Spirea, a deciduous shrub that can grow from 2 to 6 feet tall, depending on variety, thrives in full sun to light shade in most climate zones. A member of the rose family, spirea can fall prey to various pests and diseases. Here's how to identify some of the most common ones that attack spirea and what to do to treat them.
1. Aphid Attacks
Aphids are small insects that often attack spirea along with other plants in the garden. They are green, yellow, brown, red, or black, depending on species, reproduce in quantities of 80 offspring per insect, and mature in a matter of 7 to 8 days. Although aphids don’t cause tremendous damage to spirea as a rule, they can result in yellowing, curling, and distorted or stunted leaves and plants.
Controlling aphids involves application of insecticidal soap sprayed on the top and underside of leaves. This application may need to be repeated. Another method is to knock off the aphids with a burst of water. This method will need to be repeated, but it will eliminate the aphids for a while. In addition, biologic controls include beneficial lady beetles and lacewings.
If damage is minimal, simply pinch off affected leaves and new growth. Be careful to dispose of the refuse in the trash. Cut down on high nitrogen-content fertilizer. Use less soluble fertilizer with nitrogen and apply in small amounts or use time-release organic fertilizer instead of synthetic.
2. Spider Mites
Spider mites, often living in colonies of hundreds on the undersides of leaves, can cause significant damage to spirea. The first sign is a slight stippling of dots on the leaves, after which the leaves turn yellow and drop off. Webbing may appear all over the plant and damage worsens with water stress.
Control measures include ensuring adequate irrigation, as water-stressed spirea is prone to spider mite attack. Applications of water spray, neem oil, and/or insecticidal soap also help.
3. Scale Attacks
Scale attacks are caused by insects related to aphids. First sign of the scale is that leaves turn yellow and then fall off. The entire plant may be destroyed in scale attack.
Control includes application of a light horticultural spray. Or, simply scrape off the bumps of the scale with a rubbing alcohol-tipped cotton swab.
4. Fire Blight
Fire blight, a bacterial disease affecting spirea, first appears as watery, tan-colored ooze from the branch or twig that soon turns the affected area dark with air exposure. Flower stems wilt and turn black, and blight moves into stems and root systems. The disease overwinters and spreads to neighboring plants by rain and splashing water.
To control fire blight, prune the spirea’s diseased and infected areas by cutting 8 to 12 inches below. Applying copper fungicide to newly-bloomed flowers helps reduce the disease, but won’t do anything for already infected blooms.
5. Powdery Mildew
Powdery mildew is a fungus that attacks many garden plants, including spirea. It is easily recognizable by the powdery, white spore growth on leaves, shoots, buds, and flowers. New growth appears dwarfed or distorted, and leaves often die and fall off earlier than healthy ones.
Prevention is the best control for powdery mildew as the disease is highly susceptible to extreme heat and sunlight. Plant spirea in full sun, with good air circulation and avoid too much fertilizing. Apply fungicide—neem or jojoba oil, sulfur or potassium bicarbonate—at 7 to 10-day intervals before the disease begins.