Integrated Pest Management
Phytophthora Root Rot of Ornamentals
Phytophthora root rot, caused by fungi in the genus Phytophthora, is a common disease of many ornamental plants. This disease is typically induced by wet, warm soil conditions and usually results in a gradual death of the infected plant. Careful examination of the plant crown and roots is required to positively diagnose Phytophthora root rot.
The fungus attacks the roots and crowns of susceptible plants. The finer roots first begin to discolor and decay. The pathogen may eventually spread to larger roots and crowns. Infected roots become reddish-brown or black whereas healthy roots are light brown or cream in color.
Junipers and other evergreens show above ground symptoms long after the roots are severely damaged. An infected plant is generally poor in vigor and has very slow growth. The foliage discolors to shades of yellow, reddish-brown, or gray. The infected plant declines branch by branch and eventually dies in one to two years if the conditions for toot rot are not corrected.
Deciduous trees and shrubs may show above ground symptoms by suddenly wilting and dying, especially during warm weather. A gradual loss of lower leaves is also common. The plants usually become chlorotic with symptoms resembling iron chlorosis. Severely infected trees are not well anchored and can be easily rocked back and forth in the soil.
Phytophthora is a water mold. This fungus requires continually wet, warm soils to infect susceptible plants. Overwatering during the summer is conducive to Phytophthora infection. The pathogen may enter roots through wounds or through direct penetration.
Foliar symptoms may not be evident until after the roots have already been severely damaged. At this point fungicides are generally not effective in controlling the disease. Mildly infected plants may recover if the soil is allowed to dry out. The diseases can be prevented by following these suggestions:
1. Buy only disease-free plants. Look for healthy foliage with good color and vigor. Avoid plants that are defoliated, wilted, and have discolored foliage, roots, or crowns. Examine the roots of potted plants by carefully removing the pot.
2. Junipers and other susceptible ornamentals should be planted in beds isolated form turf and annual flowers so that irrigation can be properly managed.
3. Do not overwater. Established evergreens need only one or two thorough irrigations per month. New plantings may require more frequent irrigation until established.
4. Plant in well-drained soil. Avoid planting in low spots where run-off collects or near downspouts. Plants should not stand in water longer than a few hours.
5. Do not plant deeper than the soil level of the container or soil line in the nursery. Avoid injuring the roots during transplanting.
6. Fertilize properly to keep plants vigorous.
7. Remove infected plants. Do not replace with susceptible varieties.
8. In nurseries, steam or fumigate diseased soil beds.
9. Drench with Subdue, Aliette, or terrazole (Truban). Subdue and Aliette are very effective but must be applied before the root system is seriously damaged for best control. Be sure to follow label directions. These fungicides will only be effective if the cultural conditions mentioned above are followed.