Several different beetles infest roses. They may destroy the ornamental value of the plant by seriously damaging the flowers and foliage. The insects usually spend the winter as larvae in the soil or as adults in plant debris on the ground. In late spring or summer, mature beetles fly to roses and feed on the flowers, buds, and sometimes leaves. Punctured flower buds usually fail to open, and flowers that do open are often devoured. Many beetles feed at night, so their damage may be all that is noticed. Female beetles lay their eggs in the soil or in flowers in late summer or fall. The emerging larvae crawl down into the soil to spend the winter, or they mature and pass the winter as adults. The larvae of some beetles feed on plant roots before maturing in the fall or spring.

Black spot is caused by a fungus (Diplocarpon rosae) that is a severe problem in areas where high humidity or rain is common in spring and summer. The fungus spends the winter on infected leaves and canes. The spores are spread from plant to plant by splashing water and rain. The fungus enters the tissue, forming spots the size of a pinhead. The black spots enlarge, up to 3/4 inch in diameter, as the fungus spreads; spots may join to form blotches. Twigs may also be infected. Plants are often killed by repeated infection.

This common plant disease is caused by a fungus (Sphaerotheca pannosa var. rosae). It is one of the most widespread and serious diseases of roses. The powdery covering consists of fungal strands and spores. The spores are spread by the wind to healthy plants. The fungus saps plant nutrients, causing distortion, discoloration, and often death of the leaves and canes. Powdery mildew may occur on roses any time during the growing season when rainfall is low or absent, temperatures are between 70° and 80°F, nighttime relative humidity is high, and daytime relative humidity is low. In areas where there is high rainfall in spring and summer, control may not be needed until the drier months of late summer. Rose varieties differ in their susceptibility to powdery mildew.

Macrosiphum rosae Rose aphids do little damage in small numbers. Plants can tolerate fairly high populations without much effect. The aphids are extremely prolific, however, and populations can rapidly build up to damaging numbers during the growing season. Damage occurs when the aphid sucks the juices from the rose stems and buds. The aphid is unable to digest fully all the sugar in the plant sap and excretes the excess in a fluid called honeydew, which often drops onto the leaves below. A sooty mold fungus may develop on the honeydew, causing the rose plants to appear black and dirty. Ants feed on the sticky substance and are often present where there is an aphid infestation. When aphid populations are high, flower quality and quantity are reduced.

Rose rust is caused by any of several species of fungi (Phragmidium species) that infest only rose plants. Rose varieties differ in their susceptibility to rust. Wind spreads the orange fungal spores to rose leaves. With moisture (rain, dew, or fog) and moderate temperatures (55° to 75°F), the spores enter the tissue on the undersides of leaves. Spots develop directly above, on the upper surfaces. In the fall, black spores develop in the spots. These spores can survive the winter on dead leaves. In spring, the fungus produces the spores that cause new infections. Rust may also infect and damage young twigs.

These mites, related to spiders, are major pests of many garden and greenhouse plants. They cause damage by sucking sap from the undersides of leaves. As a result of their feeding, the plant''s green leaf pigment disappears, producing the stippled appearance. Spider mite webbing traps cast-off skins and debris, making the plant messy. Many leaves may drop off. Severely infested plants produce few flowers. Mites are active throughout the growing season but are favored by hot, dry weather (70°F and up). By midsummer, they have built up to tremendous numbers.