Brown spots on Knock Out Roses - Knowledgebase Question

University Park, MD
Question by LynnHButler
May 18, 2006
Hello. I planted on Easter Sunday three Knock Out Rose shrubs. They currently are in bloom and I thought doing well. I noticed a few leaves that are getting light brown spots (not black) and some of the leaves look stunted. Right now they have lots of flowers but on one, not that many bloom ready to go. I am concerned they have some kind of disease?

Also, do I need to cut off spent blooms to encourage rebloom and do I need to fertilize them?

Thank you.

Lynn Butler

Answer from NGA
May 18, 2006


Knock out is a very disease resistant and low maintenance rose.

Deadheading or removing the spent blooms is not required. It reblooms well without deadheading. If you wish, you can remove them but take only a minimal amount of foliage, or no foliage at all, when you cut them off.

Your roses will grow best in a full sun location with good air circulation and average to rich soil that is evenly moist yet well drained, meaning never sopping wet or saturated and never dried out. Poorly drained soil is one of the most common problems people tend to have with their roses.

Fertilization would ideally be based on the results of soil tests, however you could use a slow release or general purpose granular fertilizer in the very early spring per the label directions, and possibly again in early summer. Do not fertilize in late summer or fall. You can also topdress in spring with a good quality compost. Using an organic mulch several inches thick year round will also help feed the soil slowly over time.

Based on your description I am not certain what is happening to your roses. Black spot would be one of the most common problems on roses in your region, however this particular rose is highly resistant to it and I have never seen it affected by black spot. Other causes of foliage problems are planting too deeply or in poorly drained soil (if the yellowing, browning is starting at the bottom of the bush and working its way up the plant this might be the cause.) Overfertilizing can cause discoloration due to "burning" of the roots; accidental herbicide exposure or some pesticides can cause discolored foliage and also distorted leaves. Frost damage could also cause this type of discoloration, but would tend to be concentrated at the top of the plant. Aphids can cause distortion of the leaves, generally at the growing tips of the plants. So there are many potential causes for the symptoms you are seeing.

Since these are new plants I would strongly suggest you consult with a professionally trained nurseryman at the retailer where you purchased the plants, or with the University of Maryland extension service -- their contact information is available at

To make a diagnosis, they may appreciate a photo of the overall plant and closeup of the affected leaves, or a freshly cut sample kept cool and in a clear plastic bag. If it is something that requires a chemical control, the extension will have the most up to date information on what to use and how/when is best to apply it.

I'm sorry you are having trouble with the roses.

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