The brown markings look like lilac leaf blight and this is a fungal infection encouraged by rain and damp weather. Clean up any fallen leaves and remove any infected leaves if they are easy to reach. Good air circulation is key so move the lilac if it is not out in the open. Most often the lilac will still bloom and grow and the disease just shows up when rainy spring weather appears. You can try using Neem oil or a fungicide spray following instructions if the disease continues to spread. Keep growing, Marianne Binetti