White coating = powdery mildew. The hard brown "mushroom" at the base is unrelated—likely a saprophytic fungus on mulch. Same conditions (damp, warm days/cool nights) favor both.
Spray any rose fungicide (neem oil, sulfur, potassium bicarbonate) per label. Improve air flow. Water at base, not foliage. Mildew is cosmetic on Knock Outs—they'll outgrow it.
Diagnosing Holes in Knock Out Rose Leaves: Slugs vs. Caterpillars
Spindly, Curly Rose Leaves in Everett, WA: Herbicide Drift Diagnosis
Tiny White Holes in Knockout Rose Leaves: Rose Slugs (Sawfly Larvae)
Identifying and Controlling Scale Insects on Knock Out Rose Stems
Controlling Aphids on Roses: Identifying Oily Leaves and Effective Sprays
Rescuing Knockout Roses from Transplant Shock and Fertilizer Burn
Overwintering Knock Out Roses in Raised Planters: Mulch and Fertilizer Timing
Treating Powdery Mildew on Knockout Roses Naturally
Rooting Florist Roses: Low Success but Possible
Holes in Knock Out Rose Leaves: Night-Feeding Insects
Reviving Neglected Rose Bushes After Years Without Water
Saving a Broken Rose Topiary: What to Do When the Stem Snaps
Transplanting Knockout Roses in Zone 5: Timing and Winter Protection
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