis.ifas.ufl.edu/publication/PP332).

BLUEBERRY GALL MIDGE

The blueberry gall midge (BGM) is a tiny fly whose larvae feed on vegetative and floral buds. Blueberry gall midge will feed on leaf or vegetative buds, leaving young leaves deformed and misshaped. Gall midges lay eggs at any time during the growing season when the plants are making new flushes of growth. See Table 2 and EDIS publication ENY-997, “Blueberry Gall Midge on Southern Highbush Blueberry in Florida” (https://edis.ifas.ufl.edu/publication/IN1239).

CHILLI THRIPS

Chilli thrips are becoming a more pronounced problem in blueberry. Adults feed on blueberries in late spring to early summer shortly after the bushes are pruned. Chilli thrips feed primarily on young leaves, causing leaf bronzing and shoot dieback. During heavy infestation, the edges around younger leaves and stems are eaten and leaf curling occurs. The chilli thrips are smaller than the flower thrips and are approximately 0.04 inches long with dark fringed wings and dark spots across the back of the abdomen. Insecticides that can be used to manage chilli thrips include Delegate® (spinetoram), Assail® (acetamiprid), Sivanto® (flupyradifurone), and Apta® (tolfenpyrad). Conventional products that can be used to manage chilli thrips include Malathion and Sevin® (carbaryl). See Table 2 and EDIS publication ENY-2053, “Chilli Thrips on Blueberries in Florida” (https://edis.ifas.ufl.edu/publication/IN1298).

DIAPREPES (CITRUS ROOT WEEVIL)

Diaprepes larvae damage blueberry plants by feeding on the roots, including channeling and holes in the roots and feeding injury and girdling near the crown of the plant. Note that this damage is frequently observed some period of time after the larvae begin to feed on the roots, and at that point damage near the crown can appear similar to the effects of root girdling or mechanical wounding and abrasion. These injuries can kill or cause serious decline in blueberry plants and may also create an entry point for Phytophthora, causing a root rot infection. Management and control should target both the adult and larval stages. Control for adults consists of foliar insecticide sprays every 10–14 days when adults are active, beginning when 3 or more adults are found within 1-acre blocks. Larvae can be managed with insecticides either by directly drenching the soil area beneath the plant canopy or by applying them through drip or microjet irrigation systems. In addition, entomopathogenic nematodes may have potential for controlling root weevil larvae in blueberry. See Table 2 and EDIS publication ENY-999, “Diaprepes Root Weevil on Southern Highbush Blueberry in Florida” (https://edis.ifas.ufl.edu/publication/IN1241).

FLATHEADED BORERS

Flatheaded borers are a species of beetle in the Chrysobothris genus. The larvae tunnel through plant canes, ultimately killing the cane and acting as an entry point for disease. It is possible that adult beetles are attracted to stressed or damaged blueberry canes, with adult females laying eggs on the injured area, and larvae excavating tunnels just beneath the bark. This is a relatively new pest in blueberry, and research is ongoing regarding its life cycle, monitoring and trapping protocols, and control measures. See Table 2.

FLEA BEETLES

The blueberry flea beetle can cause serious damage during the summer months. Blueberry flea beetle eggs, and possibly adults, overwinter in the leaf litter of blueberry fields. Eggs are very small and orange-yellow in color, and hatching coincides with leaf bud opening. Larvae migrate to the foliage and feed on blossoms and leaf margins, giving the leaves a notched appearance. The larval stage takes 9–20 days to complete. Fully-grown larvae fall to the soil and pupate, with adults emerging approximately 15–28 days later. Adults are less than 0.25 inches in length, oval shaped, and a shiny copper bronze or metallic blue in color, and they chew small holes in the foliage. Adults mate and lay up to 200 eggs per female. The blueberry flea beetle has several generations per year in the southern United States. See Table 2 and EDIS publication ENY-411, “Insect Management in Blueberries in the Eastern United States” (https://journals.flvc.org/edis/article/view/116379).

IMPORTED FIRE ANTS

Ant baits employed in early spring as a broadcast treatment usually eliminate most but not all fire ant mounds within treated areas. Under high ant pressure, treating a second time in the fall provides better fire ant control. Most ant baits are slow acting and require up to 8 weeks to control active mounds because they interfere with reproduction, causing a gradual colony die-off. Extinguish® Professional Fire Ant Bait (0.5% methoprene) is labeled for use on all crop land sites. It is effective but somewhat slower acting than Esteem® Ant Bait (0.5% pyriproxyfen). In order for the bait to be effective, active ant foraging is essential. Worker ants must be attracted to baits so that they will carry the baits back to their colonies. Ideally, temperatures should be warm and sunny. Ant baits work best when the soil is moist but not wet. Avoid applying ant baits when conditions are expected to be cold, overcast, rainy, or very hot. Individual mound treatments are most effective when used as needed for the occasional colony that survives broadcast treatments. Mound treatments using insecticide baits should be applied in a circle 3–4 feet from the mound. Baits should not be placed directly on top of mounds so that mounds remain undisturbed. The colony will move the queen to safety if mounds are disturbed. See Table 2.

PHYTOPHTHORA ROOT ROT

Phytophthora root rot (PRR) is considered the most serious soilborne disease that affects southern highbush blueberries (SHB). Some SHB cultivars are considered tolerant and others highly susceptible, while rabbiteye cultivars are less affected by the disease. PRR is caused by the oomycete pathogen Phytophthora cinnamomi. Common aboveground symptoms associated with Phytophthora infections are reductions in plant vigor and premature fall discoloration. Symptoms at ground level and belowground include crown and root rots. Disease on susceptible hosts occurs when certain environmental conditions (primarily a saturated root zone and root wounding) trigger Phytophthora reproduction, infection and symptom development. PRR is typically more severe in low and poorly drained areas of a farm. The pathogen causes root discoloration (dark brown to black, instead of the normal cinnamon brown) and decay. Advanced stages of infection cause plant stunting, wilting, an abnormal or reduced root system, root rot, and plant dieback. Leaf discoloration also typically occurs, including yellowing, reddening with or without marginal burn, abnormal growth of new leaves, and defoliation. Plants affected by PRR may also be more susceptible to other dieback diseases including stem blight. Fungicides with the active ingredient mefenoxam, such as Ridomil Gold SL, are recommended where PRR occurs and are applied twice yearly (typically in January and June) through drip irrigation or as a band application directly to the bed. In addition to the phenylamide fungicide mefenoxam, which is applied to the soil, Aliette (Fosetyl-Al 80%) and numerous phosphorous acid products, referred to as “phites” or phosphonates, provide some control when applied as summer foliar sprays. See Table 1.

SCALE

Scale insects injure blueberries by sucking plant sap, inserting their mouthparts into a plant and remaining immobile throughout their lives. Signs of infestation are leaf yellowing (chlorosis), defoliation, fruit drop, sooty mold, branch dieback, or plant death. Soft scales, Coccidae, secrete a waxy covering over the body. They also secrete a large amount of sugary waste (honeydew) resulting in sooty mold, which can interfere with photosynthesis and slow plant growth. Of the soft scales, Indian Wax scale, Ceroplastes ceriferus (Fabricius), is the most prolific on blueberries in Florida. See Table 2 and EDIS publication ENY-2094, “Wax Scale on Southern Highbush Blueberries in Florida” (https://edis.ifas.ufl.edu/publication/IN1387).

SOUTHERN RED MITE

The southern red mite is the key spider mite pest attacking blueberry plants in Florida. Southern red mites primarily infest the lower side of the leaf, giving the leaf a bronzing appearance when the population is high. Southern red mites also produce a protective web made of silk over the infested surface to protect them from predators. The underside of leaves should be closely examined with a 10x hand lens for adults, shed skins, and webbing. Tapping foliage onto a sheet of white paper can also be used to find adult mites. A few miticides, including fenazaquin (Magister®), fenpyroximate (Portal®) and acequinocyl (Kanemite®), have recently been labelled for spider mites. See Table 2 and EDIS publication ENY-1006, “Mite Pests of Southern Highbush Blueberry in Florida” (https://edis.ifas.ufl.edu/publication/IN1284).

WHITE GRUBS (GRUBS OF ASIATIC GARDEN BEETLE, EUROPEAN AND MASKED CHAFER, AND ORIENTAL BEETLE)

White grubs are the larval form of certain beetle species, such as the masked chafer. They feed on blueberry roots, and damaged plants have the appearance of drought stress. It may take a number of years for larvae numbers to increase to a damaging level, although feeding injury on young plants can quickly result in symptoms and plant death. Masked chafer larvae are up to one inch long, with whitish bodies and brown head and legs. See Table 2.

YELLOW-NECKED CATERPILLAR

Yellow-necked caterpillars feed on the foliage of blueberry plants. Their bodies are covered with long, fine whitish hairs. The head is black, the area behind the head is yellow, and the body is covered with fine white hairs. Feeding by newly hatched caterpillars can skeletonize the foliage, leaving only the large leaf veins. In significant infestations, plant defoliation can occur. This can be minimized by pruning out infested stems. See Table 2.

 

Table 1. Disease Management Options 

 

 

 

Table 2. Insect and Mite Management Options