Phytophthora Root Rot
Phytophthora root rot fungus thrives in poorly drained soil. Symptoms include yellowing leaves, smaller new growth and early leaf drop. Roots become brittle and turn a reddish-brown color. As more of the roots become infected, the plant withers and dies. Fungicides do not cure the disease; they only suppress it. Prevent this fungus by selecting disease-resistant azaleas and planting them in well-drained beds. The symptoms of phytophthora root rot may be confused with those of soil iron deficiency.
Leaf gall is not seriously damaging to the azalea. Infected leaves emerging in April and May become thickened; they also curl and turn pale green, almost white, in color. As the infection spreads, a white powdery coating covers the galls. If caught early, the thickened curling leaves can be removed by hand. Fungicide is usually unnecessary, since plant damage is mild.
Petal blight fungus only affects the blossoms on the azalea plant. Tiny white spots appear on colored petals, and rust-colored spots appear on white petals. As the infection progresses, the spot size grows, and the petals become slimy. Cool, wet weather during flowering season increases the chance of petal blight. Pick off and destroy infected flowers and apply a fungicide such as Benomyl when the flower buds begin to open. Future treatments every five days during the blooming period may be needed.
Well-established azaleas are more susceptible to twig blight. Aggravated by environmental stresses such as drought or excessive heat, the infection causes twigs to wilt and leads to early leaf drop on infected branches. Fungicides are not effective in controlling twig blight. Instead, prune the diseased branches several inches below the discoloration. Disinfect the shears with alcohol between each cut to avoid spreading the fungus to healthy portions of the plant.
More unsightly than damaging, powdery mildew appears in the fall months. A white powder-like coating covers the leaves and causes premature leaf drop. A fungicide like Bayleton may be used if the leaf drop becomes serious. Usually two to three applications are required.
Leaf rust fungus appears in mid-summer, causing yellow spots on the tops of the leaves. Rust-colored spots on the leaf bottoms produce orange-colored spores. As the infection progresses, serious leaf drop will occur. Wet leaves increase the chances for continued infection through the late summer and early fall. Prevent the fungus from overwintering by clearing away dead leaves around the flower beds. Hemlock trees can also be infected by leaf rust fungus, causing the azaleas to be reinfected each year.