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Possible diseases of Lilacs

By Melanie Mathieson
Gardening Guru

You have followed all of the advice of the Gardening Guru but your lilac shrub seems to have something wrong with it. Is it a pest or a disease? This column will assist you in determining what disease may be bothering your shrub.

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 as leaves suddenly look scorched along the edges, then the young shoots blacken. The leaves may also have dark spots surrounded by a pale halo and 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 circulation to reduce frost damage and dry the foliage by planting your lilacs in the open and well slaced. 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 HYPERLINK “” alcohol, Lysol or a solution of 1 part household bleach to 10 parts water. Next year, use a HYPERLINK “” copper fungicide spray (available at garden supply stores) when the spring weather is warm and wet and thus favourable for infection. Avoid heavy pruning and excessive fertilizing, practices that encourage rapid, weak growth. Early-blooming lilacs 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.

Witches’ broom, is a disease that is more common in large lilac collections than on backyard shrubs but can still happen. The disease appears as a clump of very dense growth on an otherwise ordinary tree or shrub. It is known to be caused by a class of virus like pathogens called mycoplasma-like organisms (MLO), 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. Late-flowering lilacs are most susceptible to this disease. The shrub may look unhealthy, have twiggy growth and uncharacteristic out-of-season bloom or growth flushes and eventually die. A similar disease of HYPERLINK “” 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 appears 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 defences 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 lilac cultivars and hybrids are susceptible but there are some resistant species now available. Ensure all leaf litter is cleaned up and disposed of – do not compost it.

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 and ensure all leaf litter is cleaned up and disposed of – do not compost it.

Although fairly unusual, verticillium wilt can affect lilacs planted in soil where HYPERLINK “” tomatoes, HYPERLINK “” 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.

The next column will help you determine if it is an insect pest bothering your lilac.