Knowledgebase

Rose bushes with holes in leaves #754486

Asked June 07, 2021, 11:57 AM EDT

Hi, I moved to a home last August with rose bushes throughout the property, including two large bushes lining the front entry. I don't know the species of the rose. The plants were doing okay in the spring, but recently have been decimated by an insect or disease. The flowers appear unaffected, but the leaves have white spots or more commonly holes throughout (see picture). Some of the stems have black spots on them. I've prune the bushed back heavily and essentially removed anything that looks diseased. I haven't seen any bugs on the plant but not sure if I caught it too late. I'm wondering: 1) what this disease/pest might be and 2) if it's worth keeping these bushes. They can be quite pretty, but I'm not fond of pruning them and would be happy to replace with native plants without thorns that would work well in a shaded area, close to the exterior of my brick home. Thank you so much! Maggie

Montgomery County Maryland

Expert Response

 The white window paning (etched surfaces) or small holes on the foliage looks like rose slug damage. These are insect larvae, not slugs, and are in the group called sawflies. They are easy to miss, small and wormy looking and often can be found beneath the leaves. Look under the leaves. It is possible the damage may have been done. 
Here is our page on them and how to best control: https://extension.umd.edu/resource/roseslug-sawflies-shrubs 

https://extension.umd.edu/resource/common-problems-roses

Hand picking/squishing works and the most environmentally-friendly approach would be to try to blast the leaf undersides with a harsh jet of water from a hose nozzle. Sawfly larvae can't get back onto the plant after being dislodged like this. 

Otherwise, low-impact insecticides like a summer rate of horticultural oil or spinosad (an active ingredient in several products) will help control them while having minimal or no impact on beneficial insects. 

Spray upper and lower leaf surfaces. Follow label directions about how often repeat treatments can be / need to be made. Some rose slug species have more than one generation per year, so control efforts may need to be made more than once if you see damage continuing. 

Marian

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