Rhododendron root rot is a fungus that affects the roots of the rhododendron. Spores generate and attack the fine roots of the plant, then spread to the rest of the root system, rotting the system. It is carried in the soil, water and plants. Root rot thrives in soil that is waterlogged and poorly drained. It also thrives in plant debris that has fallen off the plant or that has been pruned and left near the plant.
New rhododendron plants should be quarantined for at least a year before putting them in the landscaping with other plants. If the plants show any signs of root rot, destroy the affected plants and discard them.
Drainage and Watering
Start new plants with disease-free cuttings. Make sure the containers have enough drain holes, and place the containers on a gravel bed to ensure proper drainage. Make sure the water is clean irrigation water and that the water has reduced salinity. Do not splash water on the plants, and make sure, if using a hose, to keep the hose off the ground, as the ground may be contaminated.
Keep the Area Clean
Keep the gravel beds clean. Do not allow the water that drains into the gravel beds touch the pots. If one plant is contaminated, the contaminated water will contaminate the rest of the plants. Pick up all fallen or pruned plant debris and discard it in plastic bags.
Phenylamide and phosphonate fungicides do not kill root rot, but they prevent it from becoming established. If you have had problems with root rot in the past, treat the rhododendrons with preventative fungicides.
Fumigate the containers, beds and soil with soil fumigant. Soil fumigant is available at your local nursery. Always put treated soil in treated pots. If you put the treated soil in untreated pots, you run the risk of recontaminating the soil.