A Common Rhododendron Leaf Spot disease
It's not uncommon to see dark blotches on the leaves of rhododendrons and azaleas. This unsightly fungus is referred to as Cercospora leaf spot (Cercospora handelii). Severe infections take their toll, denuding plants of their foliage and weakens plants, eventually killing them.
Lower leaves show the first signs with irregular brown spots that develop a lighter tan eye in the center. Some spots show a yellow halo while others bear tiny dark pimples inside the larger spots. Those wee dimples are the vectors that spread the infection.
A contributing factor that initiates and worsens this disease is drought. Rhododendrons have shallow roots so they dry out quickly, especially when it’s hot and dry. Accumulated drought stress over the years promotes this disease and eventually the plant will slowly decline. Water in the summer and any other time when there’s a lack of rainfall. When you do water, avoid wetting the foliage as this spreads the disease. Instead, thoroughly soak the ground past the canopy.
A thick 3 inch layer of mulch is essential for rhodo health as it protects the soil from evaporation and insulates the soil from temperature extremes.
Avoid crowding rhodos as stagnant air also contributes to this disease. Remove neighbouring shrubs if they are just too close or thin out their stems. When planting rhododendrons, know their mature width and provide them with proper spacing to encourage good air circulation.
Remove infected stems that have died back to just above a set of healthy green leaves where there is no sign of infection. Prune out entire stems if they are fully infected to their base. Don’t leave stubs: cut just above a leaf, stem, or right at a stems base. After each cut disinfect your pruners with rubbing alcohol – there’s no need to dilute or combine one part Pinesol or Lysol to 3 parts water. Place the disinfectant in a spray bottle as it makes it easier to treat the tools as you cut. Rake up infected leaves when they eventually fall off and discard so they don’t reinfect.
With proper care, the infection should lessen. Don’t expect the damaged leaves to repair themselves and turn green. Once they are infected leaf tissues will not revert back, however new leaves should be healthy.