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Double Knock Out Roses With White Spots & Holes on Leaves

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In 2005 the Double Knock Out rose (Rosa x ‘Radtko’) made its garden debut to standing ovations and has never looked back. In addition to the original cherry-red color, there is also a pink variety (Rosa x ‘Radtkopink’). Both ‘Radtko’ and ‘Radtkopink’ are hardy in United States Department of Agriculture plant hardiness zones 5 through 9. Touted as drought-tolerant, low-maintenance and resistant to black spot, these roses may nevertheless suffer from the occasional disease or insect infestation. However, if your Double Knock Out rose develops white spots and holes in its leaves, you can diagnose and treat the problem relatively easily.

What’s Wrong?

Although residues from garden sprays or diseases such as powdery mildew can cause white spots on the leaves of your Double Knock Out rose, the combination of white spots and holes is more than likely damage caused by sawfly larvae, also known as rose slugs (Endelomyia aethiops). Despite the robustness of the Knock Out rose family, they aren’t immune to rose slugs, a pest that’s common to many varieties of landscape roses.

“>Signs and Symptoms

Rose slugs eat Double Knock Out rose foliage from underneath. Young larvae cause distinctive damage when they skeletonize leaves, or eat all but the very top, translucent layer. Close up, this layer resembles a windowpane, but from a distance, these areas may look like opaque, white spots. As the larvae mature, they chew large holes all the way through the leaves. They don’t eat rosebuds, so they won’t disfigure your flowers, but severe infestations can retard your rose’s flowering and overall growth and leave the plant looking forlorn all season.

“>Pinpointing the Pests

To protect your Double Knock Out rose from damage, it’s important to positively identify the pest involved. Rose slugs range in color from light green to black and look similar to small slugs or caterpillars. Like caterpillars, these larvae have legs and leglike protuberances. Also note that they overwinter underground as pupae and mature into wasps, not flies.

“>Controlling the Problem

University of California Statewide Integrated Pest Management experts advise that natural predators may adequately control rose slugs. Treating them with garden chemicals requires catching them in the act of eating your rose leaves, though. Hosing them off your Double Knock Out rose may be all you need to do, but for heavy infestations, consider using insecticidal soap or spinosad. If you go this route, carefully read and follow all directions on the product label.

Double Knock Out Roses With White Spots & Holes on Leaves. Double Knock Out roses (Rosa x "Radtko") first appeared in gardens in 2005; in addition to the original cherry red color, there is also the Pink Double Knock Out rose variety (Rosa x "Radtkopink"). Touted as drought-tolerant, …

White spots, holes on Knock Out Roses

We had several Knock Out Roses planted in April. They have done well until now. Recently, I have noticed white spots and holes on the leaves. Is this a fungus or an insect? I have not seen anything moving and have looked under the leaves. How do I correct this problem?

Baltimore County Maryland

1 Response

This looks like damage from rose slug sawfly feeding. These insects are typically very active in May-June but they can continue to be active through the fall. Check the surfaces of the leaves (upper and lower) for very tiny green caterpillars. This damage may be from earlier in the season, so you may not see insects present. Starting in May next year, check your plants regularly to see if there are sawfly larvae present on the foliage. You can use horticultural oil or spray with spinosad to manage these insects. You will find additional information here: https://extension.umd.edu/hgic/topics/rose-slugs-shrubs

Other insects, such as Japanese beetles, can also cause chewing damage on rose foliage in June-July. https://extension.umd.edu/hgic/topics/japanese-beetles

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We had several Knock Out Roses planted in April. They have done well until now. Recently, I have noticed white spots and holes on the…