With summer comes lilac leaf spots; at the UMN Plant Disease Clinic this year, we have seen the usual fungal pathogen Pseudocercospora causing leaf spots and blight (rapid death). Due to the unusually wet summer, many lilacs across the state are experiencing issues with Lilac Pseudocercospora Leaf Spot. This pathogen can contribute to leaf drop. Environmental conditions, humidity, and stressors like drought or excessive moisture, shaded locations, or lack of proper pruning can be exacerbating 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 may be the best strategy in reducing diseases in general for shrubs and trees.
Fungicide treatments should not be considered late into the summer. If repeated severe infections occur, 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. Pruning Trees and Shrubs is a resource to help with that. 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.