Landscape and Nursery - Phytophthora Root Rot and Wilts

Phytophthora root rot and associated wilt is one of the most common landscape problems that I diagnose each year. It can kill large landscape plants and will often progress down a line of plants. It is more prevalent in wet conditions, areas that receive irrigation, or after periods of heavy rainfall, especially following a dry period when roots have some damage. The following is the first of a set of articles on this problem disease.

Phytophthora root rot is a serious, widespread and difficult to control fungus disease affecting a wide range of plants in Delaware. Plants susceptible to Phytophthora root rot caused by P. cinnamomi include azalea, rhododendron, dogwood, Camellia japonica, Pieris, Taxus (yew), deodar cedar, mountain laurel, heather, juniper, high-bush blueberries, white pine, leucothoe, aucuba, and others. Boxwood is attached by Phytophthora parasitica, a similar fungus.

On azalea the symptoms vary with the cultivar . On Kurume hybrid types, such as Coral Bells, Hinodegiri, and Hino Crimson, new leaves are smaller than normal with interveinal chlorosis, possibly some purple coloration and defoliation. This chlorosis is often confused with a deficiency of iron or other nutrients. At times light applications of iron and complete fertilizer can improve the green color of leaves but only for a short time. Excessive yellowing and loss of older leaves are the predominant symptoms on Snow azalea. Usually, large plants slowly decline in vigor and die branch by branch over a period of several months to years, but sometimes they can die rapidly. Roots are reddish-brown, brittle and often limited to the upper part of a container or soil. The reddish-brown discoloration advances to the larger roots and eventually to the main stem. On rhododendron the primary symptoms are a rapid wilting and death of leaves. Leaves droop but remain attached to the limb. Watering does not restore leaf turgor. Usually several to many shoots are affected. Root symptoms are similar to those on azalea.

On other plants the main symptoms are a gradual decline in vigor, loss of dark green color, curling of leaves and excessive loss of older leaves. Root symptoms are similar to those on azalea. Yews die rather suddenly with the foliage turning reddish-brown. Roots are also reddish-brown in color and the discoloration may extend into the main stem. White pines in the landscape often die suddenly during dry periods.

Adapted from "Phytophthora Root Rot and its Control on Established Woody Ornamentals", Ornamental Disease Note 13, by D. M. Benson, Research Plant Pathologist and R. K. Jones, Extension Plant Pathologist, North Carolina State University.