Phytophthora Root Rot

Phytophthora root rot often presents as a wilting, declining plant with yellowing or reddish foliage that doesn't respond to watering. When you excavate roots, you'll find them brown or black and rotted, beginning with the fine root tips. Larger roots develop distinct lesions with clean edges between the decayed tissue (dark and sunken) and healthy tissue (light colored, firm). Severely affected plants become wobbly and pull easily from the ground, and cankers may form at the base of the stem as the pathogen moves upward. Container nurseries in Western Washington are especially vulnerable because our wet winters create ideal conditions for this water-loving pathogen.

Phytophthora species are water-molds thriving in saturated or poorly drained soil. High moisture alone doesn't cause disease, but it greatly favors the production of swimming spores that attack fine roots. Management revolves around water management. Ensure excellent drainage in all planting beds and containers, elevating containers on gravel pads at least 4 inches deep; never place containers directly on plastic sheets, which prevent drainage and spread contaminated water between plants. Quarantine new plants for at least one year before integrating them into production areas. Select resistant cultivars when available. The pathogen can survive years in soil and plant debris, so crop rotation is essential: don't plant susceptible crops in previously infested fields for extended periods, or use new, pathogen-free soil if replanting is necessary.