More than 700 species of rhododendrons exist throughout the world. Rhododendrons are shrubs that are often confused with azaleas, a plant from the same species. Although they can grow in the wild with minimal care, garden rhododendrons do struggle with some diseases and pests. The best treatment against disease and pests is preventive care, but sometimes the best-cared-for rhododendrons can still die from diseases and pests.
Dieback is a disease caused by a fungus known as the Botryosphaeria canker. The fungus attacks leaves either at their tips or margins causing spots to form. Next, stalks and twigs are infected. Another common sign of the fungus is the reddish-brown color that can be found when scraping an infected branch. Treatment involves removing and disposing all dead or diseased parts. Use bleach on all cleaning tools.
Root rot is a disease that especially occurs in poorly drained soil. It can be prevented by ensuring plant drainage is good and plants aren't crowded. Although soil fungicides can help prevent or control the disease, usually the problem can be tolerated when just one or two plants are infected.
Flower blight causes water-soaked spots to appear on flowers that later become brown and soft. The disease is mainly caused by heat and humidity. Plants need to be kept dry during hot summer months to avoid the disease so watering should be reduced. Once the disease attacks infected leaves should be removed from a plant to prevent other fungal infections from developing.
Chlorosis is a somewhat common disease of rhododendrons. It's an iron deficiency caused by a lack of chlorophyll which causes leaves to turn pale green and then yellow. Besides plant deficiencies, the problem is caused by poor soil drainage, plant nutrient deficiencies, high alkalinity and compacted roots.
The worst pest attacking rhododendrons is the root weevil. According to Rhododendron.org, this insect attacks more than 100 different plant species besides rhododendrons. Root weevils come in dark colors of slate gray to black brown and have small white or yellowish flecks on their back. These small, yet visible insects are ¼ to ½ inch long. They're also characterized by a short, wide snout that has an attached antenna.
The lace bug is a common insect problem in warm regions. Tiny nymphs travel on the leaf's undersides to suck juices which cause yellow spotting. When a plant is infested with many lace bugs the entire leaf turns yellow and then brown before dropping. Lace bugs can be treated using insecticides such as Cygon, Malathion or Orthene.
Stem borers are pests which makes girdles on tips of growing shoots, resulting in weakened stems, causing them to break off and die. Other common pests include caterpillars that eat leaves (thrips), mites, and scale insects. Local extension agents can recommend chemicals and explain how to use them for treating pests.