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Phytophthora (Collar Rot, Crown Rot, Root Rot)
Also SeeFruit Tree Diseases and Pests: An Introduction
List of Common Fruit Tree Diseases and Pests
Phytophthora, once considered fungi, are now classified as oomocytes or "water molds." These pathogens live in the soil, and if soil moisture is maintained, they can survive for years, patiently waiting for some unsuspecting gardener to plant a tree in their neighborhood. Once a host is available, phytophthora can infect the roots and the crown (the section of the trunk at the soil line). The pathogen is spread by irrigation, rain, and soil disturbance. Infected trees can introduce phytophthora.
A tree infected by phytophthora will have an overall sick appearance. It can be difficult to distinguish this ailment from wet feet, winter injury, or nutrient deficiency. If phytophthora is suspected, the base of the tree can be inspected. Dig out some soil from around the trunk and peel back the bark from the exposed area. The material under the bark is called cambium, and when it is healthy, it is green. Infected cambium will be orange or brown. If a sickened tree is dug up completely, its roots may also be orange or brown. When phytophthora has caused crown rot, this can appear as a brown, grey, or bruise-colored canker on the lower trunk that is easily confused with fire blight. Trees may be infected in more than one location.
The two best defenses against phytophthora are water management and resistant rootstocks. Avoid prolonged soil saturation, poorly drained planting sites, and depleted soils. Healthy soil will contain beneficial antagonists.
All of the apple rootstocks that we offer are tolerant of phytophthora (wet feet): the Geneva rootstocks, B.9, B.118, P.18 and MM.111. Myrobalan plum and Mazzard cherry are also resistant to phytophthora. While tolerant of wet feet, this doesn't mean that they like it. All fruit tree rootstocks prefer well drained soil.
For further information see the Cornell Guide to Growing Fruit at Home and the Cornell Cooperative Extension IPM Factsheet on Phytophthora Root and Crown Rots.
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