See:
Rhododendron - Phytophthora Root Rot
Notes Phytophthora root rot can be caused by a number of Phytophthora species. These pathogens are fungus-like microorganisms and are also known as water-molds. Poorly drained, waterlogged soil or media, plastic ground covers, and excess irrigation favors these organisms. Fibrous roots rot first, turning brown or black, followed by the rest of the root system. The edge of the advancing lesion is distinct from adjacent, unaffected tissues which appear moist and light in coloration (white to green). Once roots are rotted, symptoms of nutrient deficiency and drought stress appear aboveground. Use many different tactics to manage root rots but focus efforts on water management. Use fungicides as preventative treatments. Although this has only been diagnosed once by the OSU Plant Clinic, it may be more common in nursery production.