Long Island lilacs dusty mold
Trimmed lilacs after blooming in spring. New growth has been healthy. Late September leaves covered with white dust. Good sun, plenty of air circulation. Spraying with commercial insecticide. Aware of impatiens mold problem on LI this year. Lilacs were close to impatiens that died. Could they have been affected? Treatment for impatiens was to bag and rid of in garden so as not to spread disease in soil.
Certified GKH Gardening Expert
It sounds like powdery mildew. This article will help: https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm