Powdery Mildew on Lilac: Identification, Life Cycle, and Control Options

A lilac has a white powdery substance on leaves—classic powdery mildew. The expert provides a detailed breakdown: it’s a host-specific fungus (won’t spread to unrelated plants), thrives in humid spring/fall, and overwinters as mycelium or sexual spores. On many trees, it causes little damage and needs no control.

Fungicide Options (if needed)

• Propiconazole (Banner Maxx)
• Myclobutanil (Immunox, Eagle, Systhane)
• Sulfur (spray or dust; avoid above 90°F)
• Triforine (Funginex)
• Triadimefon (Bayleton, Strike)
• Fenarimol (Rubigan)
• Copper fungicides

Cultural Management for Large Trees

• Rake and remove fallen leaves off-site to reduce spores.
• Improve air circulation by pruning.
• Avoid overhead watering late in the day.

For large lilacs, sanitation is often more practical than spraying.