Knowledgebase

Rose Problem #753135

Asked May 31, 2021, 7:28 PM EDT

Hello, I recently moved into a house with beautiful roses, but they seem to be sick and I'm not sure how to treat them.

Montgomery County Maryland

Expert Response

Fortunately, neither symptom appears to be causing damage significant enough to warrant treatment at this time, though minimal-toxicity insecticides are an option.

The first photo shows what looks like leaf chewing from either caterpillars or beetles. A number of insects feed on rose foliage, so it can be difficult to determine which caused these particular leaf holes. Some only feed at night and hide elsewhere during the day.

The second photo shows "window-paning," which is damage named for the effect of creating a near-hole in the foliage - where it's eaten, but not all the way - that lets light through, like a frosted pane of a window. As the insects that cause this get older and more able to chew through all of the leaf layers, their damage switches to "skeletonizing," where the leaf "skeleton" (the veins in the leaf) are all that's left when the rest of the leaf is consumed. Sawfly larvae do this type of damage, and they are quite camouflaged underneath the foliage. They are a very common pest of roses. Although they look like caterpillars, they are not, so are not always controlled by the same pesticides targeted to caterpillars. Usually, you can avoid spraying by just picking them off or blasting them off with a strong jet of water (be sure to hit the leaf undersides when doing this). Otherwise, recommendations for intervention can be found on this page: https://extension.umd.edu/resource/roseslug-sawflies-shrubs

If you use spinosad or horticultural oil as mentioned in the page above, they should also help control the insect causing the overt holes in the first photo. Overall, though, the plants appear to be in good condition from what we can see in the photos, so the damage seems minimal so far. Leaf holes don't heal, but new growth should remain protected if the control measures are working.

Miri

Hi Miri, 

Thank you. I do want to share that this leaf damage is extensive and affects almost all leaves on the bushes. I was wondering of the two, spinosad and horticultural oil, which you think may be more effective at treating for new damage since it seem to be a pervasive issue at the moment. 

Many Thanks,
Maria 


On Jun 1, 2021, at 11:43 AM, Ask Extension <<personal data hidden>> wrote:

horticultural oil

The Question Asker Replied June 01, 2021, 11:48 AM EDT

Ok, in that case, spinosad may be more effective, since it is a chemical with some degree of systemic activity. (This means the plant can absorb it and store it in leaf tissues, so it will both survive weathering for longer and will remain present for longer to expose pests to it as they continue to feed. Horticultural oil, on the other hand, is contact-only and won't impact insects it doesn't touch.) Either spray may be tricky to apply, since ideally they should be sprayed on all leaf surfaces - undersides in particular - but since spinosad can be absorbed this thorough of a coverage is a bit less critical than when applying oil.

As with any pesticide, follow label instructions about dosage (if it's in concentrate form), intervals between applications, and any other restrictions. Several applications are likely going to be needed to control what may be multiple generations of this pest throughout the summer, especially since later new leaf growth will be unprotected. Avoid applying any spray during temperatures above 85 degrees so you don't risk damaging foliage. Several products contain spinosad as their active ingredient, so we don't have any specific product recommendations. If a garden center has an organic pesticide section, it'll probably be found there.

Miri

Thank you! One last quick question - given the extent of damage, when would be the most appropriate time to prune back foliage? 

- Maria 

On Jun 1, 2021, at 12:03 PM, Ask Extension <<personal data hidden>> wrote:

The Question Asker Replied June 01, 2021, 12:09 PM EDT

For this type of damage, trimming away hole-riddled foliage is not needed and may deprive the plant of needed energy from photosynthesis. (Since the uneaten parts of the leaf are still able to feed the plant.) Usually, shrub (non-climbing) roses are trimmed in late winter (around late February) to control size and/or promote strong flowering stems. Even so, while a common practice, it's not necessary to do so. Lighter, sporadic in-season trimming is fine as well, though it's usually for the goal of promoting faster re-bloom. If you wish to trim the roses now, we suggest removing no more than about 1/3 of the plant's overall growth to avoid too much stress on it.

Miri

Great, thank you! 

On Jun 1, 2021, at 12:22 PM, Ask Extension <<personal data hidden>> wrote:

The Question Asker Replied June 01, 2021, 12:26 PM EDT

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