Phytophthora Root Rot

Phytophthora root rot is a disease that attacks thousands of types of trees and shrubs. It is caused by several species of soil-borne pathogens in the genus Phytophthora. The disease begins when these fungus-like organisms attack the plant's fine roots, gradually advancing toward larger roots and up into the lower trunk. As the health of the root system declines the tree's ability to absorb water and nutrients is negatively impacted. In severe cases, the root system can fail entirely, resulting in the death of the tree.

Phytophthora thrives in soil that is poorly drained or excessively irrigated. When soil moisture and temperature conditions favor disease development, the spread and severity of infection can increase drastically. Additionally, trees and shrubs with root rot may cause infection in both established and new plantings that are nearby. It can survive long periods in the soil, even when no host is present, so it is prudent to be careful when planting on sites that have ever experienced an outbreak.

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Affected Areas

Plant death due to Phytophthora root rot. Note how the adjacent plants are beginning to show symptoms of yellowing/discoloration.

Phytophthora root rot can affect nearly any tree or shrub species in nearly any geography. Some species show a higher susceptibility including: azalea, rhododendron, holly, boxwood, hemlock, mountain laurel, fir, and white pine. Woody ornamentals such as oak, beech, dogwood, maple, and fruit trees are also prone to infection. This disease will be more common in places where there is poor water drainage.


Curling of leaves on plant infected with powdery mildew.
Severe symptoms of root rot visible in the root system.

The symptoms of Phytophthora root rot vary depending on the host. The first sign of infection is usually only apparent in the root system. Affected roots will be discolored and brittle. As the disease progresses, smaller leaf size, thinning of the crown and branch dieback often become apparent. Unfortunately, these later, outwardly noticeable symptoms may not appear until well after the actual infection when the disease is already well-established.


The most important factor in managing and treating Phytophthora root rot is ensuring that plants are growing in proper soil conditions. Carefully monitor irrigation and confirm that the root collar and root flare of the tree are exposed and free of excess soil or mulch. Adding organic matter and fertilizing to address nutrient deficiencies are other preventative measures to take to improve overall soil conditions for healthier trees that can better withstand infection. In poorly drained soils, plant only species that are most tolerant of these growing conditions.

Treatments are available for trees and shrubs showing disease symptoms. Given the persistent nature of Phytophthora in the soil, multiple treatments are often required and should also be considered for nearby plantings.

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