The fungus Phytophthora, found in many Kentucky landscapes, is capable of causing root, crown, and collar rot diseases of woody plants, especially those in moist sites. Susceptible plants include apple, arborvitae, azalea, beech, birch, boxwood, Chamaecyparis, cherry, dogwood, elm, fir, forsythia, Franklin-tree, hemlock, horsechestnut, Japanese holly, juniper, maple, oak pear, Pieris, pine, plum, Rhododendron, sweetgum, tuliptree, and yew.

Symptoms: Root and lower trunk symptoms include death of absorptive and transport roots, death of phloem and cambial tissues on the bark at the base of the trunk and on buttress roots. Sometimes infected tissues appear as dead streaks on the lower trunk. Dead bark may be scaly or peeled back; bleeding cankers sometimes occur. Infected inner bark and cambium tissue typically turns a cinnamon brown or dark brown color. Often, when root or crown infections are well advanced, the tops of affected trees show symptoms. They include undersized, chlorotic, folded, spinastic leaves; thinning, tufting, or browning of the foliage; premature fall color; stunting; dead branches; and tree decline and death. Not all Phytophthora infections occur at the base of the tree; some result in trunk and branch cankers, shoot blight, and even fruit rots.

Cause: Root, crown, and collar rots are caused by many species of the fungus Phytophthora, including P. cactorum, P. cambivora, P. cinnamomi, P. citricola, P. citrophthora, P. cryptogea, P. drechsleri, P. lateralis, P. megasperma, P. nicotianae var. parasitica, and P. syringae. To accurately diagnose a Phytophthora root rot disease requires microscopic examination of laboratory cultures made from infected plant tissues, and to differentiate one species from another requires a specialist.

Conditions favoring disease: Poorly drained soil or wet sites favor the disease. Phytophthora produces zoospores which are motile in water allowing the fungus to swim from infected to healthy roots in flooded or waterlogged soils. In addition to aiding dispersal, wet soils stress the root systems, making them more susceptible to Phytophthora diseases.

Infection can occur when soil temperatures are in the 60's and 70's F. Soil moisture just below saturation allows sporangia to form in a few hours and motile zoospores to be released soon after. Zoospores infect feeder roots just behind the root cap. The fungus can be splash-dispersed during heavy rains or overhead irrigation. In the nursery, the fungus can be carried in run-off from plant to plant in the field or from an infected plant to the drain holes of containers of nearby healthy plants. Phytophthora overwinters in the soil and in plant debris in the form of resistant oospores enabling the fungus to survive in soil and plant debris for long periods of time.

Control:

• Begin with disease-free plants.
• In landscapes, improve soil drainage before planting by installing drain tiles; creating planting mounds, berms or drainage ditches; and amending heavy soils in planting beds to promote internal drainage. Do not plant trees and shrubs too deeply.
• Manage water and drainage in nurseries by avoiding wet fields, use of run-off water for irrigation, and overhead watering in late afternoon. For plants grown in containers, plant in well-drained growing media such as composted hardwood bark which suppresses Phytophthora. Place containers on an area that has been graded to insure drainage away from the growing area, or place containers on a thick bed of gravel or other well drained material. Group different types of plants by water requirement so that plants are not over or under watered.
• If the field previously harbored Phytophthora, avoid using it or fumigate to kill the fungus. For container production, use clean containers.
• Choose species, cultivars and rootstocks that are resistant to Phytophthora diseases.
• Fungicides such as etridiazole (Terrazole, Truban), etridiazole + thiophante methyl (Banrot), fosetyl-Al (Aliette), metalaxyl (Subdue, Subdue Maxx), or propamocarb (Banol), can be used in the nursery to protect healthy plants. Check the label for rates and application methods and for potential landscape uses suppress Phytophthora.

Return to Richard E. Bir homepage
North Carolina State University