Here are answers to some of the most common questions we receive about garden plants. You will find concise information on general gardening techniques as well as plant selection and care. For detailed information on specific plant pests and problems refer to our Common Garden Pests and Problems page.

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Horticulture Questions and Answers

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How do I control insects and diseases on my roses?

Rose diseases commonly encountered in the St. Louis area include black spot, powdery mildew, and rose rosette. Black spot is easily identified by the characteristic small black spots which develop on the upper leaf surface, usually beginning on leaves at the bottom of a rose bush. Leaf spots enlarge, are accompanied by yellowing of surrounding tissue, and ultimately leaves drop off resulting in progressive defoliation of infected bushes. Control is difficult once it becomes well established thus prevention and early treatment are important. Control requires the removal of fallen leaves and pruning of infected canes in late winter. Routine spraying is usually required. Check with your local garden center for currently available fungicides. Other ways to limit black spot is to select less susceptible cultivars, plant so the roses have good air circulation and receive morning sun, avoid wetting foliage and promptly collect and dispose of diseased leaves.

Powdery mildew, a fungal disease which often occurs in late summer or early fall, can be easily identified by the white or grayish powdery growth which occurs on the upper surface of newer leaves. To control powdery mildew provide good air circulation and spray with an approved fungicides available at garden centers if needed.

Rose rosette results in leaf distortion and wrinkling, bright red leaf pigmentation, and witches-broom. Affected canes may be excessively thorny and slow to mature. Plants usually die thus control is limited to controlling mites and destroying infected plants to preclude disease spread.

Aphids, small, soft bodied insects, are common rose pests. They feed primarily on newer leaves and on developing flower buds. Symptoms of aphid damage include malformed flowers and leaves which curl and pucker, turn yellow or brown and fall off. Control strategies include spraying roses in early morning with a stiff stream of water to wash away aphid colonies, spraying with insecticides, diatomaceous earth, or summer oils, and use of systemic insecticides. Use of predator insects such as lady bugs is also becoming more common. Dormant oil sprays in late winter help to control aphids in the egg stage as well as other insect and disease problems.

Southern red spider mites, tiny animals usually less than 1/10th inch long and more closely related to spiders than insects, are another rose pest. They feed on the underside of leaves and may not be noticed until symptoms, typically a "bronzing" and fine silk webbing on leaves, become apparent. Control is similar to that for aphids but use of miticides may be required for severe infestations. Products containing Chlorpyrifos, Malation, horticultural oils, Piperonyl butoxide, or Resmethrin are registered for controlling spider mites.