What is the rust colored powdery substance coming on the base or lower branches of our Knockout roses?
The Gardener’s Answer
Hello, Bill in North Carolina: As far as roses go, Knockout roses are quite disease-resistant. Like all other plant material, if these shrubs are not given ideal growing conditions they are more susceptible to insect and disease problems. They are sun lovers and should receive a minimum of six hours of direct sunlight each day. They prefer to grow in well-drained nutrient-rich soil. Good air circulation is important as well. If your rose is not growing in these conditions, you might consider moving it to a location where it will be happier. From what you have described, it sounds like your rose is suffering from rose rust caused by a specific fungus (phragmidium). What appears as a rust color powder is actually a cluster of fungal spores. It typically first appears on the foliage and then spreads to the canes. All infected parts should be pruned out and disposed of. Be sure to clean your pruners after you have finished to prevent disease spread. Avoid overhead watering and only water early in the day so the foliage that does get wet has time to dry before nightfall. Depending on how severe the infection is you can use a fungicide to control further spread. You can always take a sample to your County Cooperative Extension Service for the horticulture agent to look at. They will be able to give you a positive diagnosis and control options.