Lilac White Coating: Powdery Mildew or Cottony Scale?

Distinguishing the Two

White powdery coating on lilac branches that rubbed off on a dog is unlikely powdery mildew (which doesn't transfer in clumps). This suggests cottony scale (Pulvinaria spp.)—insects producing white waxy egg sacs that easily dislodge.

Simple Field Test

Pick at the white material:

• Powdery mildew: Rubs off as dust; no insects underneath.
• Cottony scale: Reveals oval, fluid-filled insects; sacs are wet/sticky.

Treatment by Diagnosis

If powdery mildew: Usually cosmetic on lilacs; ignore or spray sulfur/fungicide at first sign next spring.

If cottony scale: Apply imidacloprid soil drench (systemic) or horticultural oil at crawler emergence (late spring). Prune heavily infested branches.

Key Insight

Powdery mildew is annual and harmless; cottony scale weakens the plant and spreads. The dog test is a reliable differentiator.