Only a handful of plants rival the beauty of healthy roses (Rosa spp.) in full bloom. Curling leaves on a rose bush can ruin that beauty. Affected leaves usually have other problems, such as discoloration and lesions along the leaf. In most cases, insects and fungi are at fault. In the worst-case scenario, an incurable virus may be the culprit. Roses -- depending on the variety -- are hardy in U.S. Department of Agriculture plant hardiness zones 5 to 9.
Everything from aphids to whiteflies feed on roses. Aphids, thrips, raspberry horntail larvae and scales are of particular concern, because their damage causes leaves to curl. Most rose pests are tiny with thrips no more than 1/20 inch, aphids 1/8 inch or less and scales up to 1/4 inch in their adult size. Raspberry horntail larvae are the largest of the four at 1 inch. The insects feed in different ways. Aphids, for example, feast on the underside of leaves, while the larvae of raspberry horntails weave their way through the stems. Tiny black dots and white lesions often indicate a thrip infestation, while clusters of immobile dots with winged insects spread throughout usually point toward a scale problem. Aphids, scales and thrips all succumb to horticultural oils. Thrips may also require the use of insecticides that contain spinosad. Raspberry horntail larvae do not call for insecticides, requiring only that affected stems be pruned back.
Roses are susceptible to diseases that cause leaf damage as well, though most don't cause leaves to curl. Powdery mildew, downy mildew and botrytis blight are the exceptions. Powdery mildew-affected leaves may curl in before white splotches appear on the leaves. Downy mildew is similar, although the splotches appear reddish-brown in color and often occur on the underside of leaves. Botrytis blight attacks both flowers and leaves, causing them to brown and curl in toward the stem. The stem may die. Horticultural oils take care of both powdery and downy mildew when the diseases attack your roses. Sulfur fungicides prevent the disease from making its mark. Increasing air circulation and decreasing humidity around roses remedies botrytis blight. Watering roses early in the day prevents humidity levels from rising during the night.
Curled leaves that sport a yellow discoloration during as new growth for seemingly no reason may indicate herbicidal damage from the year before. Leaves may also appear smaller than normal. There's nothing you can do to fix the problem, but keeping herbicides -- especially nonselective ones -- away from your roses is usually the best solution. Unless the herbicide dose was high, the rose will likely live on.
Rose-leaf curl is a virus that causes leaves to curl and may also lead to a yellowing of the foliage. Designs that include yellow zigzagging lines or circles may also appear on the leaves. Planting virus-resistant varieties is the only way to prevent the virus. No cures exist. The virus is not life-threatening to the rose, just unsightly.
- University of California Integrated Pest Management: Rose--Rosa Spp.
- University of California Integrated Pest Management: Aphids
- University of California Integrated Pest Management: Raspberry Horntail—Hartigia Cressoni
- University of California Integrated Pest Management: Thrips
- University of California Integrated Pest Management: Powdery Mildew on Ornamentals
- University of California Integrated Pest Management: Roses: Diseases and Abiotic Disorders
- University of California Integrated Pest Management: Viruses
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