Phytophthora root rot, a soil-borne water mold, infects landscape plants in poorly drained soils or those subject to excess irrigation. Bartlett Tree Research Laboratories states that poorly draining, compacted soils give rise to the destructive mold. However, this fungal disease also occurs in soils that contain plant debris of previously infected plants. According to the Alabama Cooperative Extension System, phytophthora root rot is perhaps the most damaging disease of many types of field-grown and container ornamental plants.
There are many species of phytophthora fungi, and not all cause root rot. P. cactorum causes stem collar rot while P. cinnamomi causes root rot. Moreover, not one but different fungal species cause phytophthora root rot in plants. The abundant fungi of this disease adversely affect both field and nursery plants. They thrive in areas with climates between 77 and 83 degrees Fahrenheit, and in overly moist soils.
Susceptible Landscape Plants
Phytophthora causes severe damages to landscape plants including boxwood, dogwood, azalea, fir, camellia, elm, lilac, cherry, juniper, Chinese chestnut, magnolia, mountain laurel, maple, hemlock and rhododendron.
Symptoms of phytophthora species vary according to factors such as susceptibility and age of the host plant and environmental stresses of planting site. However, this disease reduces root volume that hinders the plant's ability to absorb water and nutrients from the soil. Infected roots are brittle, appear brown to brownish-red, and may be confined to one section or the entire root system. The disease causes slow decline of the infected landscape plant that ranges from months to years before the plant dies. The growth of the shoot decreases, leaf size remains small, the crown begins to thin, and branches die back.
According to Bartlett Tree Research Laboratories, prevent the occurrence of phytophthora root rot by planting landscape plants naturally tolerant to area with poorly drained soil or prone to flooding. Crape myrtle (Lagerstroemia indica), inkberry holly (Ilex verticillata), mockorange (Philadelphus coronarius) and butterfly bushes (Buddleia davidii) are landscape shrubs that withstand moist soils. Similarly, landscape trees that prefer moist soils include Amur maple (Acer ginnala), Leyland cypress (Cupressocyparis leylandii), willow oak (Quercus phellos) and white spruce (Picea glauca).
For effective control, use a combination of good management practices and approved chemical treatments. Periodically treat plants that exhibit symptoms of phytophthora root rot with an approved fungicide. Prevent the spread of the fungus from diseased plants to nearby healthy ones by drenching the soil around healthy plants with a fungicide that contains metalaxyl or etridiazole.
Make sure the root collar is free from mulch and soil.