Although Knockout Roses are engineered rose shrub varieties that are designed to be low-maintenance, cold-hardy and disease-resistant, they remain susceptible to a wide array of problems. Knockout Roses with yellowing leaves may be suffering from disease, insect infestation or a cultural problem. Determine what is causing the leaves to yellow before attempting to treat the shrub. Yellow leaves on a Knockout Rose shrub can indicate a problem as severe as an incurable disease or as simple as a lack of water.
Diagnose rose mosaic virus by looking for yellow line patterns, or mottled or ring spots on the Knockout Rose leaves, often coupled with weak growth and decrease in blooming. No cure exists for rose mosaic virus, but it isn't contagious.
Treat the Knockout Rose for windburn or drought by watering the shrubs deeply two or three times per week and spreading a 2- to 3-inch-thick layer of bark mulch on the ground over the rose shrubs' root area. Knockout Roses affected by windburn or drought will have leaf edges that turn mottled yellow to crispy brown.
Look for the Knockout Rose shrubs' leaves turning yellow and dropping off to diagnose nutrient deficiencies. Feed the Knockout Rose with a good all-purpose rose fertilizer that contains acronutrients and trace minerals such magnesium, either as a granular soil feed or a foliar-feed spray, according to the directions on the label.
Troubleshoot spider mites infesting the Knockout Rose by first looking for speckled, yellowing, drying, dying and dropping leaves. A fine webbing may form on the undersides of the leaves during warmer months. Spray the shrub with a strong stream of water from a hose two or three times per week to reduce spider mite populations and their damage.