List of Common Diseases and Insect Pests of Rhododendrons
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):
B. Abiotic Problems (caused by nonliving agents):
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.