Collar Rot

Phytophthora species

Water moldalso: Crown rot, Phytophthora collar rot, Foot rot, Phytophthora crown and collar rot

A rot of the trunk base and crown at or just below the soil line, usually caused by Phytophthora water molds in wet, poorly drained ground. It girdles the tree from the bottom, choking off the flow of water and nutrients, and is a common cause of slow decline and sudden death in fruit trees and many woody plants.

🔎 How to spot it

Above ground the first signs are vague decline: leaves yellow, growth is weak, and the canopy thins, often on one side. At the base, the bark turns dark, wet, and soft and may ooze gum or sap, and cutting it away reveals a reddish-brown rotted inner bark with a sharp line between diseased and healthy white tissue. The rot can extend down into the crown and major roots.

🥀 Damage it causes

As the rotted band widens it girdles the trunk, and the tree declines and can die, sometimes slowly over seasons and sometimes collapsing in a single hot spell once the damaged roots cannot keep up. Young trees and those on susceptible rootstocks can be killed outright.

🔬 What causes it

Soilborne Phytophthora species, which need saturated soil to spread and infect and can survive in the ground for years. Poor drainage, heavy or compacted soil, planting too deep, and mulch or soil piled against the trunk all keep the crown wet and invite infection.

🛡️ Prevent it

Plant on well-drained sites or raised berms, set trees at the right depth with the graft union well above the soil, and keep mulch, soil, and turf pulled back from the trunk so the base stays dry. Avoid overwatering and prolonged soil saturation, and choose resistant rootstocks where collar rot is a known problem.

🧯 If it is already here

Improve drainage and let the crown dry: pull soil and mulch away from the trunk to expose the affected area and remove standing water. On valuable trees a fixed-copper or phosphonate (phosphorous acid) treatment can help, but badly girdled trees rarely recover and are best removed, then replanted with resistant stock on a better-drained site.

💡 Good to know

Collar rot specifically attacks the scion bark just above the graft union, while crown rot hits the rootstock below it, but the cause and cure are the same: keep the base dry and well drained. A sharp margin between brown dead bark and white healthy bark at the trunk base is the classic confirming sign.

🌱 Plants it affects

72 plants in the library can be affected by this problem

Aunt Molly's Ground Cherry

Black Tartarian Cherry

Butterfly Blue Pincushion Flower

Crimson Cherry Rhubarb

Garvinea Sweet Glow Gerbera

Texas Scarlet Flowering Quince

Triple Crown Blackberry

Valencia Orange

Victoria Rhubarb

For educational and informational purposes only. Disease management advice is general guidance drawn from university cooperative extension sources; always identify a problem positively and read and follow the label on any product before use, especially around food crops, children, and pets.