Answer: What you describe sounds like powdery mildew, a fungal disease. It is quite common on all types of shrubs. Try using a homemade baking soda/oil spray: mix 1 tsp. baking soda and 2.5 tsp. summer oil (a lightweight horticultural oil) in 1 gallon of water. Spray this once per week, covering both upper and lower leaf surfaces. A commercial organic alternative is Soap Shield Fungicidal Soap. To avoid, try to provide full sunshine, good air circulation all around the plant, and ample soil moisture. You can remove the leaves affected with powdery mildew so it won't spread to healthy leaves. Good luck with your euonymus!
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