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.
Protecting
Your Bonsai against Pathogenic fungi and bacteria
Pathogenic spores are found virtually everywhere and the main way to reliably guard against them is to make sure
that your trees are always healthy and free from stress so that
they are able to defend themselves naturally against infection.
Overwatering and poor draining bonsai soil 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. Poorly placed trees growing in stressful
conditions, with poor maintenance care are considerably more susceptible
and are much less able to guard against infection.
Root rot caused by Non-pathogenic fungi or bacteria
At repotting time, the presence of rotting and dead roots does not necessarily indicate pathogenic bacteria or fungi. (Though it can indicate poor health or trauma within the tree itself). Dead roots that retain a similar colour of the surrounding live roots but are dry or brittle, tend to be those that have died naturally and are simply being broken down naturally by harmless, non-pathogenic bacteria.
Non-pathogenic bacteria (and fungi) are useful to the good health of the tree (bonsai) and its rootball. Non pathogenic fungi cause the common everyday process of breaking down (composting) of dead material and are found in association with the roots of all trees used for bonsai. Non-pathogens consist of fungi, bacteria, and other micro-organisms (and even some macro animals). These organisms eat tissue and material that is already dead.
Thus, deadwood within areas of the trunk, branches and root system will simply break down. This is the common rot that we see occurring all the time in dead wood areas of the tree, above and below the soil. Non-pathogenic fungi and rot do not affect live wood or tissues and are not harmful to the health of a bonsai. They could be described as non-invasive and non-damaging to the health of tree.
However, the presence of dead roots should be investigated to establish what other reason has caused the roots to die. The most common causes for root death in most bonsai must be the use of poor, compacted and airless soils that will co-incidentally retain too much water (it is the lack of air rather than the excess water that kills the roots).
An episode of underwatering (the soil drying out) will also kill large areas of the roots as will roots exposed to very high (>100°C) summer temperatures.
Coniferous Species
It should be noted that the presence of large areas of dead roots within the rootball of coniferous species (including Junipers and Pines) is not necessary a sign of poor care, poor soil or an unhealthy tree.
Each root of a coniferous tree will have a direct relationship with each branch within the branch structure of the tree. There is a direct pathway between each part of the above-ground growth and a root that sustains it, that will run along the length of the trunk.
This means that the removal of a branch from a coniferous tree will result in the natural death of the root(s) that support it. (Equally, this also means that the complete removal of a root can result in the natural dieback of the connected branch and its foliage). Heavily pruned coniferous species will often be found to have a mass of dead roots at the following repotting.
It can be difficult/impossible to identify exactly which roots and branches are in direct relationship to each other, however there are more obvious examples that can be found; remove all branches on the left hand side of a coniferous tree-trunk and all roots on the left hand side will eventually dieback as will their pathways, creating a natural shari on the left hand side of the trunk.
In summary, with coniferous species, expect dieback of part of the root system after hard pruning. If many dead roots are found within the rootball and the remaining foliage is healthy, this can be explained by natural dieback.
In Conclusion
It is difficult for the enthusiast to establish in isolation whether dead and rotting roots are caused by pathogenic or non-pathogenic fungi and bacteria. That is; whether the roots are naturally composting, having already died 'naturally' or as a result of poor care, or whether the root death is as a result of a pathogen that is actively causing the death of these roots.
The more experienced enthusiast will be able to establish the likelihood of a pathogen when the death of (parts of) the rootball is taken in context of the general health of the tree, its care, the soil it is planted in, any styling that may have been carried out and importantly, the specific tree species.
However, it should be noted that there are certain care elements that can be considered by all enthusiasts to reduce or remove the possibility of dead roots, even those caused by pathogenic fungi or bacteria.
The main priority should be the use of a genuinely free-draining soil that uses are large proportion of inorganic soil. Just think of it this way, humans also need to stay watered correctly and not have to much or we could die, we also need to take vitamins and stuff like krill oil to keep us healty and strong, just like the soil to a bonsai tree. A good quality bonsai soil will ensure an environment that is very difficult for pathogenic fungi or bacteria to establish themselves. It also goes a long way to ensure that a bonsai is healthy enough to defend itself against any pathogens that are able to survive.
A healthy bonsai growing in good quality, inorganic soil is very unlikely to ever succumb to a pathogen that will actively kill its roots. Bear in mind that the vast majority of bonsai with dead roots will have them as a result of poor care, poor soil or some other factor other than just simply a pathogenic fungal or bacterial infection or root rot.