With summer comes lilac leaf spots, and common lilac often sees a lot of leaf damage starting in summer and intensifying into September. Common diseases that cause these problems include the fungal pathogen Pseudocercospora, causing leaf spots and blight. Many lilacs across the state also experience issues with Lilac Pseudocercospora Leaf Spot, often in combination with more common diseases. Historically, Septoria leaf spot and powdery mildew also contribute to leaf blight, and in some cases, to abundant leaf drop.
Environmental conditions, humidity, and stressors like inconsistent soil moisture, saturated soils, shaded locations, or lack of proper pruning exacerbate these problems.
Recommended Management Practices
Fungal leaf spots alone rarely become severe enough to cause the decline or death of the plant. Removing and destroying leaf debris and pruning out dead branches are the best strategies for reducing diseases in general for shrubs and trees.
Fungicide treatments should not be considered late into the summer and fall. If repeated severe infections occur year after year, preventative spring to early summer fungicide applications may help prevent diseases (but won't eradicate them). Because of this and the fact that lilacs tend to be large and difficult to effectively spray fungicide on, fungicide applications are rarely warranted.
These problems are good reminders that good care and pruning can help improve shrub health in general. Learn more in this publication: Pruning Ornamental Shrubs (PDF).
Providing good, consistent care by making sure shrubs get at least 6 hours of direct sunlight a day and watering during drought periods can help lilacs thrive.
Consider renovating your established lilacs. This will increase air circulation (reducing foliar fungal diseases), improve appearance, and, as an added benefit, promote better flowering.
Other Lilac Problems (not as common)
While Pseudocercospora, powdery mildew, and Septoria are abundant this year, your lilac may be experiencing other issues. Use this information to evaluate your shrub and take recommended actions.
Phytophthora Shoot Blight
Lilacs perform well in clay soils, but slow water drainage leads to excess moisture in the root zone and is prone to root rots. With root rots, shrubs develop less defined foliar symptoms (no spots nor mildew on top) and tip die-back. Learn more in this article: Phytophthora Shoot Blight on Lilacs.
Mechanical Injury
When assessing your lilacs, it is also good to examine the stems and trunk for evidence of any mechanical injury that may be caused by animals, mowing equipment, or Lilac/Ash borers.
Verticillium Wilt
Lilacs are also susceptible to verticillium wilt, where the leaves will look droopy and water-deprived, change from green to yellow to brown, and drop. with progressive branch wilting at a slower pace. Learn more in this article: Verticillium Wilt.
Lilac Bacterial Blight
Earlier in the season we may also see lilac bacterial blight. In early spring when the weather is cool and wet, the bacterium Pseudomonas syringae can infect newly emerging shoots, flower buds, and leaves on many lilac varieties, including Chinese, Japanese, Persian, and common lilac. Learn more here: Lilac Bacterial Blight.
Contact the Plant and Insect Diagnostic Clinic with More Questions
If you need help with your plant problems, the PIDC is happy to help. Directions for submission can be found here: How to Submit a Sample.