Phytophthora root rot, commonly known as rhododendron root rot, is a fungal disease that affects several species of woody trees and shrubs, including rhododendrons and azaleas. Like most plant fungal diseases, rhododendron root rot is easier to prevent than to treat, and appropriate cultural practices will help to stop it from afflicting your plants.
A fungal pathogen found in the soil causes root rot. According to the Virginia Cooperative Extension, two species are most often to blame for infections in rhododendron: Phytophthora cinnamomi and Phytophthora parasitica. The fungus moves through water, so it infects plants growing where soils remain excessively wet for prolonged periods of time.
Rhododendrons affected with root rot begin by showing decreased growth, slightly wilted foliage and yellowish leaves. Leaves may wilt when daytime temperatures rise and recover at night. As roots begin to discolor and die, you may notice a darkening moving up the stems of your rhododendron, beginning at the ground.
Root rot varies in how long it takes for a plant to die. According to University of Connecticut Integrated Pest Management, young plants grown in containers may die in as little as two weeks, while older landscape plants may survive for a year, succumbing to symptoms only when the plant experiences stress.
Underground fungal structures give off infectious particles called zoospores equipped with a tail that allows them to move through water. Zoospores are attracted by amino acids found in plant roots and, once finding a root, settle in, shed their tails and grow into the rhododendron's root tissue, where they feed on nutrients stored inside of the plant.
The North Carolina State University Plant Pathology Extension urges gardeners to take preventative precautions against root rot; chemical controls are rarely effective once symptoms progress above ground, where they can be noticed by gardeners. Choose well-drained locations for rhododendrons. If your soil is clay or compacted, incorporate organic material such as pine bark into the soil when planting. Avoid amendments like peat or sawdust that retain water. Planting rhododendrons in raised beds also improves drainage in nonporous soils.
Once the disease has established itself in a plant, treatment is difficult, if not impossible. The Virginia Cooperative Extension recommends treating nearby plants to prevent spread of the infection. Using a fungicide approved for root rot in rhododendrons, drench the soil around healthy plants according to instructions on the label.