Phytophthora root and crown rot—Phytophthora spp.
Phytophthora cinnamomi and other Phytophthora spp. commonly cause maladies that are also called collar rots, foot rots, and Phytophthora root rot. These funguslike water molds (Oomycetes) affect many plant species. Phytophthora ramorum infects aboveground plant parts and causes sudden oak death and Ramorum blight.
Root- and crown-infecting Phytophthora cause leaves to wilt, discolor, remain undersized, and drop prematurely. Twigs and branches die back and the entire plant, especially when young, can be killed as roots and vascular tissue die. Plants infected when they are mature grow slowly and may gradually decline.
Small brownish lesions encircling feeder roots are an early symptom of infection. As disease progresses, the small, fibrous feeder roots become scarce. Where present, the small roots are black, brittle, and dead from infection. Woody roots decaying from Phytophthora alone are firm and brittle but eventually soften as secondary decay organisms develop.
Depending on environmental conditions and the species of pathogen and host plant, sap that is black, brown, or reddish may ooze from darkened areas of trunk bark. Cutting away bark from the basal trunk and roots often reveals a brown or reddish streak, stain, or canker in infected wood with a water-soaked margin separating it from the healthy whitish or yellowish wood.
Confirmation of Phytophthora requires sending an appropriate plant sample with viable infection to a diagnostic laboratory. Test kits that employ the serological (antigen-antibody) technique ELISA (enzyme-linked immunosorbent assay) are available for use in the field to confirm the presence of Phytophthora where research-based sampling recommendations have been developed.
Phytophthora spp. can survive in the soil for many years. They require high soil moisture to infect host roots, so the pathogen thrives in areas of excess soil moisture and poor drainage.
The pathogen infects the smaller (feeder) roots. Infected root tissue develops spore-forming structures (sporangia) and produces spores (chlamydospores, oospores, zoospores). Phytophthora spores spread from tree to tree easily and rapidly in water as it moves over or through the soil and the spores persist in soil and water.
Phytophthora spp. are commonly introduced into gardens and landscapes through the planting of infested nursery stock. After planting, disease develops and the introduced Phytophthora spreads in the landscape in drainage water and soil movement. Phytophthora also spreads through the movement of contaminated equipment and shoes and through any human or animal activity that moves moist soil from one place to another.
Use only nursery stock that has been certified to be free of Phytophthora spp. and obtain plants from reputable local suppliers. Inspect roots and reject plants that show decayed roots or evidence of crown rot.
Prepare the site well before planting, use excellent sanitation, and provide appropriate cultural care, especially proper irrigation. Good water management is the most important factor in reducing the threat of Phytophthora root and crown rot. Where soils are compacted, drain poorly, or are often soggy, improve drainage and plant only species not reported to be susceptible to P. cinnamomi.
Some pesticides (e.g., phosphonates) help to control certain Phytophthora spp. if combined with practices such as avoiding overirrigation and improving drainage. However, plants larger than medium-sized shrubs can be difficult to effectively treat in landscapes. For more information, see the Pest Notes: Phytophthora Root and Crown Rot in the Garden.