Although often touted as "super" roses with inviolable immunity, knockout roses can fall prey to the same problems that other roses experience. However, when grown in the right environment and given the proper care, knockout roses are more resistant than the average rose to pests and disease. If your knockout roses have problems that crop up season after season, ask a knowledgeable horticulturist about proper knockout rose care and environment or more reliable cultivars.
Powdery mildew is a common fungus that attacks roses that live in warm, humid conditions. Knockout rose plants that are afflicted with the fungus look like they have been dusted with white powder, and some of their leaves may look crinkled or deformed. To kill powdery mildew, remove the affected foliage and then treat your knockout roses with an organic fungicide according to the manufacturer's instructions.
Black spot is another rose fungus that is often due to watering overhead or prolonged periods of rain. When a knockout rose's foliage or flowers remain wet for over 8 hours, black spot fungus can take root on the foliage. Affected leaves are covered in 1/2 inch light yellow spots that have slightly fringed edges. Treat black spot by removing affected foliage as soon as the fungus is identified. Then, treat the plant with a dormant oil and sulfur spray according to the manufacturer's instructions.
Although the little spots that show up on the underside of a knockout rose's leaves may look like rust, rose rust is actually a fungus. It is easily treated by removing the affected leaves and then spraying the plant with a dormant oil and sulfur spray.
Aphids are a common rose pest that can become prob. Their presence is often signified by the sticky, shiny honeydew that they deposit on your knockout rose's foliage. Or by the black mold that grows on the honeydew when it is left on the plant. The easiest way to get rid of aphids is to introduce predatory insects like gall midges. They are available in easy-to-apply spray forms like that offered by Just Aphid Killer.
The colorful Japanese beetle most often attacks knockout roses in the Eastern United States. Japanese beetles can be identified by their copper shells, metallic green heads, and large numbers. To keep them from devouring your knockout roses, simply pick them off by hand and drop them in a bucket of soapy water.
Spider mites announce their presence by the webs that they spin all over your plant. Rid your knockout roses of the unsightly menace by spraying the plant with an organic mite and insect spray.