Dead Roots and Root Rot in Bonsai

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Phytophthora and Root Rot in Bonsai trees

Root-rot is a generic term often used in bonsai to describe roots that are found to have died and rotted away. But what is root rot exactly?

Rotting roots come in two forms; pathogenic and non-pathogenic and it can be difficult to differentiate between the two. However, while one can mean the loss of the tree (or bonsai), the other is simply a natural process that can sometimes indicate ill-health in the tree.

Root rot caused by Pathogenic fungi or bacteria

Pathogenic fungi and bacteria kill live roots as they feed off them, blocking the vascular tissue that carries moisture and sugars between the roots, branches and foliage, causing the foliage to wilt and die back of the above ground portions of the tree. The pathogenic fungi and bacteria that can affect trees and bonsai include Verticillium, Pythium or Phytophora.

However root rot caused by pathogenic bacteria/fungi is limited to a relatively limited range of conditions and usually affects a relative minority of tree species. These bacteria are often dependant on certain climatic conditions; usually during cool and wet weather in Spring.

Routine root pruning or roots that are damaged can provide open wounds for infection by pathogenic fungi where the roots are subject to poor conditions (most often poor draining, compacted and/or airless soils). Entry points for fungi can also be provided by these same poor growing conditions that can kill off areas of fine root growth.
Trees that are especially prone to infection include Yew, Cypress, Box, Apple, Acer, Beech, Azalea and Lime.

Verticillium, Pythium, or Phytophora in Woody Plants

There are over hundred's of different species of pathogenic fungi that cause disease in plants. While the Phytophora species is airborne, most pathogens are soil-borne (the pathogens exist in the soil and are spread through the re-use of infected soil). Opportunist fungi spores are able to remain in old soil or on plant debris for many years.

As the pathogen does not produce a structure that is visible to the naked eye, it is only noticeable when infection is well advanced and above-ground symptoms can be seen. If the spread of the pathogen is not halted, it will nearly always result in the death of the plant. (Fig B)

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). With some species, (in particular Acer or deciduous tree species), the presence of pathogenic root rot can be detected by the discolouration of the trunk base and branches. These areas will have a blackened areas of infected tissue that dieback and effectively girdle the trunk or branch and results in wilting leaves and the death of all growth above it.

The presence of these pathogens 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. The roots will fall apart easily and will be soft and 'mushy'. Very often there will be a quite positive foul smell as opposed to the 'earthy' smell of healthy roots. Fig C shows the visual differences between a rotting Juniper root mass (on the left) compared with a healthy rootball on the right.

 Treating pathogenic fungi or bacteria

There is no effective chemical treatment for these diseases. Discovery of fungal infection and root rot 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 live roots to regenerate and for the tree to survive. All infected soil should be burnt or binned along with any infected growth that is removed. The tree's pot must be sterilised with a disinfectant before repotting to avoid re-infection. Use of a very free-draining open soil mix (preferably with no organic matter) will make conditions for any remaining fungal spores very difficult.

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