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UC Pest Management Guidelines


A branch of a small azalea shrub in bloom.

Floriculture and Ornamental Nurseries

Azalea (Rhododendron spp.)

Disease Control Outlines

(Reviewed 3/09, updated 3/09)

In this Guideline:


Disease (causal agent) Symptoms Survival of pathogen and effect of environment Comments on control
Cutting rot and graft decay
(Rhizoctonia solani and
Cylindrocladium scoparium)
Basal rot of cuttings occurs. Under humid conditions, tops are rotted and covered with fungal strands (mycelium). In soil and plant debris. Favored by moist conditions and temperatures of 75° to 80°F. Steam or chemically treat propagating media, flats, etc. Grow stock plants in treated media and observe strict sanitation. Spray or drench cuttings in rooting media with thiophanate‑methyl or iprodione. Cylindrocladium is difficult to control with fungicides; triflumizole can be used in enclosed commercial structures.
Flower blight
(Ovulinia azaleae)
Small, round spots rapidly enlarge and cause entire flower to collapse. Rotted flower becomes soft and clings to leaves or stems. Black fungal structures (sclerotia) produced in diseased flowers and survive in soil. Favored by cool, rainy weather and by moisture on flowers. Spores are airborne. Avoid overhead irrigation. Remove and burn diseased blossoms. Mulch soil with 4-inch layer. Treat soil with PCNB several weeks before plants bloom. Protect blossoms with thiophanate-methyl or triadimefon.
Leaf gall
(Exobasidium vaccinii)
All or part of leaf becomes greatly thickened, distorted, and crisp. Also affects flowers. Infected parts are covered with a white or pinkish bloom of fungal spores. On living plants. Airborne spores produced only during wet weather. Hand‑pick galls where practical before they turn white. Avoid overhead irrigation. Protect foliage with a fungicide, such as mancozeb, during wet weather.
Root rot
(Pythium and Phytophthora spp.)
Plants are low in vigor. Leaves wilt and turn dull green and fall, so only a few terminal leaves remain on the plant. Plants frequently die. Wood under bark at soil line is discolored. Roots become discolored and rotten (Pythium). Root and basal stem rot
(Phytophthora).
Water molds occur in soil. Favored by overwatering, poor drainage, and other factors that weaken plants. Treat growing media with methyl bromide. Select cuttings from high on stock plants. Drench plants with mefenoxam, or spray with fosetyl-al. more info: Pythium Root Rot, Phytophthora Root and Crown Rots *
Septoria leaf spot or leaf scorch
(Septoria azaleae)
Dark, reddish brown, angular spots appear on leaves, which fall prematurely. Leaves yellow on some cultivars. On living and dead leaves. Favored by wet weather. Fungal spores spread in splashing water. Avoid overhead irrigation. Protect foliage with a fungicide such as mancozeb or thiophanate-methyl.
Azaleas are also susceptible to crown gall * (Agrobacterium tumefaciens), gray mold * (Botrytis cinerea), powdery mildew * (Microsphaera sp.), and web blight (Rhizoctonia solani).
* For additional information, see section on Key Diseases.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside

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