Knockout roses are an extremely popular and relatively new plant, but they are susceptible to a host of common plant diseases. Introduced in 2000, they are described as easy to grow and maintain. Knowing how to prevent and treat the plant diseases below is crucial to raising big, beautiful knockout rose bushes.
A fungal disease, Black Spot is, as the name suggests, an outbreak of dark circles with yellow edges on the leaves and stems. Black Spot is not curable, but it can be cut back to prevent the spores from spreading. If untreated, Black Spot will kill a knockout rose bush.
Orange powdery dust that first appears on the underneath of leaves, Rust is another fungal disease that spreads in moist air. Rust commonly occurs when the weather is frequently wet, leaving moisture on the rose bush that dries on the foliage and flowers.
White instead of black or orange, this fungal disease coats the leaves and stems (and sometimes even the flowers) with baby powder-like dust. Without treatment, Powdery Mildew stunts the bush's growth and the foliage dies and falls off.
Fungus-contaminated water causes these sores that swell and kill the foliage at the spot of the reddish-brown injury and everything above it. The fungus infects the cane through cuts in its skin.
Keeping moisture away from the knockout rose's leaves, stems and flowers is the simplest way to prevent these fungal ailments. Don't water in the evening and only apply moisture to the base of the plant. If one of these diseases shows itself, cut off the diseased areas and thoroughly clean away the trimmings from the soil around the base. Clean the pruning shears, too. Lastly, a few organic anti-fungal treatments like citrus oil can help, but avoid chemical applications if at all possible.