Phytophthora root and collar rots: what the extension services record

This page describes what the named sources say. It is not a diagnosis of any tree and does not say what should be done with one. None of the three pages read lists these root rots as regulated or notifiable. Each section names its source, all read on October 9, 2026. The soil-borne root and crown rots covered here are separate from the leaf, shoot and bark diseases caused by other Phytophthora species, which have their own pages: sudden oak death and Phytophthora lateralis. The full list of diseases is on the tree diseases page.

What the pathogen is

The Morton Arboretum says there are approximately 180 described species of Phytophthora, that some are found in certain regions and others are widespread, and that some are host specific while others have long host lists. UC IPM, the University of California's pest management program, says several species of soilborne Phytophthora cause root and crown rot, sometimes called collar rot. Both stress that the organisms are water moulds and not true fungi: UC IPM says Phytophthora is more closely related to brown algae, and the Arboretum says the distinction matters because the products that work on true fungi do not always work on Phytophthora.

Hosts as the sources name them

Source	Hosts named
UC IPM	Almost all fruit and nut trees and many ornamental trees and shrubs, including many California natives, when the soil stays wet for long periods or the plant is set too deep. It says different species attack different plants and a single plant can carry several species. It also lists tomatoes, peppers and eggplant.
Missouri Botanical Garden	Many species of fir, true cedar, white-cedar (arborvitae), larch, pine, spruce, yew and Douglas-fir, in its page on Phytophthora root rot of conifers.
The Morton Arboretum	Gives the hosts as most plant species, with no list of trees, and says root rot can lead to the death of the host plant.

Signs the sources describe

Above ground, the Arboretum lists wilting, stunted branches and leaves and a general loss of vigour, and says the cause is that infected roots cannot take up water. UC IPM says the leaves look drought stressed and may turn dull green, yellow, red or purplish, that trees or plants often wilt and die quickly with the first warm weather of the season, that plants in the most poorly drained spots are the usual ones affected, and that plants may collapse one at a time among healthy neighbours. It says symptoms may start on one branch and spread, and that a tree may decline over several years or be killed in one season.

At the root and crown, the Arboretum says the fine absorbing roots turn mushy and lose their firmness, go brown to black, and the outer layer of the root slips off to expose the inner core. UC IPM describes darkened bark around the crown and upper roots, gum or dark sap oozing from the margin of the diseased area, and reddish-brown zones in the inner bark and outer wood, often separated from healthy tissue by a dark line. The Missouri Botanical Garden conifer page describes a cambium that is red-brown or butterscotch coloured at the root collar, dead black feeder roots, heavy pitch bleeding from roots infected by Phytophthora cinnamomi, and trees that turn yellow-red and die, sometimes suddenly.

How it lives and spreads

The Arboretum says Phytophthora can have up to four spore stages, two of them survival spores, and that some species survive in fallow soil for decades. UC IPM says resting spores can last for years in moist soil with no host, but that their survival falls over time if the soil dries out. When free water is present the spores germinate and make swimming spores that infect root tissue; UC IPM and the Arboretum both say wounds are not needed for infection. The sources name the carriers: infected plant material, water and soil (Arboretum), and splashing rain, irrigation and runoff water, contaminated soil, equipment and plant parts (UC IPM). The Missouri Botanical Garden adds that the pathogen grows into the host, girdles the root or trunk and eventually kills the tree, and that the spores return to the soil in rotted tissue if the tree is removed.

UC IPM separates warm-season species such as P. cinnamomi, P. citricola and P. cactorum, which attack when soils are warm and moist, from cool-season species such as P. capsici, P. syringae and P. citrophthora, which attack when soils are cool and moist and are typically a winter drainage problem. It says a laboratory determination to species can be useful, and the Arboretum says identification to species needs DNA sequencing by a lab. The Missouri Botanical Garden says diagnosis can be confirmed only by isolating the organism in pure culture.

Look-alike causes the sources name

UC IPM says no mycelium is visible in trees affected by Phytophthora, which separates it from Armillaria root rot with its white fans between bark and wood and its mushroom smell, and that mushrooms or visible fungal filaments point to a fungal disease such as Armillaria instead. See Armillaria root rot. UC IPM also says plants with this rot do not recover when irrigated, unlike plants suffering water stress, and the Missouri Botanical Garden lists drought, heat, underwatering and overwatering as stresses that can weaken roots. For flooding as a separate cause of tree decline see flooding and waterlogging.

Status and control wording

The three pages stress prevention. The Arboretum calls preventing introduction the best management, UC IPM says proper irrigation is the most important prevention, and the Missouri Botanical Garden lists site choice, drainage and avoiding soil movement from infected areas. The Arboretum says the products for Phytophthora are restricted-use pesticides available only to licensed professionals. This page gives no treatment steps, product names, rates or timings. For who may apply pesticides see pesticide licences.