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Plant Diseases/blackspot on knockout roses


Hi Melissa
We have 5 knockout roses in the front of our house that are 6 years old. They have done very well in the past and are about 5 feet tall. Normally I cut them back a little in the spring but they have grown into a wall of color so I did not cut them back in the past few years. We live in Ohio and it has been a rainy humid spring/ early summer. Now they have blackspot. I have treated them with copper fungiside but it is not working. Should I cut them way back and treat them again with copper again. If so how far should I cut them back since they are 5 feet tall and 4 feet wide.I hate to loose them since they are like I said a beautiful wall of color. Thanks in advance.

Dear Christine, It seems as though the wet humid weather has taken its toll on your roses.  Although knockout roses are more resistant to black spot, they will get it if the conditions are right.  It may be a bit of work to get rid of the black spot but your persistence will pay off.  Be sure to remove as many infected leaves as you can - these will not get better.  Keep all the infected plant matter bagged separately and make sure you wash your hands between handling the plants.  Rake up and discard any fallen leaves - never mulch these.  Remove any canes that have black on them.  You might also want to try to improve the air circulation inside the bushes, so thin the plants out a bit. Also be sure to never wet the foliage when watering. Now, when the plants are as clean as possible, then spray.  You can spray with copper fungicide, or look for Funginex, or a fungicide with the active ingredient mancozeb, chlorothalonil, or trifloxystrobin.  You will have to spray several times about five days apart.  Keep removing the leaves that become diseased.  It will take some work, and it is somewhat time consuming, but it is well worth the effort, and with any luck at all from the weather, your roses should look fine within a few weeks.  I hope this information helps.  Good luck, Melissa

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Melissa Johnston


Plant diseases, landscaping, tropical plants, roses, herbs, plant care, grafting, horticulture, plant identification, anything about plants I can likely answer.


35 years experience in various plant businesses, 1984 Certified Texas Master Gardener.

Master Gardener Association of Texas charter member

none as of yet, but I have a plant q&a book I am in process of submitting.

Magna cum laude graduate of Texas A&M, 1978

Awards and Honors
Plant growing awards, highest grade for Texas Master Gardener graduates.

Past/Present Clients
Past member of and was very highly rated. Owned landscape company in the past, Almost Paradise, and was very successful despite little equipment, no help, and no advertising. Lived well for two years until 9/11.

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