If the plant is a tree or large shrub, remove the soil from around the rotted parts of the trunk and roots. Exposure to air will slow down or stop further decay.
The drying-out technique: This process involves careful observation in order to let the soil dry out as much as possible without placing plants under too much drought stress. Stop watering infected plants. To dry the soil as quickly as possible, place container plants in a well-ventilated, even breezy location. If they can tolerate direct sun, put them in a sunny spot. But if the roots are damaged badly enough that plants wilt or drop leaves, don't put them in the sun. Eventually the soil will dry out to the point where the plants would normally need another watering. Allow the soil to become drier yet to stop the fungi from continuing their decay. Take steps to minimize the water needs of the plants. Place container plants in a shaded location to reduce evaporation from the foliage. Provide shade for plants in the ground. Spray trees and shrubs that are growing in the shade with an antitranspirant, which reduces the amount of water that evaporates from the leaves. (Antitranspirants are available in many nurseries.) Further relieve drought stress by placing clear plastic tents over the plants. The tents maintain high humidity, reducing the plants' water loss. Use tents only in the shade; if used in the sun, they can heat up enough to kill plants. Begin watering again when the plant shows signs of drought stress, such as heavy wilting or yellowing and dropping of leaves.
Prevention: To help prevent future root rot problems, allow the soil to dry slightly between waterings, and provide good drainage. (For details about proper watering techniques and improving soil drainage, see Soil Types and About Watering.) Phytophthora-resistant plants may be used in wet areas that are hard to drain. For a list, see Plants Susceptible and Resistant to Phytophthora.
