Pests / disease on knock out rose

Question: I just moved into a home in the Annapolis area. We moved in at the end of March. The one knockout rosebush at the front of the house bloomed, but is now showing very damaged leaves, by pests or disease. I’m not sure. I have no history on the plant but the previous 97 yr old owner lived here for 30 years so it likely an older bush. It is about 6 feet high and 4 feet wide, and the entire bush seems damaged. Our soil has a lot of clay in it. The bush is located on the east side of the house next to a barberry and near several azalea bushes. Can I treat this now to see if it is recoverable?

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied June 12, 2023, 4:23 PM EDT
response: The symptoms pictured appear to be both accumulated Roseslug Sawfly damage plus a leaf spot infection called Cercospora  (see link below). Both are very common on roses in our region. The Knock Out series of roses do have decent inherent resistance to the disease Black Spot, but are still vulnerable to several other typical rose ailments, and having several overlap by summer is also common. Usually, shrub roses get cut back fairly drastically each winter (around March) to refresh the plant, manage its height a bit, and to remove dense, weaker growth from the center of the plant to improve air circulation through its branches to discourage disease. It's reasonable to assume the prior gardener wasn't able to do this, or may have been unaware that this is recommended. You can explore our rose diagnostic page for more information about the usual rose ailments gardeners in our area encounter.  The barberry isn't vulnerable to these issues but it is an invasive species so we encourage gardeners to remove them. If it's within about three feet of the rose itself (hard for us to tell from the angle of the photo) then it also is competing with the rose for soil resources (moisture, nutrients, growing space) so should also be taken out for that reason; don't put anything in its place in that case so the rose isn't crowded.  Azaleas thrive in afternoon shade (a part sun/part shade exposure) and roses thrive in afternoon sun (full sun, 6-8 hours or more a day in summer). Since they're growing right next to one another, we presume one might be stressed depending on what amount of direct sunlight this planting bed receives. Since the barberry is fairly red/purple, it's probably not too shady or else it would be more green-leaved.  The only way to suppress Cercospora (as with Black Spot) is to treat the plant repeatedly with a fungicide labeled for use on shrubs for leaf spot infections; follow all label instructions about dosage and intervals. This cannot cure existing disease but might help to protect new growth from becoming infected. Otherwise, if you prefer to avoid pesticides, all you can do is to dispose of any fallen rose leaves (plus consider trimming off some with the worst symptoms) and try to maintain good air circulation within the plant by removing some stems that are crowding the foliage and reducing airflow. Monitor the plant for watering needs so it's less stressed and more capable of producing new growth (and blooms).  We think the plant can recover well (though might not look too fresh and clean until next year) and should keep flowering; roses like this can bloom all summer but go through periods of "resting" where blooming can pause while they rebuild energy reserves for the next flush of flowers. Plants stressed by other problems might have a longer window of time between blooms than those that are healthier and more vigorous. Next year, if you opt to use fungicides, sprays will have to begin fairly early in the season (not long after leaves have emerged) and continued well into summer, but even then, they're not foolproof and some breakthrough infections can still appear, as can a revisit from some sawfly larvae. Fortunately, roses can be infested with sawfly larvae each season and still rebound just fine, albeit with a bit of an eyesore due to the chewed leaves.  Miri

responder: The Question Asker Replied June 12, 2023, 5:39 PM EDT
response: Hi MiriThank you so much for your detailed answer. I really appreciate your guidance.MaureenSent from my iPhone On Jun 12, 2023, at 4:23 PM, Ask Extension <<personal data hidden>> wrote:﻿

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied June 13, 2023, 9:18 AM EDT
response: You're welcome!