Knockout roses were bred to be disease resistant and they stand true to their breeding. They are low maintenance plants and rarely bother you with pest or fungi problems but there are certain exceptions to the rule. Depending on your climate, fertilizing efforts and watering practices your knockouts might end up with a few problems.
The most common insect to attack knockout roses would be aphids. These critters more likely than not would end up on the stems and branches of your knockouts come spring time. Aphids don't cause any problems with knockout roses expect a cosmetic one. The best way to deal with aphids is to squish them manually and dip them in a soap solution to be disposed. The second best thing to do would be to treat them to a hard burst of spray from the water hose. Avoid using pesticides to deal with aphids as it would harm the rose shrub and make it more susceptible to an attack by spider mites (they are more difficult to handle).
How to keep knockout roses insect free? Well the best practice involved in bug care for knockout roses are mentioned below
Though these shrubs are bred to be disease resistant your knockouts may get unlucky at times and get infested by one of the below diseases.
1. Black spot - If the humidity in your garden stays high through out and the temperature is around 75 F, it provides conditions for development of black spot on the rose shrubs. It starts with small black spots on the leaves which turn to yellow rings and eventually turn the entire leaf yellow. So if you end up with yellow leaves on knockout roses it would have to be a black spot attack.
2. Mildew - High humidity and moist conditions on the leaves result in mildew, it manifest as white powdery substance that covers the leaves. If the foliage is untreated it would shrivel up and fall off. So if you are wondering why are there white spots on the leaves of my knockout roses? The culprit is mildew.
3. Rust - A fungal infection which results in reddish orange spots appearing on the leaves of the rose shrub. Cool and moist conditions result in rust formation. The infected plant would end up defoliating with time if left untreated. So if you are asking why are the leaves of my knockout roses turning red? The villain could well be rust.
Knockout roses are disease resistant, so it would be quite rare to observe these diseases but a few good practices definitely help.
Most likely than not you will not end up with any problems with knockout roses you plant, especially with respect to leaf diseases. If you follow the above guidelines you will further reduce the rare possibility of ever encountering black spot, rust or mildew.