Plant Health Problems
Diseases caused by Fungi:

Powdery mildew, Microsphaera penicillata.
Powdery appearance of leaves indicates infection by this fungus. This disease is favored by hot, humid weather.

Powdery mildew can be minimized by improving air circulation around the plant. Chemical control is usually not necessary.

Diseases caused by Bacteria:

Fire blight, Erwinia amylovora.
See Pear for a more detailed discussion of this disease.

Insect Problems:

Spirea aphid, Aphis spiraecola.
At least two species of aphids infest spirea and probably there are several others. When needed, ultrafine horticultural oil, insecticidal soap or malathion are among the compounds registered for control of this pest in Connecticut. Apply as foliar sprays when insects are present. Another option is to apply imidacloprid as a systemic to be taken up by the roots. Consult the label for dosage rates and safety precautions.

Spirea leafroller, Olethreutes hemidesma.
In certain seasons in August the larva of this insect webs together the leaves of the new shoots of Spiraea vanhoutei (and probably other spireas) and lives and feeds inside the nests. The remedy is to clip off and dispose of the nests.

Spirea scale, Eriococcus borealis.
This scale sometimes infests Spiraea thunbergi (and probably other spiraeas), gathering in the forks of the twigs and stems. This is a white, mealy scale closely resembling the azalea scale. Among the compounds registered for control of this pest in Connecticut are ultrafine horticultural oil, insecticidal soap or malathion. They can be applied as a spray to twigs and stems late in June or early in July when crawlers are active. Thorough coverage of the plant with spray is necessary to be effective. Imidacloprid applied to the soil as a systemic can provide season-long control. Consult the label for dosage rates and safety precautions.