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How to Manage Pests
UC Pest Management Guidelines
Floriculture and Ornamental Nurseries
Azalea (Rhododendron spp.)
Disease Control Outlines
(Reviewed 1/02,
updated 1/02)
In this Guideline:
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| 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 spp., 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). |
Fungi (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. Use ELISA tests for asymptomatic 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).
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UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
R. D. Raabe (emeritus), Environmental Science, Policy, and Management (ESPM),
UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside
C. A. Wilen, UC IPM Program, UC Cooperative Extension, San Diego Co.
A. H. McCain (emeritus), Environmental Science, Policy, and Management (ESPM),
UC Berkeley
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