Solution

• A mixture of 4 tsp. of baking soda, 2 tbsp. os horticultural oil and 1 gallon of water combined in a spray bottle or hose-end sprayer is effective at controlling powdery mildew on roses, according to Good Earth Organic Rose Care. This formula is based on research conducted by Dr. Kenneth Horst at Cornell University.
  
  

Application

• For effective treatment, gardeners should spray the roses at least once a week. Soak the leaves, including the undersides thoroughly, first thing in the morning so they dry completely before nightfall.
  
  

Preventative Treatment

• Powdery mildew thrives in dry, warm climates and attacks new lush growth. Frequently dousing rose leaves and stems with water increases humidity, thwarting powdery mildew. Gardeners should wash roses in the morning so leaves can dry during the day. Sometimes over-fertilizing can promote powdery mildew growth. Good Earth Organic Rose Care advises waiting until after new growth emerges to fertilize, if powdery mildew is a problem.