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Protect your roses from fungal diseases
Roses have always been prone to diseases. Two in particular to look out for are powdery mildew and black spot. Unfortunately, once black spot appears on a leaf, nothing can be done to undo the damage. The best thing to do is to cut off the diseased leaves and spray with a fungicide to keep the new leaves healthier. In the future remember to water at the base of the plant, water sitting on the foliage is an invitation for disease.
Generally speaking, powdery mildew becomes less of a problem after the arrival of summer which typically brings with it long, hot, sunny days. The mildew problem will go away with improved weather conditions. There are a few options for correcting the problem. An application of horticultural oil should smother the spores and reduce spread of the problem. It is best to try this as soon as possible upon visible symptoms. An early sign of powdery mildew is a slight curling upwards of the foliage. You could also try trimming back the worst affected areas and wait for new, clean growth to flush out.
Ultimately, the best way to avoid fungal diseases is to plant a disease resistant rose bush. Ask your local garden center to direct you towards the best plants. Any member of the Knock Out® Family of Roses and Drift® Roses will have excellent disease resistance. The best way to avoid spraying and treating your rose for diseases is to plant the right variety.