Pests And Diseases Of Lilacs
Diseases can lessen or stop blooming. An infection may have occurred the previous year and destroyed the
flower buds. In some cases, preventive or curative measures can be taken. While
dry rural gardens may have perfectly healthy lilacs, lilacs in big cities are
susceptible to leaf-roll necrosis. Lilacs in places with dry summers and cold
winters are more likely to remain disease-free than those growing where
conditions are humid and winters are mild.
Pest problems, especially scale or borers, can cause a lilac to bloom less. The common lilac
(S. vulgaris) and its many cultivars are among the most susceptible to pests.
Pests
- Lilac borers
- These troublesome grubs are the larvae of Podosesia syringae
var. syringae, a wasp like moth with translucent brownish wings. In late spring, it lays masses of eggs on stems
of lilac and ash. The hatching larvae tunnel into the branches and feed on the wood. The first time you'll
likely notice the infestation is when all the leaves on an entire stem or branch turn yellow and wilt, usually
in spring or early summer. Large branches may swell and crack. Look closely, and you'll see small holes
about the diameter of a pencil lead a foot or two (30-60 cm) above the ground. Directly underneath,
you'll see a small amount of sawdust. The small holes are exit wounds, so the perpetrators have already left,
but chances are, more are at work. When you prune,
you may see the borer tunnel penetrating the heartwood of older branches.
Borers are most common on stressed and wounded
plants and on large, older shoots, especially on S. vulgaris. There are pesticides labeled for lilac borer.
- Lilac leaf miners
- The larvae of a small moth species (Caloptilia syringella), leaf miners tunnel between the
layers of the leaves, making the leaves look blotched in early summer. They then roll the leaves and feed
externally. Leaves eventually turn brown, so the entire plant may look burned. When the leaves first show
signs of injury, they can be drenched with a nicotine spray or neem, but once the damage is noticed, it is
generally too late to take action. Remove affected leaves, and keep the area under the shrubs clear of
leaves in fall to prevent reinfection. The damage is more aesthetic than
physiological. S. vulgaris is especially susceptible to leaf miners.
- Mites
- The eriophyid mite (Aculus massalongoi) produces a silver or rust color on lilac leaves and may
cause some leaf-rolling.
- Oyster shell scale
- Scales are odd insects; they look like flat, oval, lifeless bumps. What they do is extract
fluid from the plant while they protect themselves under a waxy coating. They lay their eggs on the bark
in fall or spring, and the eggs hatch in late spring. The young scales, which are pale yellow or orange, are
mobile and only about 0.1 inch (2 mm) long. Once they find a place to feed, their legs wither. What you may
first notice, probably while pruning, are stems that look unusually gray arid roughened. Look closely, and
you will see small bumps that can be scraped off with a fingernail or the blade of your pruning shears.
- Lepidosaphes ulmi
- The type of scale that infests lilacs, can be controlled by pruning out the most
heavily infested branches and then applying a dormant-oil spray the next spring before the leaves have
developed. This treatment should take place before bud break on a dry, sunny, mild morning. To prevent injury
to the plant, do not apply within 48 hours of a frost or at a temperature lower than 39 degrees F (4°C). If
possible, choose a calm morning when the temperature is above 60 degrees F (15°C) and frost is not forecast for
that night. Scale can also be killed by a lime-sulfur solution painted directly on the branches or by direct
contact with a spray of soapy water: a teaspoon (5 mL) of an additive-free liquid dish soap such as Ivory to a
quart (1 L) of water. This soap spray is most effective in late spring and early summer, when scales are at the
crawler stage. Spray until the foliage drips.
Diseases
- Blight
- Symptoms of bacterial blight (Pseudomonas syringae var. syringae) generally show up early in the
season, during or directly after a spell of hot, humid weather. The onset can be dramatic. Leaves suddenly
look scorched along the edges, then young shoots blacken. Leaves may also have dark spots surrounded by
a pale halo. Flower clusters wilt and die. The same strain also infects pear, cherry, maple and other
ornamentals.
Another type of blight, caused by the fungus Ascochyta syringae, shows similar symptoms and spreads
up from the soil in the wet weather of early spring. These blights will not kill the
lilac but are unsightly
and can destroy much of the new growth. Depending on the timing of the infection, blooms may be lost this
season or next.
Control is difficult. Wounded plants and new growth are most susceptible. Provide excellent air
drainage to reduce frost damage and dry the foliage. Damage is worse in low-lying areas, where cold, damp
air can pool. If you can catch blight early, remove all infected parts by cutting into the healthy growth
below them, always sterilizing the pruning shears or saw between cuts with rubbing
alcohol,
Lysol or
a solution of 1 part household bleach to 10 parts water. Next year, use a
copper fungicide spray when spring
weather is warm and wet and thus favorable for infection. Avoid heavy pruning and excessive fertilizing,
practices that encourage rapid, weak growth. Early-blooming lilacs such as S. x hyacinthiflora and the
earliest S. vulgaris cultivars are most susceptible; singles are more susceptible than doubles; and plants with
magenta, purple and blue flowers are more susceptible than those with lilac colors. S. komarowii, S. x
prestoniae and S. villosa are also susceptible, although most late-blooming species and cultivars are more resistant.
- Mycoplasma
- Witches'-broom, a disease more common in large lilac collections than on backyard shrubs,
appears as a clump of very dense growth on an otherwise ordinary tree or shrub. First noted in 1951, the
disease is still somewhat mysterious, although it is known to be caused by a class of
virus like pathogens called MLO (mycoplasma-like organisms, or mollicutes). Apparently, the organisms colonize in the sap (phloem),
interrupting its flow and thus killing a growing point, which results in the dense growth of side shoots,
usually low down on the plant. The late-flowering lilacs, particularly S. x josiflexa and S. x prestoniae, are most
susceptible. The shrub may look unhealthy, have twiggy growth and uncharacteristic out-of-season
bloom or growth flushes and eventually die. A similar disease of
ash trees called ash yellows can infect
healthy lilacs and vice versa, probably via an insect. Pruning with sterilized tools is important. Dip or wipe
the blades in rubbing alcohol, Lysol or a solution of 1 part household bleach to 10 parts water before you
move to another plant. There is no known cure. Remove and destroy plants with obvious symptoms.
- Powdery mildew
- The disease is caused by a fungus (Microsphaera alni, M. syringae) that hits when the air is warm and
humid. It usually starts on older leaves toward the base of the plant in July and spreads throughout the foliage
from mid-August until the leaves fall in October. Leaves develop unsightly white or light gray patches,
then turn yellow and drop off. New growth may be stunted. The damage is more aesthetic than
physiological, as it usually occurs when the leaves have fulfilled their purpose.
The best defenses come at planting time. Give lilacs full sun and air circulation and, if
possible, choose resistant cultivars and species. Try to avoid wetting the foliage any more than necessary.
Irrigate deeply by watering directly on the ground. Most S. vulgaris cultivars and hybrids are susceptible; the
worst include 'Buffon,' 'Henri Martin,' 'Marlyensis' and 'Mrs. W.E. Marshall.' S. x chinensis 'Metensis' is
also susceptible. If your lilacs tend to become infected, grow them behind perennials such as
lilies and
ornamental grasses that are tallest when the lilacs are disfigured. Resistant species include S. x diversifolia, S.
emodi, S. pubescens subsp. julianae, S. meyeri, S. x persica and S. yunnanensis.
- Virus
- Ring spot produces yellow or target spots on the leaves, which drop early. The plant eventually dies.
There is no cure: affected plants should be destroyed.
- Wilt
- Although fairly unusual, verticillium wilt can affect lilacs planted in soil where
tomatoes, potatoes or
eggplants formerly grew. Sudden wilting may affect one branch or one side of the plant. Eventually, the
entire plant will die, so it should be removed. Wilt is
difficult to control, so take care when choosing a planting site.
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