Do not compost infected plant debris. Temperatures often are not hot enough to kill the fungus. Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection. If cultural controls fail to prevent disease buildup or if the disease pressure is too great, an application of a fungicide may be necessary.
Chemicals are most effective when combined with cultural controls.
Follow the instructions on the fungicide label for use on specific plant species, varieties, rates to be used, timing of applications, and waiting periods before harvest. Researchers at Cornell University have discovered the fungicidal properties of this combination against powdery mildew on roses. Applications of 1 tablespoon baking soda plus 2.5 tablespoons of Sunspray oil in 1 gallon of water are still experimental. Use it at your own risk. 1 Colorado State University Cooperative Extension plant pathologist and horticulturist, Integrated Pest Management, Jefferson County.