Knockout roses were bred to resist disease and insects, but unfortunately they are not immune to issues. Inspect your knockout roses regularly to catch problems as early as possible. The various problems that can cause yellowed leaves on roses are treatable if caught soon enough.
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Inspect the yellowed leaves carefully. Use a magnifying glass to examine the underside of yellow leaves for tiny, red or white insects. If the yellowing begins as speckling, sap-sucking spider mites are the likely culprits. Physically remove the insects when possible; many insecticides are equally, if not more, fatal to the insects that feed on mites as they are to the mites themselves.
Examine the leaves that are not yet yellow and the stems for fungus. To identify black spot fungus, look for black spots on leaves that are visible to the naked eye; yellow leaves are an advanced symptom of the disease. Dispose of the infected parts of the plant in a way that prevents release of the spores.
Monitor the soil moisture carefully. Roses do not like wet feet, nor do they handle drought well, and yellowed leaves can be a symptom of either. Most rosarians suggest growing rose gardens in raised beds; these allow gardeners to keep a steady flow of water in, while allowing excess moisture to drain out quickly.
Examine the patterning of the yellow on the rose leaves, as certain mineral deficiencies manifest as different yellow formations on the leaves. Yellow tips indicate a copper deficiency, while solid yellow suggests a nitrogen deficiency. Yellow with green veins indicates an iron deficiency when younger leaves are impacted, or a magnesium deficiency when older ones turn yellow first.