Diagnosing White Powder on Dieffenbachia: Mealybugs or Spider Mites

A large Dieffenbachia developed a white powdery substance and was losing large leaves. The owner had moved it from a plant light to a natural light area with no improvement, watering weekly.

Pest Identification

• Spider mites: Tiny white dust-like particles; may produce fine webbing.
• Mealybugs: Resemble small pieces of white cotton.

Treatment

• Spider mites: Spray thoroughly with soapy water (liquid dish soap).
• Mealybugs: Use a solution of 5 parts soapy water to 1 part rubbing alcohol.

Coverage is critical: drench all leaf and stem surfaces until dripping. Repeat as needed.

Underlying Issues

Leaf loss is likely due to insufficient light, not pests. Dieffenbachias need to be very close to a moderately sunny, uncovered window. A plant light alone is inadequate. Water only when the top quarter of soil is dry—weekly watering may be too frequent in low light.

Expert advice from Will Creed, Interior Landscaper, Horticultural Help, NYC.