Powdery white on the leaves is powdery mildew, a fungal disease. White powder on the soil could be a number of things. Most likely is a saprophytic fungus and not harmful. Powdery mildew on leaves is, however, a disease that needs to be controlled. Powdery mildew spores do not need water for germination and are most severe in warm, dry climates. Summer’s warm days and cool nights create favorable humidity needs for spore growth as do shady areas. The spores are spread by the wind and can overwinter on plants and plant debris.

Prevention is the most effective strategy for controlling powdery mildew. Plant in full sun, avoid overcrowding to provide good air flow, and limit nitrogen fertilizer to slow the production of succulent tissue. Water at soil level to avoid wetting foliage which could increase relative humidity. Choose resistant plant varieties whenever possible. Powdery mildew is more severe on shaded plants. It is favored by the high humidity found in crowded plantings and damp locations.

• Hand-pick and destroy mildewed leaves to control small amounts of infection.
• Rake and destroy fallen leaves year-round to reduce infection source. Do not compost diseased materials.
• Do not overwater or overfertilize plants, as the fungus prefers succulent new growth.
• Neem oil is an effective spray.