Symptoms of Leaf Spot

• There are a number of types of fungus that can cause the disease, but they all produce similar symptoms. Infestation will generally begin with off-center brown spots with yellow outlines on the leaves. Some fungi attack only the tops of leaves, while others will infest the lower sides as well. Some fungi will cause the spots to eventually turn gray or black. Infected leaves will drop from the plant long before the fall cold sets in for the normal dormancy period to begin.
  
  

Treating Leaf Spot

• Remove all infected leaves and those that drop to prevent the infection from spreading. Prune back dead, damaged or overlapping parts of the plant to increase airflow and sunlight penetration, which will decrease the ability of the fungus to grow and spread. Always water the plants in the morning, so that the leaves are not wet in the cool of the evening; such exposure to wet, cool environments promotes fungus growth. Certain fungicides such as Benomyl are approved to treat the disease. Apply these as soon as the new flowers appear and repeat following specific product instructions.
  
  

Envrionmental Spots

• Environmental stressors can also lead to spots on the leaves. Winter desiccation is one of the primary environmental causes of leaf spots; the dry, infertile months can often lead to purple-black spots on the leaves. These spots generally cause little to no harm to the plant, rarely resulting in defoliation. Simply following recommended watering and fertilizing requirements for your rhododendron plant will help to cure these spots.
  
  

Other Causes

• Many species of rhododendron plants experience spots from viruses, including necrotic ring spot. These spots are generally red-brown and will occur only in certain years on certain plants. There is no known reason for the virus infestation at these times on these leaves, and they do no damage to the plant; they will simply fade with time. Chemicals such as soaps, paints or herbicides can also cause spotting on leaves, sometimes months after the incident. Always protect your plants with plastic sheeting when using chemicals nearby.