Plant Disease Management Handbook

Phytophthora Root Rot of Rhododendron.

Healthy rhododendron root system (left) and three rhododendron root systems with Phytophthora root rot (right).

The bark has been stripped off to show the cambium which becomes a reddish-brown color when infected. Also note the clear transition between infected and healthy cambial tissue.

Standing water can occur due to gravel settling or filling in with plant debris or spilled container media.

Pots sitting on plastic sheeting can create a depression that holds water and encourages root rot.

Well drained areas can accidentally get plugged up flooding unsuspecting plants.

Hoses that come in contact with greenhouse floors, drains or contaminated plants can spread propagules of these organisms.

Phytophthora Root Rot is a problem wherever standing water occurs such as the bottom of this hill where there are dead and dying plants.

Phytophthora Root Rot is a problem wherever standing water occurs such as where these plants get splashed everytime a vehicle goes by.

Container rhododendrons showing symptoms of Phytophthora root rot when located in low lying areas of the canyard where water collects.

80% incidence of Phytophthora root rot in field-grown rhododendron.

Phytophthora root rot beginning in low lying corner of a field where water is not draining properly. Note that rhododendrons grown closest to the water (foreground) are stunted and occupy less field space than plants further from the waterlogged area (background).