Phytophthora root rot is one of the most significant root diseases of woody ornamentals, and herbaceous plants. It is caused by a multitude of fungus-like organisms, which infect susceptible plants through their root systems. Infected plants can develop root and crown rot, particularly if the soil around the base remains damp for an extended period of time. Once established, the organisms grow within the cambium and sapwood, killing the live tissue. The eradication of the healthy tissue disrupts the plant’s vascular system, preventing it from effectively absorbing water or nutrients. This results in a gradual or rapid decline of the plant. Severe infections generally culminate in plant mortality.
Distribution & Habitat
Phytophthora root rot occurs globally. It is most common in locations that experience abundant rainfall.
Phytophthora root rot generally affects plant species that are intolerant of soils with poor drainage. Certain species of Phytophthora target herbaceous plants, while others favor woody ornamentals. Herbaceous plants that are frequently infected include andromeda, azalea, camellia, dogwood, fir, rhododendron, Japanese holly, boxwood, hemlock, mountain laurel, yew, and white pine. Woody ornamentals that are prone to infection include beech, dogwood, fruit trees, red maple, sugar maple, rhododendron, and Zelkova.
The organisms that cause Phytophthora root rot subsist in the soil. When present, they overwinter in diseased plant roots or stems. When conditions are sufficiently moist, the organisms are spread through irrigation, splashes of rain, and runoff water to nearby plants, where they initiate new infections. The disease organisms may be disseminated through contaminated soil and pruning equipment as well. The organisms are most often spread during rainy periods in early spring and late fall, when temperatures are relatively cool. Flooded and saturated soil conditions are especially conducive to the spread of Phytophthora root rot. In locations where rainfall occurs frequently, the organisms may reside in the soil for years.
Symptoms of Infection
The severity of a Phytophthora root rot infection varies depending on the susceptibility of the host, the soil conditions, and the type of Phytophthora species present. On some plant species, symptoms may be not become apparent until the infection deepens. Plants with mild root or collar rot will exhibit stunted growth, chlorotic leaves, and extensive dieback of the feeder roots. Severe infections cause a slow to rapid degradation of the host, inhibiting shoot growth, and leaf expansion. Eventually, chronic infections will result in significant foliar dieback, and plant mortality. Diseased roots will become discolored and brittle. The bark on the main trunk may appear darkened. If the bark is stripped away, it will reveal a reddish-brown discoloration on the infected wood. As the wood decays, sap may leak from the bark, and trickle down onto the main trunk.
Lesions or cankers may extend into the root collar. Lesions or cankers that enlarge can girdle the main trunk. When girdled, a plant will rapidly decline, experiencing a widespread wilting of the crown. As the leaves wilt, their color will brighten, turning from a dull green to yellow, before deepening to red or purple. The wood located beneath the outer edge of the canker may express a pink or bronze shade. Infected areas of the plant will be slightly sunken beneath the cambium. Infected trees may persist for several years before collapsing.
- When planting, select trees and shrubs that are tolerant of poorly drained soils, particularly in areas that are prone to flooding.
- Prior to planting, assess the soil composition, and make amendments to improve the water drainage. A core aerator can be used to create perforations in the soil, and reduce compaction.
- Do not allow water to collect around the collar or root system of plants.
- Apply a layer of organic mulch around the base of plants to improve the soil quality, and moderate the soil temperature. If soil or mulch has been piled up over the main trunk of a plant, carefully remove it to expose the root collar.
- Plant species rooted on sites that are subject to saturated soils may require annual to biannual fungicidal treatments to minimize new infections. Applications should be performed in early spring and late fall, when infections are most common.
Photo courtesy of Jay W. Pscheidt, 2013