See:
Maple (Acer spp.) - Leaf Scorch
Cause OSU Plant Clinic records contain many samples of lilac with an abiotic leaf scorch. It is clearly a nonpathogenic problem as no pathogens were recovered from disease tissue. Diagnosticians speculated about possible causes including sunburn, heat stress, drought, physiologic, and excess salt (from fertilizers). Sun or heat related stress will occur primarily on the south side of plants.