P. cinnamomi attacks the roots and stems of plants, destroying the root system and reducing the ability of the plant to absorb water and nutrients. In susceptible plants, the young roots become dark and rot. Above ground, symptoms include wilting, yellowing and retention of dried foliage. Infection may result in the death of the plant. Symptoms are often more severe, and death more rapid, when plants are suffering from water stress (e.g. in summer or drought).
Image: Phytophthora Dieback affecting Banksia spinulosa