Phytophthora Root Rot
Introduction
Phytophthora root rot is a general name for a group of diseases caused by species of Phytophthora belonging to a group of fungus-like organisms. The pathogen attacks the roots of many trees and shrubs, particularly the fine roots that absorb mineral nutrients from the soil. The pathogen prefers poorly drained soil and produces motile spores that can swim through saturated soil for short distances. In North Carolina, the disease is particularly important in Christmas tree production, fruit trees, and many ornamentals.
Identification
Phytophthora root rot can often be difficult to diagnose and laboratory tests are usually required to confirm the presence of the pathogen.
Above-ground, symptoms mimic nutrient deficiency, drought, or decline such as leaf chlorosis, leaf necrosis, wilting, dieback, or death. In Fraser fir, needles will turn yellow-green and may wilt before turning brown. Symptoms often begin in lowest branches progressing upward, or may only affect one side or section of the tree. Plants may decline over several months or several years.
The bark on large roots of infected trees may only be loosely attached; root tissue beneath bark is often discolored red, brown, grey, or black. Feeder roots are often fewer in number or even completely absent. The bark on small roots is easily pulled off the center core; small roots may be mushy, reddish- brown, or black. The lower stem or root crown of severely diseased plants may be sunken in, swollen, pitch-soaked, or have roughened bark. On Fraser fir, removal of bark from the lower stem may reveal butterscotch, brown, or black colored sapwood. Dead or dying trees often occur in groups or in low- lying areas. Seedlings of many trees and shrubs can be killed rapidly by Phytophthora under the right conditions; this is known as “damping off.” Phytophthora root rot is most often confused with damage caused by soil inhabiting grubs and abiotic disorders.
Management
No single practice will prevent Phytophthora root rot; an integrated management approach must be taken. Do not plant susceptible species on sites where the disease has been known to occur; utilize resistant and/or disease-free plants. Obtain plants and soil from reputable sources. Remove and destroy diseased plants (including the root system). Plant in well drained soil; utilize site preparation techniques to improve soil drainage. Promote overall plant health by providing adequate water, fertilization, and maintaining proper soil pH. Avoid over-watering and use of nitrogen rich fertilizers. Crop rotation with resistant plant species is often a successful way to reduce or eliminate soil inoculum. Pesticides are available for specialized applications.
