Blackspot on Knockout Roses

VetlorAugust 9, 2014

We have 3 knock-out roses in the middle of a boxwood hedge. (see photo) My wife has fought blackspot fungus for years and is ready to give up and pull them out. Someone suggested that the reason this supposed resistant plant is so plagued is because of the surrounding hedge. (The Japanese Beetles also drive us crazy but they have been worse on the okra this year and really really loved the edamame plants last year.) My request is for suggestions for another colorful plant to place in the middle of this boxwood ellipse. We love the multiple colorful blooms of the knock-out roses but they are far from being carefree.

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dublinbay z6 (KS)

Could you take a close-up pic of the leaves in question? We sometimes have gardeners who think they have incurable BS when in actuality it is a different kind of fungus. A close-up can help us diagnose the problem for sure.

My opinion is that if Knock Out is hopeless there, there is no rose that is more carefree.

I assume that spot is getting at least 6 hours of sun and getting enough water also--or those could be problems.

How old are those KOs? My neighbor's KO was more like 5-6 feet tall and 4 ft wide when it was in her garden about 3 years.


    Bookmark   August 9, 2014 at 2:40PM
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michaelg(7a NC Mts)

KO roses in Asheville get cercospora spot, which is probably what you are seeing. In some seasons the foliage gets ratty looking by this time of the year or later, and infected leaves may eventually die. However, compared to blackspot, it doesn't do any great harm. You can safely ignore it.

    Bookmark   August 9, 2014 at 3:30PM
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jim1961 Zone 6a Central Pa.

How about the Red (Homerun) rose...?
Wonder how that rose performs in NC?

    Bookmark   August 9, 2014 at 3:58PM
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Thank you for the prompt replies. Well, actually they do not look too bad now. : ) The leaves turn black near the bottom. The plants are not very big because she has cut them back over the years because they would get so "sick" appearing. They have been in place for nearly seven years. In the past she has drenched with the Bayer rose multi-symptom product. I may have to wait until they look bad and then post again.

    Bookmark   August 9, 2014 at 4:14PM
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One more photo

    Bookmark   August 9, 2014 at 4:16PM
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sam 4b Adirondacks

Mike of whyy you bet your garden would say it is caused by the wood mulch. He would instruction to rake it and use yard waste compost and another mulch like pine straw. WHYY you bet your garden questions a thru Z .roses

    Bookmark   August 9, 2014 at 11:37PM
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sam 4b Adirondacks

It could be an airflow problem. A Canadian explorer would do well. cape diamond or Champlain.

This post was edited by sam4949 on Sun, Aug 10, 14 at 2:43

    Bookmark   August 9, 2014 at 11:43PM
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michaelg(7a NC Mts)

Scorching of the edges looks like it might be from over-fertilizing. There is fertilizer in the Bayer drench, so adding anything else could be harmful. If there are excessive fertilizer salts in the soil, flooding with 2" of water will leach them out.

I would need a sharper closeup to diagnose the fungus.

Cutting the plants back repeatedly during the season will weaken them.

Nobody here recommends the Bayer combo drench. It contains cheap fertilizer and an insecticide that is not needed and possibly harmful. The fungicide is less effective than if it were sprayed on. I am not sure whether that fungicide controls cercospora--I think it does not-- but it is very effective against blackspot when sprayed. The spray concentrate is Bayer Advanced Disease Control for Roses, Flowers, and Shrubs. For cercospora you might try Soap Shield (copper).

    Bookmark   August 10, 2014 at 11:42AM
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cecily(7 VA)

Vetlor, your landscaping is lovely. I'd like to see more photos of your property. The burgundy foliage of the Knock-out looks very nice with the green hedge. Unfortunately you've not had much success with Knock-outs in that location so perhaps another shrub with purple/burgundy foliage would be best. There's a short crape myrtle with dark foliage that might work, sorry I don't remember the name, it was advertised in Southern Living magazine and I also saw it at Lowe's this summer. Good luck.

    Bookmark   August 10, 2014 at 11:59AM
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dublinbay z6 (KS)

I personally think that if you do the following (as others above have pointed out) that in a year you too will have easy-care Knock Outs growing there.

1. Quit pruning the poor things.
2. Quit feeding the poor things (the Bayer Combo that has fertilizer in it--stop it.) Next spring you can give them a light sprinkle of a standard fertilizer--like Rose-Tone or Plant-Tone. Do that only ONCE next spring. In fact, they probably don't need any feeding. My KOs take care of themselves without me having to feed them more than once a year.
3. Water them more. And water them on a regular routine (unless it rains). You don't want them standing in water, but most people I talk to who complain that their KOs are not performing well are trying to grow them as though they were desert plants. They are NOT. They are water-hogs, in that they do not want to dry out--and certainly not for weeks at a time. Keep the soil moist.
4. Quit spraying the poor things. That is plant-abuse when you keep spraying them for BS, and they probably do not even have BS--but you keep on spraying anyway! No wonder the poor things are not growing well.
5. Give us a close-up of a "problem" leaf--and I bet we will diagnose it as cercospora spot, not blackspot. In which case, just ignore it. Period.
6. Did you forget to water them again? Water, water, water--just don't leave them standing in water.
7. Have you forgotten to water them regularly? If so, start watering them.
8. Repeat 6 and 7.

I see lots of good healthy (red) growth on your KOs. With regular watering, they will probably start blooming beautifully in a couple weeks. And remember most roses have bloom cycles followed by rest cycles--so don't panic and think something is wrong when the bush goes into "Rest" phase--which might last several weeks.

Whatever you do, quit fussing over the poor things and give them a chance to do their own thing on their own steam. Just don't forget to water. OK? : )


    Bookmark   August 10, 2014 at 12:39PM
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sandandsun(9a FL)

In my humble opinion as a gardener, I believe that you are asking too much of the roses. The first photo clearly reveals an initial planting designed to be evergreen with minimal care, and once established to require minimal water. That is not the environment roses prefer (even KnockOuts) - they do best with regular water and little root competition. The boxwood roots are champion competitors.

I would look for a more appropriate site for the roses or just remove them entirely.

And I would instead install something of the "hardscape" persuasion in that spot. It would be excellent for some folly - a tasteful sculpture, piece of outdoor art, bird houses, etc.

If this isn't within your budget or to your liking, then I recommend looking for plants that have survival requirements similar to box. This would mean that in terms of the desire for another colorful plant that colorful foliage be considered. A variegated plant, for example, that would do well there is almost certain to exist. Or one with burgundy leaves as the post above suggests.

Good luck.

    Bookmark   August 10, 2014 at 12:39PM
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michaelg(7a NC Mts)

Kate nails it.

    Bookmark   August 10, 2014 at 1:10PM
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