Yard and Garden: Rose Diseases and Pests

A wide variety of roses are available for sunny garden areas. Proper care from planting through pruning will show them off to their best advantage. Iowa State University Extension and Outreach horticulture specialists say to confront insect and disease problems in roses by correctly identifying the pest and using appropriate control measures. Home gardeners with additional questions can contact the experts by calling or emailing the ISU Extension and Outreach horticulture hotline at 515-294-3108 or hortline@iastate.edu.

How can I control blackspot on my roses? 

Blackspot is a common fungal disease of roses. The disease is caused by the fungus Diplocarpon rosae. Symptoms of blackspot are circular black spots on the leaves. Infected leaves turn yellow and drop prematurely. Initially, symptoms develop on the lower leaves and gradually move upward. 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 varieties differ widely in their susceptibility to blackspot. When purchasing roses, select rose varieties that are resistant to blackspot. When selecting a planting site, choose a 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. Reduce the amount of overwintering fungi by carefully cleaning up the leaf debris in fall. When watering roses, apply water directly to the ground around the plants. Do not wet the foliage. Fungicide applications must begin at the first sign of disease symptoms. 

Small, green worms are eating the foliage on my roses. What should I do? 

The small, green “worms” are probably the larvae of the rose sawfly. Rose sawfly larvae (commonly referred to as roseslugs) have tapered bodies, may be up to ½ 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 the veins. The thin layer of tissue that remains eventually turns light brown. Foliage damaged by roseslugs has a window-pane or skeletonized appearance.  

Roseslugs weaken affected plants. However, the damage is mainly aesthetic. The damaged plants will continue to grow and should look better latter in the growing season.  

Small numbers of roseslugs can be picked off by hand and destroyed. Larger infestations can be controlled with insecticides, such as insecticidal soap, carbaryl (Sevin) or permethrin (Eight). 

There are round holes in the foliage of my roses. What is responsible for the damage? 

Leafcutting bees are probably responsible for the holes in the rose foliage. Leafcutting bees resemble honey bees, but are often darker in color. Female leafcutting bees 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. Control efforts are rarely justified or necessary.  

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PHOTO: Rose with blackspot

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