Azaleas (Rhododendron) are common deciduous or evergreen shrubs in the heather family (Ericaceae). According to the Azalea Society of America, there are more than 10,000 azalea varieties. This means that azaleas come in a wide range of colors, sizes and blooming times. While azaleas are overall hardy plants, they can still suffer from a few common diseases and pest problems.
Phytophthora root rot is a soilborne azalea disease caused by the Phytophthora cinnamomi fungal pathogens. While the specific symptoms vary according to azalea species, most infected plants suffer from stunted leaf growth and interveinal chlorosis, or yellowing between the main leaf veins. Some azalea varieties defoliate, while other species have foliage that turns a distinct purple color. Many infected azalea plants gradually weaken and die. The Phytophthora pathogens thrive in warm, moist environments. This root rot is more prevalent with azaleas planted in clay soils. Control includes planting disease-free varieties in well-drained soils.
Exobasidium leaf gall is a fungal disease (Exobasidum vaccinii) that attacks an azalea plant's emerging leaves. The affected leaves curl, thicken and turn light green to white in color. As the disease matures, the leaves become covered with a powdery, white substance. Control includes pruning out and discarding the infected leaves as soon as symptoms appear. While this azalea disease makes affected plants look unhealthy, it rarely causes any serious damage.
Stunt nematodes (Tylenchorhynchus claytoni) often attack stressed azalea plants. These soilborne parasites feed on the roots, causing stunted plant growth, yellowing leaves, root decay and wilting foliage. Severe cases cause the plant to defoliate. Affected azaleas eventually fail. Control includes planting nematode-free stock and regularly watering the plants.
Ovulinia petal blight is a common fungal disease (Ovulinia azaleae) that thrives in cool, wet climates. Petal blight symptoms typically include small, pale spots forming on the petals. White petals usually form tiny, rust-colored spots. These spots grow quickly, causing the petals to become soft and the blossoms to collapse. This azalea disease more commonly affects the later blooming varieties. Control includes applying fungicide as soon as the petals begin to show symptoms.
Phomopsis twig blight is an azalea disease caused by the Phomopsis fungi. This twig blight causes affected plants to suffer from wilting twigs and dying leaves. Some infected branches turn red-brown in color. Extreme cases cause the branches to defoliate. Twig blight disease is typically more severe following extended heat and drought conditions. Control includes removing and destroying the infected branches.
Leaf rust is a fungal disease (Pucciniastrum vaccinii) that commonly affects deciduous azalea varieties. Leaf rust symptoms typically include small, yellow circles forming on the surfaces of the upper leaves, while rust-colored spores form on the undersides. Leaf rust pathogens overwinter in fallen leaves. Control includes raking up and discarding fallen foliage.
Azalea lace bugs (Stephanitis pyrioides) commonly infest azalea plants. These cream-colored pests earned their name because their wings look similar to lace. The lace bug typically feeds on the undersides of leaves, causing them to turn yellow, stippled or bleached out in spots. The damaged leaves gradually dry and drop from the plant. Indigenous to Japan, azalea lace bugs more commonly attack plants in the eastern regions of the United States (U.S.). Control includes spraying the affected plants with hard sprays of water from the garden hose. Applications of insecticidal soaps often rid plants of these pests. Planting azaleas in shady areas help to reduce the risk of lace bug infestations.