Lilac bush

Question

Why do the leaves on my Lilac bush curl & wrinkle after blooming period ends ?

Answer 1

Many types of powdery mildew only cause vague symptoms, including curled or distorted leaves, long before the characteristic white powdery fungal bodies appear, making early identification tricky. Longer-standing infestations may cause leaves to die and drop early.
https://www.gardeningknowhow.com/ornamental/shrubs/lilac/powdery-mildew-on-lilac-bushes.htm
https://www.gardeningknowhow.com/ornamental/shrubs/lilac/common-lilac-problems.htm