Viburnum Diseases & Insect Pests

Pesticides updated by Joey Williamson, HGIC Horticulture Extension Agent, Clemson University, 08/13. Images added, 08/13. Prepared by Joey Williamson, HGIC Horticulture Extension Agent, Clemson University. (New 12/07.)

HGIC 2057

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In general, most viburnums are relatively pest-free. Occasionally disease or insect pest problems do occur, and usually it is during those times that the plants are under stress or growing in less than ideal conditions.

Fungal Leaf Spots

A variety of leaf spots caused by the fungi Cercospora spp., Phoma spp. and Phyllosticta spp. and the disease anthracnose, that appears as black, sunken lesions and is caused by various fungi, are fairly common on viburnum foliage. These foliar diseases are not usually serious.

Fungal leaf spots on viburnum typically are angular to irregular-shaped, and the leaf tissue in the spots is sunken and dry. Spots may begin small, but enlarge or merge together, and may be reddish to grayish brown. Fungal leaf spots typically occur during warm, moist summer months, and initially will occur on older foliage.

Prevention & Control: Many foliar problems can be prevented by keeping leaves as dry as possible. Avoid overhead irrigation and improve air circulation with adequate plant spacing and selective branch pruning. Prune overhanging trees around diseased shrubs to reduce humidity levels and speed the drying of foliage. Hand remove spotted leaves on lightly diseased plants. Rake up and destroy infected fallen leaves. The removal of this leaf material will minimize the chances of the disease reoccurring the next season.

If chemical control is needed, most fungal leaf spots and anthracnose can be controlled with sprays of fungicides containing chlorothalonil, thiophanate-methyl, myclobutanil or mancozeb. Apply when symptoms first appear and repeat every 10 to 14 days as needed.

Algal Leaf Spot

Algal leaf spot, caused by Cephaleuros virescens, may occur especially during cool, moist conditions. Leaf spots start as small, pale green circular spots, and eventually become light brown or reddish brown with age. Often the spots appear raised and velvety with feathered edges. When the spots become reddish-brown, they are producing their reproductive structures called sporangia. These sporangia are spread to adjacent foliage by wind and splashing rain. This pathogen will overwinter in leaf spots.

Prevention & Control: Algal leaf spots are common on several ornamental shrubs and trees, including camellias, magnolias, azaleas, aucubas, gardenias and rhododendrons. Monitor the plants for disease problems, practice good sanitation as stated with fungal leaf spot control, and treat any other landscape plants in the area that have algal leaf spot.

Algal leaf spot can be controlled with sprays of copper fungicides (see Table 1 for specific products). Apply when symptoms first appear and repeat every 10 to 14 days as needed.

Powdery Mildew

Viburnum species may become diseased with powdery mildew, caused by the fungus, Erysiphe sparsa (formerly Microsphaera sparsa). The occurrence and spread of this disease is favored by a combination of warm days, cool nights, and humid conditions, but is inhibited by rain. Powdery mildew is worse on plants in the shade.

Powdery mildew of viburnum primarily affects young leaves and shoots. Affected plant tissues develop a powdery white to light gray growth of fungal mycelia. The fungus is mostly found on the upper leaf surface, but also may be found on the lower leaf surface. The disease typically appears in the summer and reaches its peak in late summer. Developing leaves may be deformed by severe infections.

Prevention & Control: Since high relative humidity is an important factor favoring disease development, certain cultural practices can help prevent the disease or decrease its severity. Sanitation and measures discussed for fungal leaf spot control will aid in powdery mildew control. Additionally, some cultivars, such as Viburnum burkwoodii 'Mohawk' and V. carlecephalum 'Cayuga' exhibit some resistance to powdery mildew.

Powdery mildew can be controlled with sprays of fungicides containing myclobutanil, triadimefon, propiconazole, thiophanate-methyl, triforine or horticultural oil (see Table 1 for specific products). To prevent foliar injury, apply horticultural oils for powdery mildew control only if temperatures are below 85 °F. Although sulfur is sometimes used for powdery mildew control, it is not recommended for sulfur-sensitive plants such as viburnums.

Downy Mildew

Downy mildew on viburnum is caused by the fungus, Plasmopara viburni. This foliar disease occurs and spreads rapidly during cool to warm weather conditions coupled with periods of leaf wetness. Initially, this disease appears as light green spots on the upper leaf surfaces. The spots enlarge to form angular patches between the leaf veins. On the lower leaf surfaces downy grayish white fungal growth appears.

Downy mildew disease is different from powdery mildew in that the fungal growth is observed on the lower rather than on the upper leaf surface. The infected areas redden and then turn brown as the leaf tissue dies. Infections of the foliage in the spring can result from splashing spores produced by the diseased foliage remaining on the ground from the previous year.

Prevention & Control: As with other foliar diseases, downy mildew can be prevented by keeping the foliage as dry as possible. When planting viburnum or other nearby plants, allow for adequate plant spacing. Prune back adjacent shrubs or overhanging tree limbs. These steps will improve air circulation around the plants and aid in the drying of foliage. Rake up and destroy infected fallen leaves.

If fungicides are necessary, sprays should adequately cover both the upper and lower leaf surfaces. Fungicides labeled to control downy mildew include mancozeb, chlorothalonil, or copper fungicides (see Table 1 for specific products).

Botryosphaeria Dieback & Canker

The fungal disease called Botryosphaeria dieback and canker, caused by Botryosphaeria spp., is most likely to occur on plants suffering from drought stress, bark injuries, pruning wounds or other environmental stresses. Healthy plants are much more resistant to infection by Botryosphaeria spp., as they will wall off the fungus and prevent its spread through the branch.

Upon entry via a wound, the fungus kills cambium and sapwood tissue, causing sunken dead areas called cankers. The cankers are small initially, but enlarge or coalesce (merge) into large areas that girdle the branch or trunk. Water movement is stopped beyond that point and results in a rapid wilting or browning of foliage. Branches with cankers may fail to leaf out in the spring.

Prevention & Control: Water shrubs weekly during the growing season if insufficient rainfall occurs (see HGIC 1056, Watering Shrubs & Trees). Mulching shrubs helps to avoid mechanical injury to trunk and limbs by weed trimmers and lawn mowers. Mulch shrubs with a 2- to 4-inch layer of bark, pine needles or ground leaves, and avoid piling the mulch against the trunk.

Prune any branches with cankers back to green healthy wood. If entire branches must be pruned, cut the limb just outside the branch collar and not flush with the trunk. Disinfest pruners between every cut with a 70% alcohol or 10% bleach solution. Dispose of all prunings, as this plant material is a potential source of disease for viburnums as well as other woody shrubs. No fungicides are recommended for the control of fungal cankers, but pruning wounds may be sprayed for protection with a benzimidazole fungicide, such as thiophanate methyl (see Table 1 for specific products).

Armillaria Root Rot

Armillaria root rot is also known as shoestring root rot, mushroom root rot, and oak root rot. It is caused by the fungus Armillaria mellea which is common in landscape and garden settings. This fungus can rot the roots of many different kinds of plants. Most often this disease is found on trees and shrubs such as oak, pine, rhododendron and dogwood, but hundreds of plant species, including viburnums, are susceptible. Typically the symptoms of this root rot occur over the whole plant. Above-ground parts of the shrub generally appear stunted and yellowed, and leaves may drop. The unhealthy foliage may become more sparse over a period of several years. However, there may be no evidence of any problems, and suddenly the shrub will die. The cause of the unhealthiness or death may be difficult to determine, as similar symptoms may be caused by environmental factors such as weather stress or a general lack of plant care.

Armillaria root rot (Armillaria spp.)
Armillaria root rot (Armillaria spp.)
Andrej Kunca, National Forest Centre – Slovakia, Bugwood.org

Armillaria root rot can be distinguished from other root rots, or from drought or excess moisture injury, by examining the crown (lower trunk) and upper roots of the plant. If Armillaria is responsible for the plants decline, white felt-like fungal growth can be seen under the bark if the bark is carefully peeled back. If sufficient bark is removed, the leading edge of the fungal growth will be found, and this white growth has a characteristic fan-shape. The Armillaria root rot fungus also forms black, string-like fungal strands about 1/16-inch in diameter or less. These strands may often be seen between the bark and the wood, or on the surface of the roots, or in the nearby soil. These string-like fungal strands are called shoestrings, and look very similar to roots.

Prevention & Control: Provide good growing conditions for the viburnum, especially additional water during droughts, good soil drainage, and proper fertilization.

An infected shrub whose entire root system or trunk is diseased cannot be saved. When a shrub dies from Armillaria root rot, the large roots in the vicinity of the trunk as well as the trunk itself should be removed and destroyed. Soil in the immediate vicinity should also be removed. Avoid replanting the same species as the one removed.

Aphids

Snowball aphids (Neoceruraphis viburnicola) most often occur on European cranberry bush and snowball viburnums. They can cause twisting and curling of the young growth. These aphid are gray to dark green and feed in clusters at the tips of the branches, causing leaf curl. They feed by piercing plant tissue and sucking plant sap.

Prevention & Control: They usually cause little or no appreciable damage. Viburnums can be sprayed with insecticidal soap or horticultural oil to control aphids. Soaps and oils must be sprayed onto the aphids to be effective. Spray the foliage thoroughly, including the upper and lower leaf surfaces. Repeat spray three times at 5- to 7-day intervals. Only apply horticultural oils or insecticidal soaps if temperatures are below 85 °F.

If higher toxicity insecticides are deemed necessary, sprays containing acephate, bifenthrin, carbaryl, cyfluthrin, esfenvalerate, lambda cyhalothrin, malathion, neem oil, permethrin, or pyrethrin will control aphids. Soil drenches or granular applications of imidacloprid, dinotefuran, or disulfoton will control aphids and last longer within the plant to prevent future infestations (see Table 1 for specific products).

Flower Thrips

Flower thrips (Frankliniella tritici) and various other thrips species are pests of viburnums. Thrips are slender, dark-colored insects, with fringed wings. Adults are less than 1/16-inch in length. To see these fast-moving pests, you need a magnifying lens. Thrips are typically found on leaves and between flower petals. Both adults and nymphs (immature insect stage that resemble the adult, but are smaller) feed by scraping surface cells to suck plant sap. Thrips feed on expanding leaves, which creates purplish red spots on the undersurfaces and causes foliage to severely curl or roll, then drop prematurely.

Flower thrips (Frankliniella spp.)
Flower thrips (Frankliniella spp.)
Jack T. Reed, Mississippi State University, Bugwood.org

When they feed on flower buds, the flower may die without opening. With a light infestation, their feeding causes leaves to have silvery speckles or streaks. With severe infestations, leaves and flowers are stunted and distorted and may turn brown and die. As a result of their small size, thrips are difficult to detect before damage is obvious. To sample for thrips on viburnum foliage, hold a sheet of stiff white paper under injured leaves, and then shake or tap the branch. Gently tip the paper to remove any bits of trash and then examine the paper in bright sunlight. Any thrips present will move around on the paper.

Prevention & Control: Several naturally occurring enemies feed on thrips. To avoid killing these beneficial insects which reduce thrips populations, insecticides should be avoided as much as possible. Grass and weeds in the area should be kept mowed or removed when possible.

If it becomes essential to spray an insecticide, the following are available in homeowner size packaging: acephate, bifenthrin, cyfluthrin, esfenvalerate, lambda cyhalothrin, permethrin, or spinosad. Spray when thrips are present and again in 7 to 10 days. Insecticidal soaps will help control thrips, but thorough coverage is necessary. The soap spray must contact the pest to be effective, and may require three sprays at 5- to 7-day intervals. Soil drenches or granular applications of dinotefuran, disulfoton or imidacloprid will give some thrips suppression. See Table 1 for specific products.

Spider Mites

The southern red mite (Oligonychus illicis) is a dark reddish or brown-colored mite that is a common pest in the eastern US. Azaleas, camellias and hollies are the primary hosts for this mite, but it has been found on many other species including viburnums. The southern red mite is active in the cool weather of spring and fall, and overwinters as eggs.

Mites have needle-like piercing-sucking mouthparts. Damage to the foliage begins on the lower leaf surface where feeding begins, but as populations increase, the upper leaf surface is fed upon as well. Over time the leaf tissue collapses, the foliage turns grayish-brown, and the damaged foliage drops.

Prevention & Control: Spider mites can be removed with strong sprays of water, if applied on a regular basis. Insecticidal soap or horticultural oil are the least toxic alternatives to beneficial insects, people and the environment and can provide control when applied before population numbers get too high. Both the lower and upper leaf surfaces must be sprayed for good control. Two or three applications may be required at 7- to 14-day intervals. To determine if additional spray treatments are needed, shake or tap branches over a piece of white paper. Then look for reddish-brown specks that move around.

The following pesticides are labeled for use by homeowners against spider mites: fenbutatin oxide or tau-fluvallenate (see Table 1 for specific products). These miticides should be applied when mites are present and again in 7 to 10 days.

Scale

Armored scales, such as the oystershell scale (Lepidosaphes ulmi), can infest viburnum and cause branch dieback. If the infestation is severe enough, they may kill the shrub.

The oystershell scale can overwinter as full grown females that are attached to the bark, or as eggs that are beneath the adult scale covering. The adult female is 1/8-inch long, brown or gray, and generally the shape of an oyster shell.

Oystershell scale (Lepidosaphes ulmi)
Oystershell scale (Lepidosaphes ulmi)
Whitney Crabshawm Colorado State University, Bugwood.org

Crawler (immature stage) activity often coincides with the flush of new plant growth in the spring. These crawlers are pale, smaller than a pinhead and are the only mobile stage of the scale life cycle. Within a few hours the crawlers will settle in a suitable spot to begin feeding and excreting a waxy covering for protection.

Prevention & Control: Light infestations of scale can be scraped off by hand. Prune out and dispose of any heavily infested branches. A horticultural oil can be sprayed in the early spring before new growth begins to kill overwintering adults and eggs. Horticultural oil may be sprayed when temperatures are between 40 and 85 °F.

Monitor the crawler emergence in the spring with sticky cards, double-faced tape wrapped around a branch, or by putting an infested shoot or leaf into a baggie and watching for crawler movement. Spray with horticultural oil in the spring after the plants begin growing and the danger of cold weather has passed. Repeat this application after 10 days to better control the crawlers, adults and eggs by smothering them.

Avoid using more toxic insecticides unless the plant is seriously damaged from the scale infestation. These insecticides will often kill the naturally occurring predators of scale. If insecticides are going to be used, spray when crawlers are observed, as this is the only stage in the life cycle that is controlled by contact insecticides. Insecticides labeled for homeowner use against scale crawlers include acephate, malathion, cyfluthrin and carbaryl (see Table 1 for specific products).

Root Weevils

Root weevil adults, such as the black vine weevil (Otiorhynchus sulcatus), feed on foliage, and the injury shows up as ragged notches on the leaf edges. Although the foliar damage is not generally severe, it can be very unsightly. The adult weevil is a black, wingless weevil about 3/8-inch long. The adults are noctural feeders and move up onto the plant at sundown. On sunny days they are found in the leaf litter beneath the viburnum canopy.

Root weevil larvae cause damage by chewing and girdling roots, and damage usually begins in spring to early summer and continues through the growing season. The larvae are white, legless grubs with a brown head and C-shaped appearance. They are found in the soil around the viburnum roots.

The grubs cause more significant damage than do the adults. Plant growth may become stunted, and the foliage may become pale green or yellow. Root and crown feeding by the grubs may kill the shrub.

Prevention & Control: For successful control, treatments should be directed at the adult weevils. Once foliar damage is observed, insecticidal sprays should be made at 2- to 3-week intervals, where three sprays are usually sufficient. Along with the foliar applications, also treat the surface of the soil or mulch immediately beneath the plants (as a spray, not a drench) because this is where the adults will hide during the day. Insecticides labeled for adult weevil control include bifenthrin, acephate, permethrin, and cyfluthrin. A soil drench around the base of the shrub with imidacloprid can be used to control the larvae (see Table 1 for specific products).

Table 1. Insecticides & Fungicides for Viburnum Insect Pest & Disease Control
Active Ingredient Brand Names & Products
*RTU = Ready to Use (pre-mixed spray bottle)
**RTS = Ready to Spray (hose-end applicator)
***Drench = Add to water and pour around base of plant 1Other products containing acephate include: Ortho Systemic Insect Killer Concentrate, and Ortho Orthenex Garden Insect & Disease Control Concentrate.
2Other products containing imidacloprid include: Bayer Advanced Dual Protection Azalea, Camellia & Rhododendron Insect & Disease Control Concentrate (Drench), and Bayer Advanced 3-in-1 Insect, Disease & Mite Control Spray Concentrate (also in RTU).
3Other products containing permethrin include: Spectracide Immunox Plus - Insect & Disease Multi-Purpose Concentrate.

Note: As with all pesticides, read and follow all label instructions and precautions.
Acephate1 Ortho Japanese Beetle Killer Concentrate
Bifenthrin Ortho Bug-B-Gon MAX Lawn & Garden Insect Killer Concentrate
Hi-Yield Bug Blaster II Bifen Concentrate
Bayer Advanced Rose & Flower Insect Killer RTU*
Carbaryl Garden Tech Sevin Concentrate (also in RTS** and RTU*)
Bayer Advanced Complete Insect Killer for Gardens RTU*
Chlorothalonil Hi-Yield Vegetable, Flower, Fruit & Ornamental Fungicide Concentrate
Ferti-lome Liquid Fungicide
Ortho Garden Disease Control Concentrate
Copper Hi-Yield Copper Fungicide
Camelot Fungicide/Bactericide Concentrate
Bonide Liquid Copper Concentrate
Cyfluthrin Bayer Advanced Garden Power Force Multi-Insect Killer Concentrate
Bayer Advanced Vegetable & Garden Insect Spray Concentrate
Dinotefuran Spectracide Systemic Rose & Flowering Shrub Insect Control Fertilizer
Concentrate (with 10-8-8 Fertilizer) (Drench***)
Spectracide Systemic Tree & Shrub Insect Control & Fertilizer
Concentrate (with 10-8-8 Fertilizer) (Drench***)
Disulfoton Bayer Advanced Garden 2-in-1 Systemic Azalea, Camellia &
Rhododendron Care RTU Granules (with 16-8-8 Fertilizer)
Esfenvalerate Ortho Bug-B-Gon MAX Garden & Landscape Insect Killer Concentrate
Fenbutatin oxide Ortho Systemic Insect Killer Concentrate (with Acephate)
Ortho Orthenex Garden Insect & Disease Control Concentrate (with Acephate & Triforine)
Horticultural oil Ferti-lome Horticulture Spray Concentrate
Lily Miller Superior Type Oil Spray Concentrate
Southern Ag Parafine Horticulture Oil Concentrate
Bonide All Seasons Spray Oil Concentrate
Imidacloprid2 Merit 2 Granular
Bayer Adv. Garden Tree & Shrub Insect Control Concentrate(Drench***)
Ortho Tree & Shrub Insect Control Granule
Insecticidal soap Bonide Multi-Purpose Insect Control Soap Concentrate
Safer Insect Killing Soap Concentrate; & RTU
Schultz Safe Insecticidal Soap Insect Killer RTU*; Concentrate
Natural Guard Insecticidal Soap Consentrate
Lambda cyhalothrin Spectracide Triazicide Once & Done Insect Killer Concentrate (also in RTS**)
Mancozeb Bonide Mancozeb Flowable with Zinc Concentrate
Malathion Ortho Malathion Insect Spray Concentrate
Southern Ag Malathion Insect Spray Concentrate
Spectracide Malathion Insect Spray Concentrate
Myclobutanil Spectracide Immunox Multi-Purpose Fungicide Concentrate (also in RTS**)
Neem oil Southern Ag Triple Action Neem Oil Concentrate
Ferti-lome Rose, Flower & Vegetable Spray Concentrate
Green Light Rose Defense Concentrate
Bonide Bon-Neem Insecticidal Soap Concentrate
Green Light Neem Concentrate
Green Light Neem II RTU (with Pyrethrins)
Ferti-lome Triple Action Plus with Neem Oil RTU*
Permethrin3 Bonide Eight Insect Control Vegetable, Fruit & Flower Concentrate
Bonide Borer-Miner Killer Concentrate
Hi-Yield Kill-A-Bug II Concentrate
Bonide Eight Yard & Garden RTS**
Bonide Eight Garden & Home Insect Control RTU*
Propiconazole Banner Maxx Fungicide
Ferti-Lome Liquid Sytemic Fungicide
Bonide Infuse FungicideConcentrate; RTS
Pyrethrin Southern Ag Natural Pyrethrin Concentrate
Spectracide Garden Insect Killer Concentrate (with Pipernyl Butoxide)
Bonide Garden Insect Spray Concentrate
Spinosad Green Light Lawn & Garden Spray Spinosad Concentrate; & RTS**
Southern Ag Concentrate Naturalyte Insect Control Concentrate
Tau-fluvalinate Bayer Adv. 3-in-1 Insect, Disease & Mite Control Concentrate (also in RTU*)
Thiophanate-methyl Cleary's 3336
Ferti-lome Halt Systemic Fungicide
Southern Ag Thiomyl Systemic Fungicide Concentrate
Triadimefon Southern Ag Bayleton 50 of T&O
Triforine Ortho Rose Pride Rose Pride Disease Control Concentrate

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This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.