Phytophthora root rot is caused by the fungus Phytophthora cinnamomi. It is most often found in indoor plants in soil that is poorly drained, and it usually kills the plants. While it is less aggressive in outdoor plants, it is nevertheless almost always lethal. When excess water deprives roots of oxygen they need, the fungi take over and the roots rot.
It's easy for a gardener or grower to overlook the symptoms of Phytophthora root rot because they can look like overwatering, drought stress or lack of nutrients. Plants in containers show a yellowing of the leaves followed by wilting before they die. Plants in landscapes may decline slowly for a year or two before they die.
The fungus causes the fine roots to turn brown and die. It spreads into larger roots and upwards into the crown of the plant. From there, it moves up the stem, sometimes girdling the plant. The surface of the stem turns brown followed by the vascular phloem and xylem. The leaves turn yellow and curl, then gradually wilt. Landscape plants can die a year after symptoms occur or they might die without showing any symptoms at all.
Many flowers planted in containers are susceptible to Phytophthora root rot. Apple trees, azaleas, birch, cherry, elm, juniper, lilac, magnolia, maple, mountain laurel, oak, pear, pine, plum, privit, rhododendron, sycamore, and walnut are among the numerous woody plants that are susceptible to Phytophthora root rot.
Conditions for Infection
Phytophthora root rot most often occurs in wet, warm soils. It is found more often in poorly drained soils and in heavy clays than in well-drained sandy soils. It is common in areas where rain water from roofs or run-off water collects around the base of the plants. Shallow soils above compacted hard pans and rock are susceptible. Over-watering or prolonged periods of heavy rain will cause root rot.
How it Spreads
Phythophthora root rot fungi are transported by infected cuttings, soil and potting mixes. The fungi are present in the stock of many nurseries. They are spread by crop debris, from pot to pot by propagation, and by flowing or splashing water.
The best way to control Phytophthora root rot is to prevent infection.
Buy plants that are disease free from a reputable nursery. Do not buy plants that look wilted in the morning or don't look very green. Do not buy evergreen plants that lack normal foliation or that have discolored, dark roots.
Plant in soil that is well drained. If the soil does not drain well or is heavy, plant in raised beds and mix bark or other porous material 8 to 12 inches into the bed. Placing gravel 6 to 12 inches under the surface can help drain heavy or clay soil.
Do not plant a new plant deeper than the soil level in the container that it came in or the soil line in the nursery that it grew in. Make the soil firm beneath the root ball so the plant won't settle deeper.
If you are planting in an area where plants have died of root rot, replant with plants that are not susceptible to the fungus.
Fungicides protect the root systems. They will not kill the root rot fungi present in roots. Apply fungicide drenches or foliar sprays when the new roots start to appear. Applications after that are unlikely to serve any purpose.
Horticulturalists at North Carolina State University recommend applying fungicides containing the active ingredient mefenoxam or fosetyl-Al. Apply approximately 10 square feet around an individual plant. Treat 20 to 30 square feet for large shrubs. In addition those chemicals, plant scientists at the University of California, Davis, recommend fungicides containing phosphoric acid.