Lilac Summer Woes

This summer has been tough on lilacs (Syringa). There have been repeated reports across the state of dying plants, brown and curled leaves, white speckled leaves, and general demise of the bushes. As a rule, lilacs are relatively disease free, so what is causing the problem?

Lilacs are native to Eastern Europe, but were introduced to the United States in mid-1750’s. They are a beloved garden favorite with over 1800 cultivars available in a vast array of colors, bloom periods, fragrances, and plant sizes. No longer are gardeners limited to the simple lilacs of their grandmother’s day. There are few pests and diseases that effect these stunning beauties, but this year the ones that do cause problems seem to be proliferating.

Last year south central Wisconsin suffered from drought conditions. This was not beneficial to any plants, lilacs included.  This was followed by excessive and abundant rain this spring. Abundant rain is a good thing, right?  Not when it is excessive, and not when the humidity is also unbearably high.  When the ground is saturated such that the air pockets that are normally in the soil are all filled with water, it creates an unfavorable condition for many plants, leading to root problems. Lilac roots in particular are susceptible to rot if they are allowed to stand in perpetually wet soil.

Another common problem with lilacs is powdery mildew.  If the plant’s leaves look like they have been sprinkled with powdered sugar, this is the culprit.  Fortunately, this problem is purely cosmetic.  It is most often a problem during a hot humid year.  The best defense is to plant resistant cultivars, prune and space properly so that there is adequate air flow between and within plants, and clean up all debris in the fall.  Treatment with fungicides is not recommended.

Of a more serious nature is bacterial blight.  This disease is more of a problem early in the season, especially is the spring is wet and cool. At first the leaves will look healthy and a short time later they will develop dark black streaks as if a strong flame has been held near them.  Dieback of twigs and blossoms will also occur.  To control this disease, prune (during dry weather) infected branches 4-6 inches below obviously dead areas, disinfecting pruners after each cut.  Destroy the plant debris; do not compost it. To help avoid this disease, do not fertilize late in the season and do not overfertilize.  A soil test will help determine the appropriate amount of fertilizer needed.

Leaf scorch was a significant problem this year. Because of the excessive rain, saturated soil suffocated portions of the root systems. This resulted in an inability of the plant to support vegetation. Leaves quickly dried up, but remained on the plant. Many of these bushes will recover next year, but flowering may be affected.

To maintain a healthy lilac, prune them appropriately. Most of the lilacs that suffered this year were poorly pruned or not pruned at all. Cut out one-third of the branches larger than one- and one-half inches in diameter at ground level using loppers or a pruning saw.  This will maintain good air flow and allow younger, new growth to flourish.  The reward will be a healthy plant and a spectacular flower show.

Carol Shirk

Certified Master Gardener