Diagnosing White Patches on Lilac: Powdery Mildew vs. Insects

Question: My 'Mme Lemoine' lilac has recurring white patches on limbs—not scale, possibly whiteflies—that return despite neem oil and insecticidal soap sprays. Second outbreak this summer. Help!

Answer: Likely Powdery Mildew, Not Insects

First, verify: Are the white patches living insects or a fungal coating? Lilacs are highly susceptible to powdery mildew, a fungal disease favored by damp, humid conditions. It appears as white, powdery spots on leaves and stems—not moving insects.

If Powdery Mildew (Most Likely)

• Apply a copper-based fungicide (e.g., Soap Shield, available from Gardens Alive)
• Many effective fungicides sold at garden centers
• Improve air circulation: prune crowded branches, avoid overhead watering
• Preventative sprays in early summer work best

If Confirmed as Insects (Whiteflies, Mealybugs, etc.)

• Use pyrethrum (organic, chrysanthemum-derived)
• Non-toxic to humans/pets; effective against most insects
• Available at garden centers or online (Planet Natural, Gardens Alive)

Correct diagnosis saves time and money—treat the right problem!