Aphids (there are several kinds which attack roses) are the worst pests. Colonies of green, brown or pink insects cluster together on young, sappy shoots and flower buds. Thrips, also sap-sucking insects, are just visible to the naked eye and damage both buds and flowers. Two-spotted or red spider, a small mite, infests the lower surface of leaves, causing yellowing or browning and premature ageing. Some caterpillars chew holes in leaves and others roll leaves together. There are also several scale insects that attack roses.
The most serious diseases of rose plants are black spot and powdery mildew. Black spot causes small blackened areas, yellowing leaves and premature leaf fall. Humid weather, moderate temperatures and heavy night dews favour its spread. Powdery mildew produces a white powdery growth on leaves, stems and buds. It is most active in warm (but not too hot), humid weather.
Pests and diseases can be effectively controlled by sprays of insecticides, miticides and fungicides. Many gardeners prefer to use an all-purpose rose spray to control both pests and diseases rather than use individual chemicals. A number of such all-purpose sprays and dusts are available. The specially formulated Rose Shield or ready-to-use Yates Rose Gun Black Spot & Insect Pest Killer are recommended. After winter pruning, while bushes are still leafless, spray roses with Lime Sulfur at winter strength. It will help control fungal spores and rose mites.
This information is from the Yates Garden Guide: fully revised & updated 44th edition, HarperCollins, $39.99. You can have this information and so much more at your fingertips by purchasing the Yates Garden Guide, available at all leading bookstores and Bunnings stores.