The showy spring blooms of healthy azaleas can add interest to the garden year after year. Unfortunately, several diseases can affect azaleas, causing damage to the flowers, leaves or root system. Following good cultural practices in the garden can prevent many diseases, but in some cases gardeners may need to apply chemical controls or remove infected foliage and branches.
Leaf gall attacks azaleas in early spring. Leaves grow thick and curl, turning pale green or white. A powdery layer appears on the leaves during the late stages of leaf gall. The leaves also may turn completely brown. Although unsightly, this disease does not result in long-term damage. Remove and destroy the affected leaves. If you have several infected azaleas in your garden, apply fungicides containing mancozeb or triadimefon to fight this disease. Native azaleas are more susceptible to leaf gall than hybrid varieties.
Root and Crown Rot
Caused by the fungus Phytophthora, root and crown rot often often attacks azaleas planted in poorly drained soil during wet weather. Azaleas affected with root rot display wilted, possibly yellow, leaves and soggy, dark-colored roots. Crown rot discolors the woody base of the azalea. Chemicals cannot control root and crown rot. Examine azaleas before purchasing and avoid choosing wilted plants or plants with darkened roots. Plant azaleas in well-drained soil or raised beds. Amend clay soils with compost to increase aeration. Some azaleas, such as Indian and Korean azaleas and the "Hampton Beauty" and "Polar Sea" hybrids, have shown some resistance to root and crown rot.
The fungal disease petal blight causes small spots to form on flowers of azaleas. White spots appear on colored flowers and brown spots appear on white flowers. As the spots grow, the flowers begin to rot. This disease spreads by splashing rain, wind and insects and over-winters in the soil. Avoid overhead watering of azaleas, and remove and destroy any affected flowers. Replace any mulch around diseased plants to keep the fungus from surviving in the soil. Apply fungicides containing chlorothalonil, maneb or triadimefon according to product directions.
Phomopsis twig blight tends to attack stressed azaleas after periods of drought or extreme heat. This disease causes the leaves of affected plants to wilt. Defoliation of entire branches may occur. Under the bark, the tissue appears reddish-brown. Remove branches by pruning several inches below any discoloration. Disinfect pruning tools with rubbing alcohol between each cut to prevent spreading the disease. Discard all diseased branches away from the garden.
Leaf rust often attacks plumleaf and swamp azaleas. Small yellow spots appear on the top of the leaves, and rust-colored spores form on the bottom. Repeated bouts of leaf rust can lead to defoliation of the infected azalea, but the disease does not cause serious damage. Do not allow leaves to remain wet overnight ,and regularly clean up fallen foliage to prevent the spread of the disease. "Balzac" and "Red Letter" cultivars have some resistance to leaf rust.