By Richard Jauron, Kendall Evans
There are several common disease problems that can damage a rose’s natural beauty. Disease organisms can cause unsightly leaves, reduced flower production and result in the death of leaves or even the entire plant. Iowa State University Extension and Outreach horticulturalists give advice on how to keep roses healthy and protected from these diseases. To have additional questions answered, contact ISU Hotline at hortline@iastate.edu or call 515-294-3108.
There are round holes in the foliage of my roses. What is responsible for the damage?
Leafcutting bees are most likely responsible for holes in rose foliages. Leafcutting bees resemble honey bees but are often darker in color.
Female leafcutting bees tend to make nests in rotted wood or the stems of plants. The sides of the nesting cavities are lined with round pieces of foliage. After lining the cavities with leaf discs, pollen and nectar are placed in the nest cells to serve as food for the immature bees.
Leafcutting bees may remove discs of foliage from many plants. However, they prefer rose, green ash, redbud, lilac and Virginia creeper. Holes in the leaves are typically one-half inch or less in diameter. The cuts are clean, as if they were “punched out” with a paper punch.
Leafcutting bees are beneficial pollinators. Damage to roses and other plants is usually minor and control efforts are rarely justified or necessary.
Small, green worms are eating the foliage on my roses. What can I do?
The small, green “worms” are probably the larvae of the rose sawfly. Rose sawfly larvae, commonly referred to as roseslug, have tapered bodies, may be up to a half-inch in length, and are pale green in color. The larvae somewhat resemble slugs, hence the common name of roseslug.
Rose sawfly larvae usually feed on the undersides of the rose leaves. They consume most of the green tissue of the leaf, leaving behind a thin layer of tissue and veins which eventually turns light brown. Foliage damaged by roseslugs has a window-pane or skeletonized appearance.
Small numbers of roseslugs can be picked off by hand and destroyed. However, larger infestations can be controlled with insecticides, such as: insecticidal soap, carbaryl (Sevin) or permethrin (Eight).
How can I control blackspot on my roses?
Blackspot is a common fungal disease of roses. Symptoms of blackspot are circular, black spots on leaves. Initially, symptoms develop on the lower leaves and gradually move upward. Infected leaves will turn yellow and drop prematurely. By late summer, severely infected plants may be nearly defoliated.
The blackspot fungus overwinters on fallen leaves and infected canes. Spores are splashed onto newly emerging foliage in spring. Blackspot development is favored by warm, wet weather. Careful rose selection, cultural practices and fungicide treatments can be used to control blackspot on roses. Rose cultivars differ widely in their susceptibility to blackspot.
When purchasing roses, select rose cultivars that are resistant to blackspot and choose a planting site that receives full sun and provides good air movement. Full sun and good air movement promote drying of rose foliage and discourage blackspot infections. When watering roses, apply water directly to the ground around the plants. Do not wet the foliage.
Reduce the amount of overwintering fungi by carefully cleaning up leaf debris in fall. Fungicide applications must begin at the first sign of disease symptoms.