When large rhododendron shrubs are incorporated into landscapes for their shape, form and brilliant flowers, the last thing any gardener wants is to find that her flowering perennial has been affected by a fungal disease. Concerns associated with a rhododendron's diseased state include how did it become vulnerable or stressed, was it watered or fertilized properly and was it planted at the proper depth. When the answers are no to any of these questions, you may find that your rhododendron is vulnerable to a fungal disease.
Phytophthora Root Rot
This disease is caused by the Phytophthora cinnamomi fungus that affects the roots of rhododendron shrubs and other plants. The fungus affects the roots by changing their color to a red-brown tone and it spreads until it reaches the shrub's main stem, according to North Carolina State University's Plant Pathology Extension. Among the ways to prevent phytophthora root rot is to plant the rhododendrons in well-drained soil and prevent water from collecting around the base of the shrub.
Petal blight is caused by the Ovulinia azalea fungus. It starts by forming white to brown spots on the flowers' petals. As the fungus grows and spreads, it weakens the petals and causes the flowers to die, usually within three days. North Carolina State University suggests preventing the formation of this fungus by applying Terraclor to the soil and triadimefon to the plants once the flowers begin to bloom. If the disease is noticed on a few flowers, pick and discard them away from the rhododendron plant.
The Microsphaera penicillata fungus causes discoloration to the leaves of rhododendrons. According to the Botanic Gardens Trust of Sydney, Australia, this white fungus growth eventually covers the entire surfaces of the affected leaves. This fungus affects young or shaded rhododendrons growing in humid climates.
Potted rhododendrons are commonly affected by several Phytophthora fungi that cause the leaves to dieback and fall off the stems. The fungi form as a dark-colored discoloration on the leaves, and they attack both leaves and stems, particularly of succulent plants since they have high-water content.
This fungal infection can also be called Botryosphaeria dieback since its fungus, Botryosphaeria dothidiae, causes older woody parts of rhododendrons to dieback. It forms as a brown color on a part of the stem and then spreads along it. The fungus causes the branches to die and the leaves to turn from green to grayish to brown before rolling up and dying back.