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 usually do not recommend a fungicide application as damage to the plant is negligible. However, if you would like to use a fungicide as a preventative next year, myclobutanil is labeled. Myclobutanil is found in Immunox; F-Stop Lawn & Garden Fungicide and Monterey Fungi-Max. (Ward Upham)