Damage to roses is caused by the larva girdling the tender growth or new rose stems, causing the tips to droop. The resulting damage to the rose stem is distortion of the stem and foliage dying at the tips. Eggs are deposited inside the tender growth of the rose stem by the adult's ovipositor, so some control can be had with a systemic insecticide when the telltale drooping of the canes is first noticed. The egg-laying period is usually about two weeks before the last killing spring frost and can last through midsummer. Diseases Powdery mildew, blackspot, anthracnose, rust and verticillium wilt are all caused by fungi.
Powdery mildew is probably the most serious disease affecting roses in Colorado.
It is at its worst during periods of warm, dry days and cool, humid nights, such as mid-August until frost. Symptoms include a whitish coating on leaves and young shoots, curled leaves and distorted buds. This disease is best controlled by planting resistant varieties and a preventive spray program with a fungicide. Funginex with Triforine as the active ingredient and Benlate, used as directed on the label, will give reasonable control. Blackspot, anthracnose, rust and verticillium wilt are not serious problems on roses in Colorado.