We have had a number of contacts regarding lilacs that were virtually defoliated with leaves showing many spots on the leaves. Cercospora leaf spot appears to be the causal agent.
This is one of those diseases that only shows up during wet, humid summers. Though the shrubs look bad, loss of leaves this late in the season will not harm otherwise healthy plants.
Therefore, there is no need to apply a fungicide. Also, an application this late would not help anyway. However, cleaning up dead leaves will reduce the amount of inoculum for next year and is recommended.
This disease is weather-dependent and requires wet, humid weather at certain times to flourish. Therefore, lilacs that were hammered this year may not become infected next. Even if we do have another wet summer that encourages this disease, we do not recommend a fungicide application as damage to the plant is negligible.
Lauren Fick is the Horticulture Extension Agent for the Cottonwood Extension District. She is at firstname.lastname@example.org and 785-628-9430 or 620-793-1910.