Azalea and Rhododendron Diseases

Botryosphaeria canker

Rough, sunken, dark-brown areas form around wounds or natural openings in the bark. The wood of a recently killed branch is lighter brown than the pith. Dead bark falls off the cankered area. Leaves on affected branches wilt as affected branches die. Chocolate-brown cankers enlarge along the branch more quickly than around its circumference. Tiny black fungal fruiting structures that pepper the dead bark are most easily seen on the light tan bark.

Botryosphaeria dothidea

Irrigate plants to prevent drought stress, which predisposes the plants to this canker. Prune infected branches, cutting back to where growth will resume. Do not leave large stubs of non-growing tissue. Disinfest the pruning shears frequently. No chemicals adequately control this disease. Rhododendron hybrids considered to have some resistance include Boursault, Cunningham's White, English Roseum, Lebar's Red, Roseum Elegans, and Roseum 2.

Cercospora leaf spot)

Circular to irregular brown spots up to 1/2 inch in diameter form on leaves, especially lower leaves. Spots may become tan in the center and may have a yellow halo. Dark-brown, pimple-like fungal fruiting structures form within the spots. Infected leaves may fall.

Cercospora handelii

Inspect new plants and do not use if found to be infected. Rake and destroy fallen leaves. Avoid overhead irrigation. Apply chlorothalonil, myclobutanil, cupric hydroxide, mancozeb + thiophanate methyl, or triadimefon to protect leaves before infection in the spring.

Leaf and flower gall

Young leaves and flowers become swollen, fleshy, and pale green. These become white due to the formation of spores by the fungus on the surface. Later, the galls become hard and brown. Infection occurs in the spring. The new spores formed on the surface of the galls are dispersed but do not cause more galls to form during that same season. They remain dormant until the following spring.

Exobasidium vaccinii

Remove and destroy all galls before they become white with new spores. If many plants had the disease in previous years and galls were too numerous to pick, apply mancozeb to protect new foliage and flowers as they emerge. Applications can cease when the leaves reach their full size.

Phytophthora root rot and top dieback

Plants are stunted and wilted. Leaves yellow, and the entire plant eventually dies. Roots with few feeder roots die. Stem wood at the soil level has red-brown discoloration. In the top dieback phase, leaves have dark-brown spots. Shoots die from the tips back as dark-brown cankers form. No obvious fungal fruiting structures are formed. Compare these symptoms to those of Botryosphaeria described above because these diseases are readily confused.

Phytophthora spp.

Purchase disease-free plants. Especially inspect southern-grown, containerized material before planting. Use clean, disinfested tools. Avoid overhead watering. Following a diagnosis confirming that the plant is infected, remove infected plants. To protect healthy plants, apply mefenoxam, etridiazole, etridiazole + thiophanate methyl, fosetyl-Al, potassium salts of phosphorus acid, fluopicolide, or propamocarb. Fungicide applications to infected plants merely delay eventual death.

Rhizoctonia web blight

Small tan to black spots form on leaves and expand to engulf the entire leaf. Leaves fall. Webbing may develop. Small plants may die.

Rhizoctonia solani

Maintain good air circulation. Avoid late afternoon watering. Apply triflumizole, thiophanate methyl, chlorothalonil, fludioxonil, or iprodione.