Roses are hosts to many insect pests. Aphids, mites, rose stem borers, and gall wasps were the four most common insect. Leaf miners, leafhoppers, thrip and other chewing and skeletonizing insects are seen occasionally.

Aphids

These small insects are one of the more common pests of roses. They are soft-bodied, usually lime green, and are found on cane tips, flower buds, and on the bottom of new leaves, where they puncture the plant to suck juices. As they feed, they excrete a sticky and glossy residue called “honeydew.” Severe infestations can lead to drying and curling of new leaves. Pesticides and natural predators, such as ladybugs, provide control for aphid infestations.

Mites

Mites are common on roses, but are difficult to see because of their very small size. They pierce foliage and suck plant sap from leaves. The yellowing and mottling of leaves that result are often the first symptoms observed. Mites also form fine webs on the undersides of leaves where they feed. Severe infestation causes defoliation. Mites are most often seen under dry conditions.

Cultural practices that result in vigorously growing roses are a gardener’s best defense against mites. Pesticides are commonly used to eliminate severe infestations. Dormant oil, which smothers mite eggs, is often applied to dormant bushes. Natural predators of mites can also be purchased for biological control. Repeated water sprays of infested plants, especially at the undersides of leaves, will help control but not eliminate mites.

Rose Stem Borer

The rose stem borer, Agrilus aurichalceus, damages rose canes when larvae tunnel in a spiral fashion beneath the bark, girdling and killing the canes. Their presence is indicated when a cane dies above the point of borer tunneling. Leaves on the infected cane turn brown as they die, creating a “flag” among healthy, green-leaved canes. On close observation, a swelling or gall on the infected cane can be seen below the dead tissue, indicating where the borer’s tunneling occurred. The gall formation weakens canes and it is common to see infected canes broken off by wind. The quickest and most effective control is removal and disposal of infected canes in fall.

Mossy Rose Gall

Mossy rose galls are caused by Diplolepis spinosa, a cynipid gall wasp. These galls are common on wild roses of North America, from Ontario to Alberta in Canada and throughout most of the northern United States. The presence of these insects is indicated by the formation of spherical, golf ball-size, spiny galls on the canes of host plants.

The development of these galls is stimulated in the spring by newly hatched larvae. The galls encase the larvae until adult wasps emerge the following spring. The galls are unsightly and alter the plant’s shape. They also stress the host plant, behaving like nutrient sinks, drawing nutrients away from the rest of the plant. Large numbers of galls on a plant can kill the plant.

Insecticides have no effect on the wasp that causes mossy rose gall. The most effective control is physical removal and disposal of galls in autumn after leaves have dropped and galls are visible. It is important to dispose of all galls since even a single missed gall can produce and reintroduce 30 to 40 mature wasps to the garden the following spring.

Many diseases infect roses. The most common and serious, from an aesthetic standpoint, are foliar diseases. These include blackspot, powdery mildew, leaf spots, and rust. Severe infections of any of these diseases reduce plant vigor. It is best to choose varieties which are resistant to common diseases. Selection of disease resistant roses reduces the incidence of infection and increases survival of the plants..

Blackspot

Blackspot is diagnosed when small, circular, black spots with feathery margins develop on upper leaflet surfaces. Spots are 1/8 to 1/2 inch (2 to 12 mm) in diameter. The leaf tissue surrounding these spots turns yellow and this chlorosis spreads until the leaflet drops from the plant.

Less noticeable black spots can also occur on a plant‘s petioles, stipules, peduncles, fruit, and sepals. Flower petals may be distorted and red flecks may occur. Raised, purple-red blotches that later blacken and blister develop on the immature wood of first-year canes.

Expanding leaves between 6 and 14 days of age are most susceptible to blackspot infection. The optimal temperature for disease development is 75° F. Conidia, the infecting spores, must be continuously wet for at least seven hours for infection to occur.

The fungus overwinters in infected canes and in fallen leaves. Protective fungicidal sprays and planting of resistant cultivars are the best means of blackspot control.

Powdery Mildew

Powdery mildew is another widely distributed and serious disease of roses. Young tissues are the most susceptible and the disease is typically diagnosed when white, powdery patches of fungal growth appear on young leaves. These leaves will often fold inward or become twisted and distorted. New stem growth and flowers can also be attacked.

The fungus overwinters in rudimentary leaves in buds or in the inner bud scales. Fungicide sprays provide the best control.

Leaf Spots

There are several leaf spot diseases of roses, including those caused by Alternaria, Cercospora, Colletotrichum, and Sphaceloma rosarum.

Spot anthracnose is caused by Sphaceloma rosarum. Young spots are red, occasionally brown or purple, and occur on upper leaf surfaces. Spots are circular and up to 1/4 inch (.5 cm) in diameter. The center of spots turn gray or white. The center of the spot often falls out, leaving a shot-hole appearance. Heavy infections of spot anthracnose can result in yellowing of infected leaflets and will cause defoliation.

Conidia are formed in spring and continue to form on rainy days through the growing season. Spores are spread by splashing water.

Rust

Nine species of the rust fungus Phragmidium occur on roses. Rust is typically diagnosed when reddish orange pustules, containing spore masses, appear on lower leaf surfaces. As the pustules develop, they appear on the upper leaf surface as chlorotic spots, giving this surface a mottled appearance. Leaves growing at the base of plants are most commonly infected.

Temperatures between 64° and 70° F (18° to 21° C) are optimal for rust development. Continuous moisture for two to four hours is necessary for infection to occur