Penn State College of Agricultural Sciences

Department of Plant Pathology

Cooperative Extension
Plant Disease Facts


Gary Moorman
Professor of Plant Pathology

Azalea and Rhododendron Diseases

Disease Symptoms Pathogen/Cause Management
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-colored 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 nongrowing tissue. Disinfest the pruning shears frequently. No chemicals adequately control this disease. Plant resistant cultivars in areas prone to drought. The following hybrids have some resistance: Roseum Elegans, English Roseum, Lebar's Red, Cunningham's White, Roseum 2, and Boursault.
Botrytis Blight Smal,l water-soaked lesions from on petals. Gray fungal growth covers infected petals. Botrytis cinerea Space plants and provide ventilation to avoid excessively high humidity. Remove fading flowers and yellowing leaves. Apply copper, fludioxonil, iprodione, or chlorothalonil to protect healthy tissue.
Cercospora Leaf Spot Circular to irregular brown spots up to 1/2 inch in diameter form on lower leaves. Spots may become tan in the center and 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 triadimefon, thiophanate methyl, chlorothalonil, myclobutanil, or mancozeb to protect leaves before infection in the spring.
Cylindrocladium Blight Plants die rapidly. Leaves turn brown to black and fall in 3 to 4 days. Stems have brown spots on which white masses of spores form later. Or roots die and plants wilt without having leaf spots form. Cylindrocladium scoparium Pot and propagate in pasteurized media. Do not reuse propagation media. Use clean, disinfested tools. Discard infected plants. Remove all crop debris. Apply triflumizole, chloroneb, or thiophanate methyl (may be toxic to some cultivars).
Damping-off Cuttings fail to root, defoliate, and die. Pythium spp. Pot and propagate in pasteurized media. Use clean, disinfested tools. Discard infected plants. Do not leave cuttings in mist beds for excessive periods. Pot as soon as rooted.
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 a fungicide to protect new foliage and flowers as they emerge. Applications can cease when the leaves reached their full size.
Ovulinia Petal Blight Pale white to rust-colored spots from on petals. Spots enlarge rapidly. Petals become slimy and fall apart easily. Ovulinia azalea Remove crop debris. Apply chlorothalonil, myclobutanil, PCNB, triadimefon, ziram, or triforine as blossoms open. Chlorothalonil is phytotoxic to some cultivars. Water in a manner that keeps plant surfaces dry. Space plants, heat, and ventilate to maintain low humidity.
Phytophthora Root Rot and Top Dieback Plants stunted, wilted, and leaves yellow. Plants die. Roots with few feeder roots die. Stem wood at the soil level has red-brown discoloration. In top dieback phase, leaves have dark-brown spots. Shoots die from the tips back with dar- brown cankers forming. Phytophthora spp. Pot and propagate in pasteurized media. Use clean, disinfested tools. Discard infected plants. To protect healthy plants, apply etridiazole, etridiazole + thiophanate methyl, mefenoxam, or metalaxyl. Use composted tree bark as the potting mix. Avoid overhead watering. Plant resistant cultivars (see list below).
Powdery Mildew Faint yellow areas form on expanded leaves. White fungal growth forms on the yellow areas of some cultivars while only small dead spots with no fungal growth forms on other cultivars. Erysiphe polygoni or Microsphaera penicilata Apply Ampelomyces, azoxystrobin, paraffinic oil, myclobutanil, triadimefon, or triflumizole.
Rhizoctonia Web Blight Small tan to black spots form on leaves. Spots expand to entire leaf. Leaves fall. Webbing may develop. Small plants may die. Rhizoctonia solani Maintain good air circulation. Avoid late afternoon watering. Apply PCNB, fludioxonil, or chlorothalonil.

The following list is from: R. K. Jones and D. M. Benson. 1982. Phytophthora root rot and its control in nurseries. Plant Pathology Info. Note #202. Dept. of Plant Pathology, North Carolina State University, Raleigh.

Resistance of azalea cultivars to root rot caused by Phytophthora cinnamomi:

Resistant Susceptible Highly Susceptible
Formosa, Fakir, Corrine Murrah, Merlin, Hampton Beauty, Higasa, Glacier, Rose Greeley, Polar Seas, Redwing, Chimes, Alaska, New White, Shin-Ki-gen, Rachel Cunningham, Pink Gumpo, Eikan, Sweetheart Supreme, Morning Glow Barbara Gail, White Gumpo, Rentschler's Rose, Dorothy Gish, White Gish, Pink Hiawatha, Margret Douglas, Gaiety, Gloria, Kingfisher, White Christmas, Sensation, Prince of Orange, White Jade, Copperman, Hexe, Massasoit, Martha Hitchcock, China Seas, Warbler, California Sunset, Amaghasa, Pride of Summerville, Hinodegiri, Flanders Field Robinhood, Hershey Red, Herbert, Fortune, Catawba, Marion Lee, Snow, Royalty, Kow-Ko-Ku, Rosebud, Mrs. G. G. Gerbing, Coral Bells, Treasure, Pat Kraft, Saint James, Carror, Purple Splendour, Pinocchio, General MacArthur, Pink Pearl, Johga, Sunglow, Hino Crimson, Elaine, Emily, Pink Cloud, Adelaide Pope, Jane Spaulding

COMMON NAME TRADE NAME
Ampelomyces quisqualis AQ-10 (biocontrol)
azoxystrobin Heritage
captan Captan
chlorothalonil Daconil, Exotherm Termil
chlorothalonil + thiophanate methyl ConSyst
copper Kocide, Phyton 27, Nu-Cop, Camelot
etridiazole Terrazole, Truban
etridiazole + thiophanate methyl Banrot
fludioxonil Medallion
fosetyl-Al Aliette
iprodione Chipco 26019
mancozeb Dithane, FORE, Pentathlon, Protect
mancozeb + thiophanate methyl Duosan, Zyban
mefenoxam Subdue Maxx
metalaxyl Subdue
myclobutanil Systhane
paraffinic oil SunSpray Ultra-Fine
PCNB Terraclor, Defend
triadimefon Bayleton, Strike
triflumizole Terraguard (greenhouse or enclosed structure use only)
thiophanate methyl Clearys 3336, Domain, Systec 1998
triforine Funginex