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
Email: Sharon.Douglas@ct.gov
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
Summary
This fact sheet lists the common insect and disease problems that occur on rhododendrons in Connecticut.