Treating Common Lilac Problems: What To Do For Pests And Diseases Of Lilac

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By: Kristi Waterworth

Shakespeare memorialized the sweet smell of the rose, but obviously he hadn’t so much as sniffed a lilac, the undisputed perfumed queen of the spring. These beautiful, hardy bushes are a great addition to your landscape because they tend to be easy to care for, and the problems with lilac bushes are mostly minor. Even so, it’s best to be prepared if you have a run in with lilac pests and diseases, so we made up a list of common lilac problems you may encounter.

Common Diseases of Lilacs

Although lilacs are a hardy bunch, they can succumb to problems like any other landscape shrub. Be on the lookout for these diseases:

Bacterial blight – The bacteria Pseudomonas syringae causes early shoot and branch dieback, distorted leaves and leaf spots that start out olive green, but soon develop water soaked areas. Those spots turn brown with yellow margins and begin to die. Blossoms may become limp or turn brown suddenly. Pruning away the diseased material and thinning the inside of the shrub is the best way to control this disease, but if the infection is widespread, a copper fungicide will help kill it quickly.

Powdery mildew – Powdery mildew is probably the most common problem in lilacs. It’s caused by a variety of fungal pathogens that result in leaves with a powdered appearance, either in tightly organized spots or spread across the surfaces. Increasing the air circulation around infected leaves is the best treatment, so make sure to thin your plants yearly.

Leaf spots – Leaf spots are another fungal problem caused by a variety of pathogens. When you see tan spots appear on your lilac leaves, with or without causing the leaves to fall, you’ve likely got one of the many leaf spot diseases on your hands. As with powdery mildew, this problem is a result of high local humidity, so thin that shrub and clean up all fallen debris to prevent future infections.

Common Lilac Pests

Lilacs attract just a few serious pests, most of the caterpillars and leaf miners that may visit aren’t anything to be worried about. However, if either of these pests appear, it’s time for action:

Scales – Scales can be difficult to detect. Many species look like cottony or waxy growths on the stems and branches of landscape shrubs. If you lift their covers though, you’ll find very small, brightly colored insects underneath. Scales are best treated with repeated applications of neem oil, spaced seven to 14 days apart. When they’re clustered together in one section of the plant, pruning them out is an excellent option.

Borers – The larvae of the clearwing moth is a boring insect that prefers to feed on lilacs. These tiny caterpillars spend most of their lives inside the stems and branches of your plant, only emerging to mate. Effective management centers around keeping the lilac healthy and happy, since sick plants are much more likely to attract borers. They have a number of natural enemies that will pick them off when the lilac is stronger and less appealing.

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Problems With Lilac Bushes - Learn About Common Lilac Pests And Diseases - garden

Lilacs are hardy plants. Most insect pests do not bother them to any serious degree. For occasional insect problems, such as aphids or borers, treat with an insecticidal soap or a mild insecticide, like Sevin. Ants will appear, sometimes in large numbers, during the blooming period. They are seeking the sweet nectar in the flower, and do no harm to the flower or plant.

Lilacs are susceptible to Cicadas, which occasionally emerge in huge numbers, in areas east of the Mississippi. If large broods emerge in your area, you will want to cover them with 1/4" pest netting during the emergence.

    Mice and voles (see moles and voles) are one of the biggest pests of lilacs. During hard winters, they will chew on the bark of the stems at or near ground level, and can kill the plant. They harbor under the mulch you amply provided, and feed on your plant, especially in the harshest of winters.

    Lilacs are susceptible to a couple of plant diseases. Most common is powdery mildew. Powdery mildew occurs most frequently during hot and humid weather. Treat any outbreak early. Apply a general purpose fungicide two or three times about a week apart, as soon as the problem is spotted. For major outbreaks, trim away infected branches and dispose of them.. Do not add them to the compost heap. While this disease can cause major problems with more tender fruits, flowers and vegetables, it will generally not cause long term or serious problems for your bush. Powdery mildew's unsightly appearance is the biggest negative for Lilacs.

    Buy Lilac Bushes select from popular Lilac varieties

    Prune out infected twigs and branches only after dry weather returns. Lilac blight is easily spread in damp weather. Prune the branches 6 inches below the infected parts, and dip the pruning shears in a solution of one part chlorine bleach to three parts water between cuts to disinfect the blades.

    Rake up all leaves and plant debris on the ground. Discard or burn all branches and debris. Do not compost them or leave them on your property.

    The best bacterial blight treatment is spraying lilac bushes in the fall with a fungicide containing copper sulfate. Spray them again in the spring before bud break.

    Forgo applications of nitrogen fertilizer in the spring if lilac blight is a concern. In most cases, lilacs need only a small amount of fertilizer, if any, in the spring when new growth emerges. Too much fertilizer reduces blooms and can promote disease and insect problems.


    Symptoms of lilac tree diseases tend to be characterized by their visual effects on the tree. For example, leaves may appear scorched and brown around the edges. They may also develop a variety of dark rings and spots. Flowers also tend to wilt and die. Additionally, some diseases cause a white, black or gray substance to take over the plant, spreading along branches and onto leaves and flowers. You may also notice that parts of your plant are turning black or have developed large, lumpy growths.

    How to recognize, treat and avoid lilac bacterial blight

    CORVALLIS, Ore. - Do new buds and branches on your lilac look blackish, like they've been scorched by a blowtorch? Your bush might have a bacterial plant disease called lilac blight.

    A cool, wet, rainy, spring season favors development of lilac blight, especially if rains follow a late frost or winter injury. Oregon State University Extension plant pathologists are warning that this might be a favorable year for the disease.

    Actually known to plant pathologists by the complete name of "lilac bacterial blight," this disease is caused by a bacterium Pseudomonas syringae pv. Syringae. The same organism is the source of bacterial blight on pear, blueberry, cherry, maple, and many other woody plants and the symptoms of lilac blight are similar in appearance to fire blight in fruit trees.

    At first, leaves look perfectly healthy and then a short time later they look as though someone has placed an open flame near them. The dark black streaks on one side of young shoots show the progression of the disease. The flowers will wilt and turn brown and unopened flower buds become blackened.

    Do not fertilize late in the growing season. Do not over fertilize young plants.

    Lilac blight is difficult to control and it is recommended that you buy blight-resistant varieties whenever you plant new lilacs.

    It also helps to space and prune your lilac plants so they are not rubbing against each other and air can circulate freely between the plants. Do not fertilize late in the growing season. Do not over fertilize young plants because high nitrogen favors disease development, explained Melodie Putnam, OSU Extension plant pathologist.

    If your lilac bush does have infection, prune and burn all infected parts as soon as you notice them. A spray of copper sulfate during the early spring each year should help prevent the problem before the buds begin to break.

    It also helps to space and prune your lilac plants so they are not rubbing against each other and air can circulate freely between the plants.

    Lilac blight bacteria over-winter on diseased twigs or on healthy wood. Factors that weaken or injure plants – wounds, frost damage, soil pH, poor or improper nutrition and infection by other pathogens – predispose them to the disease.

    Sources of this disease can include old cankers, healthy buds, leaf surfaces and nearby weeds and grasses. Wind, rain, insects, tools and infected nursery stock spread the bacteria.

    Some species of lilacs have shown resistance in western Washington including S. josikaea, S. Komarowii, S. microphylla, S. pekinensis, and S. reflexa. Most cultivars of S. vulgaris are susceptible, but some have been observed with less disease when planted in a garden those include 'Edith Cavell', 'Glory', 'Ludwig Spaeth', and 'Pink Elizabeth'. Note that 'Ludwig Spaeth' is highly susceptible under intense nursery production systems.

    If your lilac bush does have infection, prune and burn all infected parts as soon as you notice them. Spray copper sulfate during the early spring.

    The disease starts as brown spots on stems and leaves of young shoots as they develop in early spring. A yellow halo may also be around the spot. Spots become black and grow rapidly, especially during rainy periods. Further infectious development depends on the age of the part of the plant attacked.

    On young stems, infection spreads around the stem and girdles it so the shoot bends over at the lesion and the parts above it wither and die. Infections on mature wood occur only on cherry trees, not on lilacs.

    Young, infected leaves blacken rapidly starting near the margin and continuing in a wedge-shaped pattern down to the petiole. Eventually the entire leaf dies. On older leaves, spots enlarge slowly. Sometimes, several spots will run together, and the leaf may crinkle at the edge or along the mid-vein. Flower clusters also may be infected and rapidly blighted and blackened. Buds may fail to open or may turn black and die shortly after opening. Symptoms are similar to those of winter injury.

    To see photos of this disease, visit OSU Extension's PNW Plant Disease Management Handbook. The handbook also reiterates these cultural controls for treatment:

    • Maintain adequate spacing between plants and prune to provide good air circulation within the canopy.
    • Prune out and burn all affected tissues immediately.
    • Plant resistant species or cultivars.
    • Do not fertilize late in the growing season. Do not over-fertilize young plants.

    Growing lilacs for Minnesota landscapes

    Lilacs are a favorite, long-lived, spring-blooming shrub. They are primarily grown for their fragrant flowers ranging from white to pink to deep purple.

    Lilacs are native to Eastern Europe and Asia. The genus Syringa comes from the Greek word syrinx meaning pipe or tube and refers to the lilac’s stem which can be hollowed out.

    In the landscape, lilacs make excellent hedges, foundation plants, specimen plants, large borders and group plantings. They bloom for 10 to 14 days, depending on the weather, and have a distinctive sweet fragrance.

    Lilacs also provide habitat for small birds and pollinating insects.

    Growing lilacs

    • Hardiness zone: 3 to 7 depending on the species.
    • Full sun (6+ hours) less sun reduces flowering and increases powdery mildew.
    • Moist, well-drained soil avoid wet soils with poor drainage.
    • Soil pH 7.0 to slightly alkaline. Have your soil tested by the U of M Soil Testing Lab.
    • Space new plants based on mature size.
      • Planted too close, lilacs will require more pruning and will reduce bloom.
      • Proper spacing increases air circulation and reduces powdery mildew.
    • Once the lilac is established (2 to 3 years after planting), fertilize every few years with an all-purpose shrub fertilizer.
      • Avoid applying lawn fertilizers near or around lilacs as the high nitrogen content will increase leaf production and decrease flower production.
    • Water after planting. Make sure the soil drains easily and does not pool.
      • Poor drainage can result in stunted growth, fewer flowers and root rot.
    • This video (28:44) from U of M Morris gives selection, planting and growing information.

    Like other spring blooming shrubs, lilacs develop flower buds for the next year after blooming during the current year. Lilacs should be deadheaded immediately after blooming to encourage good bud development and flowering the following spring.

    • Deadhead: Use a hand pruner to cut off dead flower heads down to a pair of leaves, or use a hedge trimmer for larger plantings. Shear lightly, taking off only the dead flower heads.
    • Renewal pruning: Renewal pruning allows more light throughout an older plant and encourages new stems to grow and flower. Newly planted lilacs usually do not need pruning for 2 to 3 years and it will take 1 or 2 years before a lilac blooms. Years later, lilac stems can grow very large and tree-like and they will tend to shade out new growth at the plant base.
      • Use a lopper or hand saw to remove a third of the thickest stems at the base.
      • Do this every year until all large stems are removed.
    • Rejuvenation pruning is a technique for smaller, densely-branched lilacs like Korean or Meyer lilacs (Syringa meyeri) that have become overgrown.
      • In late winter, use a saw or hedge trimmer to cut the entire plant to the ground.
      • New growth will grow from the root area in spring.
      • The plant may not bloom or sparsely bloom for 1 or 2 years.

    • Powdery mildew is common and cosmetic only choose disease resistant cultivars.
    • Lilac borer is the larvae of the clearwing wasp.
    • Bacterial blight is more common in cool, wet weather.
    • Causes for a lack of flowers:
      • Plant is not yet well-established. After planting, a lilac’s energy reserves will be used for developing a strong root system rather than flowers or leaves. Some blooming may begin in years 2 and 3. Be patient and continue to care for your lilac.
      • Plants are not receiving enough sunlight. Make sure your lilac has 6 or more hours of full sun per day. If the lilac has become shaded by another plant over time, consider pruning the other plant.
      • Too much nitrogen fertilizer is being applied causing the plant to put energy into growing leaves, not flowers. Don't apply lawn fertilizers (high in nitrogen) around the base of lilacs. Plant lilacs in garden beds, not lawns.
      • Pruned at the wrong time of year and cut off flower buds. Lilacs set flower buds for the next year right after blooming is finished, so prune immediately after flowering.
    • For more about lilac issues and solutions see What’s wrong with my plant?

    Species and cultivated varieties of lilacs for Minnesota

    The following section is not a complete list of cultivars available commercially. For a wider selection:

    • Visit your local garden center. Have a list of plant features you want in a lilac, especially the size of your planting space.
    • Use our plant selection program. It is free and takes just a few minutes to set up and confirm your user account. Instructions can be found at Plant Elements of Design.
    • Read Lilacs for Cold Climates by Dr. Laura Jull, associate professor in horticulture, University of Wisconsin Extension.

    The following lilac species and cultivated varieties are organized by bloom time (early to late). Plant sizes are listed height x width. Hardy in zones 3-7 unless otherwise indicated.

    Early Spring (April to mid-May)

    Syringa x hyacinthiflora, Early flowering lilac, Hyacinth lilac: 7-12 ft x 8-10 ft. Good for foundation plants. Disease resistant. Cultivars: ‘Excel’, ‘Maiden’s Blush’, ‘Pocahontas’

    S. oblata, Early lilac, Broadleaf lilac: 8-12 ft x 8-10 ft. Compact form. Disease resistant. Cultivars: ‘Betsy Ross’, subsp. dilatata Korean early lilac, ‘Cheyenne’

    S. x chinensis, Chinese lilac: Zone 4a. 7. 8-12 ft x 8-12 ft. Prolific blooms, dense leaves and flowers down to the ground good for a large screen. Diseases: powdery mildew, bacterial blight. Cultivars: ‘Saugeana’, ‘Lilac Sunday’

    S. x laciniata, Cutleaf lilac, Cutleaf Persian lilac: Zone 4b. 5-6 ft x 5-8 ft. Rounded, open form, little pruning needed. Disease resistant.

    S. meyeri ‘Palibin’, Meyer lilac, Palibin lilac, dwarf Korean lilac: Zone 4a. 4-6 ft x 4-6 ft. Forms a good, low screen. Disease resistant. Cultivars: ‘Palibin’

    S. x persica, Persan lilac: 4-8 ft x 5-8 ft. Sterile, no seed capsules produced after blooming. Disease: powdery mildew, bacterial blight.

    S. pubescens ssp. patula, Manchurian lilac: 5-7 ft x 4-6 ft. Good for foundation plants. Cultivars: ‘Miss Kim’, ‘Klmone' Miss Susie TM

    S. vulgaris, common lilac, French lilac: 8-15 ft x 6-12 ft. Good for hedges, living fences, tall borders, wildlife. Diseases: powdery mildew, bacterial blight some cultivars are resistant. Cultivars: ‘Sensation’, ‘Mme. Lemoine’, ‘Wonderblue’, ‘Ludwig Spaeth’, 'Beauty of Moscow’, ‘Primrose’, ‘Alba’, ‘President Grevy’, ‘Edith Cavelle’, ‘Katherine Havemeyer’, ‘Charles Joly’, 'Mrs. W.E. Marshall’, ‘Mme F. Morel’, ‘Montaigne’.

    S. baibelle PP12,294, Tinkerbelle® lilac: 4-6 ft x 4-5 ft. Disease resistant. Developed by Neal Holland, Sheyenne Gardens, ND. Other plants in the Fairytale® series introduced by Bailey Nurseries (MN): S. Baillina, ‘Thumbelina®, S. Bailming, Prince Charming®, S. Bailsugar, Sugar Plum Fairy®, Baildust, Fairy Dust®.

    S. x prestoniae, Preston lilac: Late blooming, more red in flower color. Cultivars: ‘Miss Canada’, ‘Minuet’, ‘Hiawatha’, ‘Donald Wyman’

    Watch the video: Top 5 Most Beautiful Lilacs.

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