I pruned my overgrown lilacs aggressively about a month ago and now they look terrible. The leaves are turning brown and falling off the shrubs. Did I make a mistake pruning them so hard after they had flowered?
— Brendan Kamp, Bannockburn
Lilacs form flower buds in late summer for next year’s flowers, so your timing should be good to preserve flowers for next year, even though it reduced the size of the shrubs significantly. It’s best to do this pruning right after the lilacs finish flowering, so you were a bit later than ideal, but there’s nothing to worry about. Your description sounds a lot like powdery mildew, which is an easy disease to recognize, and it is common in the landscape at this time of year. My lilacs at home are showing the same symptoms as yours, and I did not prune them this summer. Powdery mildew affects annuals, perennials, shrubs and even some trees, like maples. Powdery mildews are caused by fungal pathogens that are specific to a host and thrive when the days are warm to hot with cool nights and high humidity. Just because one type of plant in your garden has powdery mildew does not mean it will spread to other plants in your garden. Generally, powdery mildews are more severe on plants that are crowded or growing in a shady site with poor air circulation. It is best not to use mildew-prone cultivars, especially when a site is shady.
The first disease symptoms are distinctive white or grey powdery patches on leaves, buds, flowers, fruits or stems, which you probably would not have noticed. These patches can grow until they cover the entire leaf on one or both sides with a felt-like white mildew growth. Leaves are the most affected part of a plant. Generally, just the tops of leaves are affected. Small spherical, dark brown to black fungal structures may be seen mixed in with the mildew. You can also find infected leaves that are stunted, curled and chlorotic in more severe cases of powdery mildew. Leaves can turn brown and drop off plants early, hastening fall dormancy while leaving plants that look dead late in the season. Look for buds on your lilacs to help ascertain whether the plant is dead. I bet you will see healthy buds along the branches of your lilacs.
Damage from powdery mildew is mostly aesthetic and, in most situations, will not harm the long-term health of the plant unless the infections are severe year after year. If any of your perennials are completely brown, go ahead and cut them back if you do not like their appearance, but leave your lilacs alone. Death of infected plants is rare. The flowers may be reduced in number and quality due to this disease and plants will look unsightly in this growing season. Powdery mildew covering the leaves may also lower the plant’s photosynthetic efficiency, resulting in reduced growth. Infections have also been associated with increased winter injury to perennials. Severely affected perennials and annuals can be cut back to “restart” them if you notice the disease earlier in the season. Fungicide treatments are an option for plants that consistently show significant damage. My approach is to replace mildew-prone plants with more resistant cultivars or with completely different plants. Also, consider putting the plants in an area of the garden that has better air circulation. Powdery mildew is common on lilac, bee balm, phlox, squash, cucumbers and zinnia. I use David phlox and raspberry wine bee balm, both of which have some resistance to powdery mildew. As the cucumbers and squash with powdery mildew stop production, the vines should be removed, but it is okay to plant them again next year. Most years, there is powdery mildew on these plants in the late season in my garden. Your lilacs will most likely be okay next year without taking any special actions. Some years, my lilacs have had an ugly outbreak of powdery mildew and other years they are fine. It’s another thing gardeners learn each year in our gardens.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at [email protected]. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.