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 |
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