By Bev Johnson

Master Gardener

Lilacs are usually tougher than woodpecker lips. They will grow in most any kind of soil and bloom every spring. They seem to thrive on being ignored. Often the only thing left to tell there once was a house out in the country is a lilac shrub blooming all by itself. They are a tough plant; however, climate change is raising particular heck on them.

Brett Arnez, director of the University Plant Disease Clinic says experts have seen or suspected six lilac problems in the past few years. They are Septoria leaf spot, verticillium wilt, phytoplasma, pseudomonas, lilac borers and powdery mildew.  The most common is Septoria. This is a fungal disease that turns lilac leaves yellow and brown before dropping off. It mostly attacks the common lilac although the Japanese tree lilac may also be susceptible.

This disease usually affects only part of the lilac. Some branches will look healthy while the rest will have browning leaves and dieback. If we have a wet summer the symptoms can show up as early as July although it is usually not until August. So, what to do to save your shrub? First prune off any affected branches. Since this is a fungus, it is persistent. Rake up all debris under the shrub. Spores of this fungus can persist for years in leaf and twig litter and can spread to other lilacs. 

Bag or burn this litter then mulch under the shrub to keep any spores hiding under there, just waiting to infect the shrub next year, from getting that chance. If the lilac has significant leaf blighting, you may need to treat it with a fungicide. Don’t wait until the leaves are already turning brown, get it early.

Verticillium wilt can affect lilacs and other shrubs although not as much as Septoria. A good thing as there is no cure for it. If you suspect you have this problem, don’t send a twig, send a good-sized hunk of a branch for proper analysis. This fungal disease blocks access to water and nutrients to the shrub. Again, sanitation is important to prevent this disease. If your shrub dies, replace it with a resistant variety.

The three other diseases, Phytoplasma, caused by parasites, and pseudomonas, a shoot or blossom blight are here but are not common. Both conditions cause brown dying leaves. So far, lilac borers haven’t shown up in Minnesota. In other words, don’t sweat these three—yet but do keep a lookout for them.

Powdery mildew seems to show up on our lilacs every summer. It looks really icky but is usually not very serious. It usually shows up in warm humid weather. The common lilac is quite susceptible. Planting lilacs in a sunny area with good air circulation, and proper pruning will help keep this stinker down.  Or plant Korean lilacs, they are resistant. Do remember resistant doesn’t mean a plant can’t get a disease. It means it is harder for the disease to infect that plant. 

Powdery mildew affects quite a few of our plants. Here is a homemade spray to try.  Add a cup of whole milk to enough water to make a gallon. Add a squirt of sticker; liquid dish soap works well for this. Spray on affected leaves. This will not kill the mildew. It does prevent it from continuing to grow. Apparently, the milk solids kill the fungus. Apply early in the morning or late in the evening so it doesn’t promptly evaporate.