Powdery Mildew

• Powdery Mildew
  
  Powdery mildew has a white, powdery appearance and prefers warm days and cool nights. The fungus prefers humid conditions. Crowded plantings or damp, shaded locations are also ideal conditions for development.
  
  

Downy Mildew

• Downy mildew appears as thick, grey fuzz and is highly destructive. It can cause rapid, severe defoliation. It is not as common as powdery mildew and is active only in cool, damp conditions.
  
  

Black Spot

• Black Spot Disease
  
  Black spot appears as circular black spots with irregular edges, surrounded by yellow tissue. Plants with a severe case may lose all of their leaves if not treated. The disease develops in periods of high humidity or excessive rain and can be spread by splashing water, garden tools, clothing or your hands.
  
  

Rust

• Rust is most prevalent in the western United States. Like other diseases, rust on roses in most prevalent in humid, cool conditions. Leaves develop reddish orange bumps on their upper surface.
  
  

Prevention

• Good sanitary habits help prevent the spread of these diseases. Mulch your plants to prevent infected material from coming into contact with the soil and discard the mulch at the end of the season. Do not compost. Pick up any infected leaves and discard in the trash. Choose a location with plenty of sunshine and sufficient air circulation.
  
  

Treatment

• If you detect fungi, treat with a fungicide. Gardeners have also had some luck treating their plants with baking soda. Add 1 tbsp. of baking soda and 1 tbsp. of horticultural oil to 1 gallon of water and spray.