Knock out roses are prized for being tough. They're named Earth Kind roses because they are disease-resistant, drought-tolerant, hardy and relatively hassle free. Their plentiful blooms, mounding shape and self-cleaning habit -- they drop their spent blooms -- make them an ideal landscape plant. A few things cause damage to these roses, including holes in the leaves.
Rose slugs are actually not slugs, but the larvae of sawflies. Still, they're aptly named, and will chew holes in rose leaves. Get rid of them by treating the roses with insecticidal soap. Leaf cutter bees also plague knock out rose leaves when they're cutting away holes in the leaves to use in building their nests. They rarely take much from the plants, however, and their season is short. Knock out roses usually spring back well from this damage.
While knock out roses are highly prized for their disease resistance, it is not impossible for them to become infected. If black spots on the leaves precede the holes, the problem is likely black spot disease, a plague of many rose varieties. The fungus Diplocarpon rosae causes the disease, and you can usually treat it with anti-fungal sprays.
Granular fertilizers are concentrated mixes of necessary plant nutrients, such as nitrogen, phosphorus and potassium. But these fertilizers also tend to have a high salt concentration, and coupled with the nitrogen content it will cause burns the shape of the granular pellets on knock out rose leaves if it is left there. After time, the damaged areas might crumble away, leaving only holes.
Other random sources of holes in knock out rose bush leaves are harder to pin. It might be a combination of high winds and a neighboring thorny plant, or perhaps cats are chewing on the lower leaves -- they often like to rub and taste leaves as they pass. Search first for the other three problems because insect, disease and chemical damage are potentially life threatening for knock out roses.
- "Maryland Home and Garden"; A Rose is a Rose... Not Always, A Look at 'Knock Out®' Landscape Roses; Ray Bosmans; 2011
- Purdue University, Diseases of Landscape Plants; Rose Black Spot; Janna Beckerman; September 2007
- "The Organic Gardener's Handbook of Natural Pest and Disease Control: A Complete Guide to Maintaining a Healthy Garden and Yard the Earth-Friendly Way"; Fern Marshall Bradley et al; 2009