National Rose Month – Vol. 4/4 – Insect and Disease Control


Roses are especially susceptible to diseases, particularly powdery mildew and black spot. These are the most severe where humidity or rainfall is high, or where air circulation is poor. Well-maintained plants are less likely to succumb to diseases than weak ones. Follow the recommendations given in our past blog posts for planting, watering and fertilizing, and pruning your roses.

Watering your roses by drip irrigation can help control black spot and other fungal diseases, which quickly grow under moist conditions. If you must water from above, do so in the morning to give leaves and flowers a chance to dry by nightfall.

Applying mulch prevents water from splashing onto plants from the ground during rain or irrigation and contaminating them with disease spores from the ground.

To enhance air circulation, do not plant your rose bushes too close together, and do not let them grow into one another if they become large. It also helps to keep the centers of the plants open through pruning.

There are 2 types of insecticides: contact and systemic. Contact insecticides are absorbed through the insects’ bodies and must be sprayed onto them directly. Horticultural oil and insecticidal soap are types of contact insecticide that kills insects by smothering them or their eggs with a film. Systemics are applied to plants and taken up through their roots or leaves; insects are poisoned as they feast on plant parts.




Young leaves are distorted and foliage may be flecked with yellow. Flower buds are deformed and often fail to open. Petals of open blossoms, especially those of white or light-colored varieties, are often covered with brown streaks and red spots. If a deformed or streaked flower is pulled apart and shaken over white paper, tiny yellow or brown insects fall out and are easily seen against a white background.

Solution: Immediately remove and destroy infested buds and blooms.


Tiny green or pink, soft-bodied insects cluseter on leavse, stems, and developing buds. When insects are numerous, flower buds are usually deformed and may fail to open properly. A shiny, sticky substance often coats the leaves. A black, sooty mold may grow on the sticky substance. Ants may be present. When populations are high, flower quality and quantity are reduced.

spidermitesonroseSPIDER MITES

Leaves are stippled, bronzed, and dirty. A silken webbing may be on the lower surfaces of the leaves or on new growth. Infested leaves often turn brown, curl, and drop off. New leaves may be distorted. Plants are usually weak. To determine if a plant is infested with mites, examine the bottoms of the leaves with a hand lens. Or hold a sheet of white paper underneath an affected led and tap the leaf sharply. Minute specks the size of pepper grains will drop to the paper and begin to crawl around.


White, cottony masses; brown or black crusty bumps; or clusters of flattened white, yellowish, or brown scaly bumps cover the stems and leaves. The bumps can be scraped or picked off. Leaves turn yellow and may drop. In some cases, a shiny, sticky substance coats the leaves. A black sooty mold often grows on the sticky substance. Heavy infestations kill stems.


Holes appear in the flowers and flower buds; open flowers may be entirely eaten. Affected buds often fail to open, or they open deformed. Stem tips may be chewed, or the leaves may be notched or riddles with holes. Red, green-spotted, brownish, or metallic green beetles up to 1/2 inch long, are sometimes seen on the flowers or foliage.


borersinrosesSeveral or all of the larger canes and stems wilt and die. If the bark is peeled back or if dying stems are sliced open, white to yellowish worms or legless grubs up to 3/4 inch long may be revealed. Affected stems may be swollen at the base. Includes sawflies, beetles, horntail wasps, and carpenter bees.

Solution: Prune and destroy infested rose stems. Make the cut several inches below the poitn where the stem is wilted or swollen.


Yellow to brown spots, up to 1/4 inch in diameter, appear on the upper surfaces of leaves, starting in the spring or late fall. The lower leaves are affected first. On the undersides of leaves are spots or blotches containing a red, orange, or black powdery material that can be scraped off. Infected leaves may become twisted and dry and drop off the plant. Twigs may also be infected. Severely infected plants lack vigor.

Solution: At the first sign of rust, pick off and destroy the infected leaves and spray.

blackspotrosesBLACK SPOT

Circular black spots with fringed margins appear on the upper surfaces of leaves in the spring. The tissue around the spots or the entire leaf may turn yellow, and the infected leaves may drop prematurely. Severely infected plants may lose all of their leaves by midsummer. Flowr production is often reduced and quality is poor. Twigs may also be infected.

Cause: Is a severe problem in areas where high humidity or rain is common in spring and summer.

powderymildewrosePOWDERY MILDEW

Young leaves, young twigs, and flower buds are covered with a layer of powdery grayish-white material. Infected leaves may be distorted and curled, and may turn yellow or purplish and drop off. New growth is often stunted, and young canes are killed. Infected flower buds don’t open properly. In late summer, tiny black dots may be scattered over the powdery covering like ground pepper.

Cause: May occur on roses any time when rainfall is low or absent, temps are between 70° and 80°F, nighttime relative humidity is high, and daytime relative humidity is low.

RoseRxWe recommend Bonide Rose Rx 3 in 1 to control insects and disease on your rose bushes.

Bonide Rose Rx is an insecticide, miticide, and fungicide all in one that kills eggs, larvae and adults.

Insect control – aphids, scale, beetles, etc
Disease control – rust, black spot, powdery mildew, etc

An excellent choice for organic gardeners

Download and print our Tips & Info sheets:

Rose varieties

Rose Care & Pruning

National Rose Month – Vol. 4/4 – Insect and Disease Control