DISEASES AND INSECTS ON ROSES

DISEASES AND INSECTS ON ROSES

-Buy plants that are free of diseases and insects. -Keep your rose garden free of weeds, fallen rose leaves, and disease or insect-infested canes. -Apply insecticides only as needed. -Apply a fungicide weekly.

Three types of pesticides are used on roses: fungicides for diseases; insecticides for insects; and miticides for spider mites. They are usually applied as sprays. Few sprays come ready-to-use on roses. It is usually necessary to prepare sprays by mixing wettable powders or liquid concentrates with water.

Select pesticides by studying this section and pesticide container labels. Follow label directions for dilution and care on handling. Commercial pesticides and an insecticide are available.

Diseases

Of the many diseases that attack roses black spot, canker, crown gall, powdery mildew, rust and viruses are the most serious.

Black Spot (fungus): Circular black spots, frequently surrounded by yellow and fall prematurely. Severely infected plants may be almost completely defoliated by mid- summer. The plant is weakened, becomes subject to winter injury, dieback and stem cankers.

Black spot is spread by water that remains on leaves for at least 6 hours before infection takes place. Prevent black spot through careful cultural practices.

Severe pruning in spring eliminates some infected canes on which the disease overwinters. Apply the first spray when leaves are half grown, then spray weekly throughout the growing season.

Cankers (fungus): Cankers commonly occur on plants that have been weakened by black spot, winter injury or poor nutrition. They first appear as small reddish spots on the stem, and enlarge and eventually encircle the stem, causing the cane to die.

To control cankers, keep bushes free of black spot, provide them with proper winter protection, and use care when pruning. When pruning, make clean cuts near a bud. Prune all cankered canes. Disinfect pruning tools with alcohol after use on a cankered shoot.

Crown Gall(bacterium): Galls begin as small swellings, usually at ground level, but sometimes on the upper part of the stem or on the roots. They slowly increase in size. Infected plants become stunted and may die.

Control is a matter of prevention. Buy plants free of crown gall and plant and plant them in soil that has been free for at least 2 years. If crown gall appears, remove the infected plants for at least 2 years. If crown gall appears, remove the infected plants and soil. Do not drop any part of a gall in the rose bed. Disinfect tools that come in contact with galls.

Powdery Mildew (fungus): White powdery masses of spores appear on young leaves, shoots and buds. Young shoots may be swollen or distorted and foliage may be stunted. Unopened buds may be covered with powdery masses of spores.

Rust (fungus): Yellow or orange pustules appear on leaves and the plant may be defoliated. The disease may also attack young stems.

Rust overwinters on fallen leaves and is spread by wind. Therefore, clean up debris in the fall. Cool, humid summers and mild winters favor disease development.

Viruses: Rose viruses are spread by propagation of infected plants. The diseases do not seem to be spread by insects or by handling.

Viruses cause small, angular, colorless spots on foliage. Ring, oakleaf and watermark patterns also may occur. Infected or they may be slightly to severely dwarfed.

The only control for viruses is prevention. Buy plants that are free of virus disease symptoms.

Major Insects & Mites

Roses are attacked by many different insects. The most common are aphids, Japanese beetle, rose chafer, rose midge, rose stem borers, spider mites and thrips.

Aphids: Several species of aphids attack rose stems, leaves and buds. Large numbers of aphids may stunt rose plants by sucking plant juices. Aphids often appear on rosebuds. They also secrete a sticky honeydew that accumulates on foliage. Sooty mold fungi that grow on the honeydew may make aphid-infested leaves appear brown or black.

Japanese Beetle: The Japanese beetle attacks rose flowers and foliage during July and August. This beetle is about 3/8 inch long and is metallic green with coppery-brown wing covers. In areas of moderate infestation, protect against the Japanese beetle with frequent insecticide applications.

Rose Chafer: Yellowish-brown beetles known as rose chafers are often abundant during June and early July, especially in areas with light, sandy soil. They are about 1/2 inch long and have long, spiny legs. They appear suddenly on rose petals where they feed, and may destroy entire flower. Insecticides are available to control this pest.

Rose Midge: The rose midge is sometimes a serious pest of roses. This tiny yellowish fly lays its eggs in the growing tips of rose stems. The maggots that hatch from the eggs destroy tender tissue, killing tips and deforming buds.

Cut and destroy infested tips daily for 1 month to eliminate the maggots before they complete their growth and drop to the ground. Use an insecticide to help control this pest.

Rose Stem Borers: The stems of garden roses are occasionally infested with one of several kinds of borers. These stems usually die back, and those infested with the stem girdler develop a marked swelling at the point of injury. The only control is to remove and destroy infested with the stem girdler develop a marked swelling at the point of injury. The only control is to remove and destroy infested stems. Seal the ends of canes after pruning to help prevent borers.

Spider Mites: The two-spotted spider mite and related species suck juices from rose leaves, which soon become stippled. As injury progresses, leaves turn brown, curl, and drop off. When mites are abundant, they spin a web over the leaf surface.

Spider mites are usually greenish with brown spots, although some are dark red. They are almost too small to be seen without a magnifying glass. The mites overwinter as adults on leaves of living weeds or perennial garden plants. They become abundant in hot, dry weather. They are especially common on miniature roses grown indoors.

To control spider mites, remove trash and living weeds in early spring. Miticides are available as a control measure. Also, spraying leaves with water may provide some control of spider mites.

Thrips: For several weeks each summer, petals of garden roses, especially white varieties, may become brown. This injury is caused by flower thrips and related species that enter the opening flower. You can observe the tiny yellow or brown insects by shaking an infested flower over a sheet of white paper.

No fully satisfactory control is available because the rapidly expanding flowers cannot be kept adequately covered with an insecticide. To protect flowers from damage, place cheesecloth cages or bags around prized blooms.

Minor Insects & Mites

In addition to the more common insect pests listed above, four others may also occasionally attack roses. They are leaf-cutter bees, rose galls, rose scale and rose slugs.

Leaf-cutter Bees: Leaf-cutter bees cut circular pieces from rose leaves and other plants and store them as food for their young in burrows they dig in rose stems, broken branches or plant crevices. The tunneled stems usually die back several inches.

No satisfactory insecticides control is available for these bees, which are valuable pollinators of alfalfa and other plants. To prevent bees from entering and tunneling into the stems, apply a dab of white glue or tree wound paint to the end of the cut stem at pruning.

Rose Galls: Several species of wasp-like insects lay their eggs in rose stems. As larvae develop, they cause large swellings or galls. One species makes a gall that resembles fibrous moss on the stem. Another causes a large, wart-like gall near the soil surface. These galls may be confused with crown gall, which is caused by bacteria. However, if insect galls are cut open, numerous larvae - or the cells in which they develop - are visible.

No known insecticide will control gall-producing insects. The best control is to prune infested stems and bury them promptly to destroy larvae before they emerge.

Rose Scale: Old rose stems sometimes become encrusted with white insects called rose scale. These insects called rose scale. These insects suck sap from plants. Waxy adult insects are protected from insecticides.

Control scale insects by treating tiny scale crawlers before they become covered with wax, or with a systemic for covered scale adults. If scales persist until fall, prune heavily infested stems.

Rose Slugs: Rose slugs, the larvae of three species of sawflies, feed on rose leaves, causing them to appear skeletonized. The insects appear suddenly and do their damage quickly, so be alert and act quickly to ensure the effectiveness of control measures.

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