The Arborvitae, a popular evergreen from the Thuja genus, is widely valued in landscapes for its dense, fast-growing foliage, often used to create privacy screens and hedges. Despite its general hardiness, this plant is highly susceptible to root rot, a serious and frequently fatal disease that compromises its entire vascular system. The infection is especially common in residential settings where planting conditions are not optimal for this species. Recognizing the subtle signs of infection and taking corrective or preventative action are necessary steps to maintain the health of these conifers.
The Causes of Arborvitae Root Rot
Root rot in arborvitae is primarily caused by soil-borne pathogens, most commonly water molds belonging to the genus Phytophthora. These organisms are not true fungi but thrive in conditions that deprive the plant roots of essential oxygen. Phytophthora species produce motile spores called zoospores that use water films in the soil to swim and infect nearby roots.
The disease is directly linked to environmental factors that create saturated soil for extended periods, which starves the roots of oxygen and creates an anaerobic environment. Poor soil drainage, heavy clay soils, and compaction contribute to this lack of oxygen, stressing the plant and allowing the water molds to proliferate. Excessive or improper watering is a frequent trigger. Once the roots are damaged, Phytophthora organisms colonize the compromised tissue, moving upward into the root crown.
Identifying Symptoms of Root Rot
The first signs of root rot are often visible in the canopy, mimicking drought stress because damaged roots cannot absorb water effectively. Initial signs include a subtle dulling of the foliage, progressing to yellowing (chlorosis), particularly in the inner, older growth. As the disease advances, the foliage turns reddish-brown or bronze, followed by wilting and branch dieback that often starts at the tips or on one side of the plant.
To confirm root rot, a physical inspection of the root system is necessary, typically by gently excavating the soil around the base of the trunk. Healthy arborvitae roots should appear firm and white. In contrast, diseased roots will be dark brown or black, soft, and mushy, easily peeling away or breaking off when handled. In advanced cases, the bark at the base of the trunk, near the soil line, may also show discolored, dark areas where the infection has moved into the root collar.
Managing Active Root Rot Infections
If root rot is confirmed, success is not guaranteed, especially if a majority of the root system is affected. The first step is to stop all supplemental watering immediately to allow the soil to dry out and re-introduce oxygen to the remaining healthy roots. Improving surface drainage is also crucial, which involves removing excess mulch and temporarily exposing the root crown to air for several inches to facilitate drying.
Pruning is essential for salvaging the plant, requiring the removal of all dead or severely discolored branches to limit the spread of secondary pathogens and reduce the canopy’s water demand. For soil compaction, vertical mulching can be employed by drilling holes around the canopy drip line and filling them with organic matter to improve aeration and drainage. Targeted fungicides, such as those containing phosphite (Group P7) or mefenoxam (Group 4), may be used as a last resort to slow the progression of the pathogen. These chemical treatments are more effective at preventing new infections or delaying symptom development rather than curing an established, severe infection.
Preventing Future Root Rot
Proactive measures that ensure proper drainage and adequate soil aeration are the most effective strategies for preventing root rot recurrence. Selecting a planting site with naturally well-drained soil is paramount, as arborvitae will struggle in heavy clay or areas where water pools after rain. Before planting, soil structure can be improved by incorporating organic amendments like compost, which increases porosity and water percolation.
Proper planting depth is another factor, requiring the top of the root ball and the root flare to be level with or slightly above the surrounding soil surface. Planting too deep buries the root flare and creates a basin where water collects, making the plant highly susceptible to crown and root rot.
Once planted, establish a deep and infrequent watering schedule that encourages roots to grow downward rather than remaining near the surface. Check the soil moisture by inserting a finger two to three inches deep, only watering when the soil feels mostly dry at that depth. Finally, apply a two-to-three-inch layer of organic mulch, but ensure a small ring of space is left completely clear around the trunk to prevent moisture buildup at the base.