Rootrot is a generic term used in bonsai to describe roots that are found to have died and rotted away. But what is rootrot exactly and why does it continue to kill the root system even when a tree is replanted into fast draining soil and watered properly?
Rootrot is actually the result of a condition that is caused by fungi that are able to enter damaged or wounded roots. Routine root pruning or roots that are damaged or cut in some other way provide open wounds for infection by fungi. Entry points for fungi can also be provided by poor growing conditions that kill of areas of fine root growth, such as compacted or poor draining soils and over or under watering. Other common causes of root damage are overfeeding and the effects of high pot temperatures in summer.
By far the most common fungi that infect woody plants are species of the Phytophthora genus. Trees that are especially prone to infection include Yews, Cypress, Box, Apples, Acers, Beech, Azaleas and Limes though almost all woody plants or trees are susceptible.
Phytophthora in Woody Plants
There are over 80 species of Phytophthora fungi that cause disease in plants and they are distributed worldwide affecting numerous types of plant. The Phytophthora species that causes potato blight is airborne but most are soil-borne and are one of the most frequent causes of death of trees and shrubs in gardens and nature. Opportunist fungi spores are able to remain in soil or on plant debris for many years.
Though they mainly affect roots, these fungi are also able to attach to trunks and branches. As the fungus does not produce a structure that is visible to the naked eye it is only noticeable when infection is well advanced and plant symptoms can be seen. Visible symptoms include dull foliage (particularly with conifer species), smaller, yellow or sparser than normal foliage and branches dying back for no apparent reason. (see Fig A). If the fungus is not halted it will nearly always result in the death of the plant. (Fig B)
The presence of Phytophthora can be confirmed by the discovery at repotting time of dead and dying roots. Major roots will be found to have bark that covers a soft and decaying inner layer. In severe cases the rot will have extended to the trunk base or even higher.
Phytophthora itself does not cause the decay but affected roots are soon rotted by other micro-organisms naturally occurring in the soil. (Fig C) The disease is then encouraged by poorly drained, waterlogged soils, wet and warm weather.
Treating Phytophthora and rootrot
There is no chemical treatment for this disease at the time of writing. However, discovery of fungal infection and rootrot should be treated immediately whatever time of year it is found. Trees should be lifted from their pots and ALL affected roots and woody growth should be removed back to healthy wood. Hopefully enough live tissue will remain for the tree to survive. All of the soil that the tree was potted in should be burnt or binned along with any infected growth that is removed. The trees' pot must be sterilised using Jeyes Fluid or similar before repotting to avoid re-infection. Use of a very free-draining open soil mix (preferably with no organic matter) will make conditions for remaining fungal spores very difficult.
Protecting Your Bonsai against Phytophthora and rootrot
Phytophthora spores are virtually everywhere, especially in wet soils and organic composts and there is no way to reliably guard against rootrots other than to make sure that your trees are very healthy so they can deal with attacks.
Overwatering (particularly after root pruning) and poor bonsai soils provide access points for infection and are also ideal conditions for the spores to grow. This is why it is essential to provide bonsai with gritty, free-draining compost and to avoid over watering.
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