Before I continue, I should explain that it is not yet known if the two samples of Phytophthera are of the same strain. It is also important to know that rhododendrons that have not been infected do not transmit the disease. The likelihood of infection of rhododendrons in the landscape, relative to oaks, is not yet known, but treatment of infected plants is much more manageable. Infection of rhododendrons only occurs on foliage during rainy periods and only near the ground. Spores of the fungus are airborne, but seem to complete part of their life cycle in wet soil before being splashed onto low foliage.
Infection causes leaves to turn black and rot.
It easily spreads to nearby leaves, into petioles and eventually into small stems. Rhododendrons should be periodically inspected during and after prolonged wet weather. Although leaves may discolor for a variety of reasons, it is best to remove any that appear damaged or infected, regardless of the symptoms, especially those close to the ground. Any stems that die should be pruned back to healthy stems. Leaf litter below plants should be removed.