Identification

• Host-specific fungi cause powdery mildew, although the symptoms are the same in all plant species. Certain types of fungi cause powdery mildew to occur in lilacs but not in rose or dahlias. Powdery mildew fungi develops when the nights are cool and the days are warm but not hot. High humidity is an important ingredient in powdery mildew development. Lilacs are prone to powdery mildew development, as are dahlias, roses, dogwood, flowering crabapples, phlox and zinnias.
  
  

Effects

• Lilacs with powdery mildew develop a white coating of fungus that looks similar to powdered sugar or talcum powder. Powdery mildew fungi causes the leaves, stems and flowers to become almost completely covered in this white fungus. Young, tender leaves are especially susceptible to powdery mildew fungi, often developing leaf curling before the fungus is noticeable. Lilacs with severe powdery mildew infections often develop yellow leaves, bronzed leaves and distorted flowers. Powdery mildew does not usually cause lilac to die, but it greatly diminishes the beauty of this plant.
  
  

Cultural Controls

• Powdery mildew can be prevented by cultural control methods. Avoid planting lilacs in shaded areas, as this promotes high humidity and powdery mildew fungi. Space your plants far enough apart to promote good air circulation. Provide your plants with water early in the morning, allowing your lilacs to completely dry before nightfall. Avoid using overhead watering systems or wetting lilac foliage when watering. Remove any infected plant debris from around your lilacs, as this may promote the spread of disease.
  
  

Chemical Control

• Some lilacs become infected with powdery mildew each year and suffer no serious long-term effects. Lilacs that develop severe powdery mildew may need applications of fungicides to control symptoms. Fungicides are most effective if applied when the first symptoms of powdery mildew appear. Spray your lilacs on a regular schedule, especially if the weather conditions are favorable for powdery mildew. Cover the entire plant completely and follow the instructions on the product label for the most effective means of disease control.