Thanks to the introduction of many new rose cultivars with built-in blackspot resistance, this fungal problem isn't what it used to be. But what if you are nursing a rose that isn't quite so sturdy?
As the name suggests, blackspot appears as circular dark blotches surrounded by a ring of yellow on infected leaves. It cannot be cured, so it's best to remove and destroy infected leaves and canes. On still-healthy areas of the plant, take these actions to prevent Blackspot from completely owning your rose:
Because blackspot is a fungal disease that feeds on high humidity, it's important to monitor your watering habits. First, don't irrigate late in the evening before water has a chance to evaporate from leaf surfaces. Better yet, water at ground level.
Also, make sure there is plenty of air flowing around and through each plant. Fungus loves to grow in moist, stagnant air, so prune your roses periodically, especially after each bloom period. (Tempting as it may be to cut roses for a bouquet after a refreshing rain, you should always avoid touching plants when they're wet. This can also spread fungus spores.)
A teaspoon of baking soda per quart of water is a fairly effective fungus-preventing foliar spray. Some say spritzing milk on the plant works well too. My personal preference is a of spraying Neem oil, which packs a one-two punch as both an insecticide and fungicide, but isn't harmful to beneficial insects.