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List of Common Diseases and Insect Pests of Rhododendrons

PP038 (12/97R)

By Dr. Sharon M. Douglas
Department of Plant Pathology and Ecology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P. O. Box 1106
New Haven, CT 06504-1106

Telephone: (203) 974-8601 Fax: (203) 974-8502

There are pest problems on rhododendron that occur almost every year in Connecticut.

I. Common Problems:

A. Biotic Problems (caused by living agents):
1. Diseases
2. Insects

B. Abiotic Problems (caused by nonliving agents):
1. Cultural
2. Environmental

II. Pest Control:

A. Accurate Diagnosis

B. Severity of Problem:
1. Nature of pest problem (i.e., type of insect or disease)
2. Level of disease or infestation (i.e., loss threshold)

C. Control Options:
1. Culture- This includes cultural methods that modify the plant's growing conditions; maintaining plant vigor by proper watering, fertilizing, appropriately timed pruning and transplanting, adequate spacing, improving the tilth of the soil, adjusting soil pH.
2. Sanitation- This involves use of healthy, disease-free cuttings or plants; pruning of affected plant parts; raking and removing affected plant parts such as fallen leaves; disinfesting tools such as pruning shears and spades.
3. Resistance- This involves use of resistant cultivars, when available.
4. Chemical- This involves proper selection and timing of pesticides which are registered for use in control of specific diseases or insect pests.

III. Diseases Caused by Fungi:

A. Root Rots:
1. Phytophthora Root Rot- Phytophthora cinnamomi Phytophthora spp.
2. Armillaria Root Rot- Armillaria mellea
3. Damping-off and Root Rot- Pythium spp. Rhizoctonia solani

B. Diebacks, Blights, and Leaf Spots:
1. Botryosphaeria Dieback- Botryosphaeria dothidea
2. Phomopsis Dieback- Phomopsis spp.
3. Phytophthora Dieback- Phytophthora spp.
4. Leaf and Flower Gall- Exobasidium spp.
5. Powdery Mildew- Microsphaera penicillata Sphaerotheca pannosa Erysiphe cruciferarum
6. Leaf Spots- Many different fungi
7. Rusts- *Chrysomyxa spp. (10 species) Aecidium spp. (2 species) *Pucciniastrum spp. (1 species) Puccinia spp. (1 species) * reported in United States

IV. Diseases Caused by Bacteria:

A. Crown Gall: Agrobacterium tumefaciens

V. Diseases Caused by Viruses:

A. Rhododendron Necrotic Ringspot Virus

VI. Insect Pests:

A. Black Vine Weevil: Otiorhynchus sulcatus

B. Rhododendron Borer: Synanthedon rhododendri

C. Rhododendron Tip Midge: Clinodiplosis rhododendri

VII. Abiotic Problems:

A. Winter Injury and/or Drying

B. Nutrient Deficiencies

VIII. Unusual Problems:

A. Leaf Flooding

B. Vole Injury


This fact sheet lists the common insect and disease problems that occur on rhododendrons in Connecticut.