Leaves Curled and Distorted by Aphids
Aphids, also called "plant lice," are soft-bodied, pear-shaped sucking insects about the size of the head of a pin. They sometimes cluster in large masses on tender new leaves and buds of spirea. Their feeding retards plant growth, and causes the leaves to curl and turn yellow. Ants, attracted by the aphids' honeydew secretions, wander over the plants and protect the aphids from natural predators.
Check stem tips and leaf undersides for small groups of aphids. Mash isolated clusters between thumb and forefinger or spray light infestations with a vigorous water spray 3 times, every other day or so, in the early morning. Eliminate nearby ant nests if possible. If the aphids are all over the shrub, spray them directly with an insecticide product featuring insecticidal soap according to label instructions. Since most types of spirea rarely have pest problems, consider if yours may be under stress which makes it vulnerable, and address this situation.
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Leaves Rolled into Tubes by Leafrollers
Leafrollers are the larvae of small moths. They protect themselves while feeding by rolling leaves into tubes and binding them with strands of silk. The oblique-banded leafroller sometimes attacks spirea leaves. Adult moths are brown or gray, 1/4 to 1/2 inch long. Larvae are dark to light green or cream to yellow caterpillars, 3/8 to 1 3/4 inch long.
If there are not too many leafrollers, handpick the larvae in their leafy tubes and crush them. Control larger infestations by spraying or dusting foliage with a product containing Bacillus thuringiensis, or Bt, as soon as you see the feeding caterpillars and the rolled leaves on your spirea. They will ingest the bacterium and later die. Follow the instructions on the package label. Because Bt is rapidly inactivated by sunlight and rain, you may have to repeat the spray.
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Shrub Stunted, Leaves Discolored by Root Knot Nematodes
Spireas infested with Southern root knot nematodes look sickly, wilted, or stunted. They develop yellowed or bronzed foliage, then they decline slowly and die. All these symptoms result from nematodes, whitish, translucent, roundworms about 1/50 to 1/10 inch long, which attack their roots. Infested roots are poorly developed, show knots or galls, and may be partially decayed. Nematode activity is most obvious in hot weather, when wilting shrubs fail to recover from the heat.
Control nematodes by adding lots of compost (especially leaf mold) to the soil around the spirea plants to encourage beneficial fungi that attack nematodes. Pour diluted liquid fish emulsion onto the soil around affected roots. It is toxic to nematodes. Remove dead or dying shrubs with the soil around their roots and discard them in the trash.
Stems, Leaves Encrusted with Small Bumps Indicate Scale
The first sign of a scale attack is that some of the spirea’s leaves turn yellow, then drop off. Infested new growth in spring is stunted and severe infestations can kill a young shrub. Some species of scale excrete honeydew, which attracts ants and encourages the growth of sooty mold on the leaves. If you notice any of these symptoms, look for the scale insects themselves. They suck sap from plant foliage protected by rounded waxy shells which make telltale bumps on stems and leaf undersides. The bumps may be white, yellow, or brown to black, and are about 1/10 to 2/5 inch in diameter.
Simply scrape small patches of bumps off spirea leaves or stems with your fingernail or a cotton swab dipped in rubbing alcohol. Spray more heavily infested shrubs with light horticultural oil according to the directions of the label to smother the insects and any eggs.
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Leaves and Twigs Die Back Due to Fire Blight
This bacterial disease is spread by insects and rain. It is one of the most destructive diseases of some spireas, blighting young twigs and limbs. Leaves die, hang downward and cling to the blighted twigs. Dying branches, which appear scorched, are conspicuous during the summer and ruin the shrubs' ornamental value.
Prune out infected branches, cutting at healthy tissue well below blighted areas. Spray shrubs with a streptomycin antibiotic wettable powder product to protect blossoms. Do not over fertilize, because excess nitrogen makes the shrub more blight-prone. This disease is rarely serious unless the spirea is planted near pears, apples or hawthorns which are also susceptible to it.
Leaves Coated with White Powder Due to Powdery Mildew
Powdery mildews caused by various fungi sometimes attack some kinds of spireas, coating their leaves and growing tips with whitish patches. While this disease is not fatal, it does mar the appearance of the shrub. Since control is time consuming, most homeowners choose to live with it.
To protect healthy foliage on an infected shrub from the spread of the mildew, spray it thoroughly with a general garden fungicide containing sulfur according to instructions on its label. Collect and discard all dead leaves, branches and mulch in the fall to reduce overwintering mildew spores. Spread fresh mulch for the winter.
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Discolored Blotches on Leaves Caused by Fungal Leaf Spot
Yellow, brown or black blotches, which frequently run together, appear on leaves affected with fungal leaf spot. Heavily infected leaves turn yellow or brown and fall prematurely. Some fungal spots are surrounded by flecks or black dots, the spore-bearing fruiting bodies. You can easily distinguish leaf spot diseases from spots caused by winter injury or ice that focuses sunlight, burning the leaves.
Cool, moist weather promotes fungal diseases, especially when new leaves are developing. Pick off and discard infected leaves. Shake out all fallen and diseased leaves from the center of the shrub and destroy them. Cut down and trash (do not compost) seriously infected shrubs together with the soil ball. Mulching helps prevent splash-borne infection in outdoor plantings. Spray healthy foliage every 7 to 10 days with a general garden fungicide to protect it from the spread of the infection.
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Foliage Burned by Dog Urine
Dog urine is mildly toxic to most ornamental plants, as well as lawns. It may discolor spirea foliage and even kill lower branches that dogs visit repeatedly. Spray vulnerable foliage with an anti-transpirant spray to provide some ground-level protection. Also try screening the plants or spraying foliage with a repellent spray. Prune out damaged areas to stimulate regrowth of new, healthy stems and foliage. For more information see the file on Dealing With Dogs and Cats