Diseases

• Root rot and crown rot are common rhododendron diseases. Poorly drained soils that become waterlogged encourage the fungi that cause these diseases. Stem dieback is a fungal disease that causes cankers that strangle the stem. Stems, twigs and foliage above the canker discolor and drop off. Several fungi cause spots on rhododendron leaves and flowers.
  
  

Preventions

• Providing rhododendrons with the right conditions goes a long way toward ensuring the health of the plant. Rhododendrons need well-draining soil with lots of organic matter, a pH below 6, and partial shade. When using a sprinkler, water early in the day so that the plant has time to dry before nightfall. When watering by hand, wet the soil, not the plant.
  
  

Treatments

• Prune off all infected parts of rhododendrons below the source of infection and remove any diseased leaves and stems that have fallen from the plant. Disinfect pruning shears with a 70 percent alcohol solution or a 10 percent bleach solution between cuts. If the disease becomes so advanced that the plant can't be saved, remove and destroy it, and don't plant new rhododendrons in the same area. Fungicides may be helpful with stem dieback and leaf and flower spots. Use a fungicide containing copper hydroxide, copper salts of fatty acids, or chlorothalonil.