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How To Save A Bonsai From Root Rot

How To Save A Bonsai From Root Rot
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Root rot is a commonly used term in bonsai that is generally used to describe the infection of roots by an invasive pathogen, such as a bacteria or fungus. In reality, some cases of 'root rot' are completely normal and part of the ecosystem of the bonsai pot.

In this blog, we'll take a look at what root rot is, how you can tell if it is normal or invasive and how to save a bonsai from root rot.

Normal rotting roots in a bonsai tree

When taking care of a bonsai tree, it's important to be able to determine if something in your tree is part of normal growth or actually a disease. In the rootball of all bonsai trees, there are many types of bacteria and fungi growing. Most of these are what are known as non-pathogenic micro-organisms, meaning they do not cause disease in the tree.

These healthy micro-organisms are involved in the normal process of decay and composting of dead material in the rootball. These bacteria and fungi feed on things that are already dead.

It is very common and can be normal to find dead and rotting roots when repotting a bonsai tree. These rotting roots are most often roots that have naturally died and are now decomposing with the help of healthy, non-pathogenic bacteria and fungi.

This is particularly common in tree species that have a linear relationship between roots and branches. If you remove a branch from such a tree, either via pruning or creating deadwood, then the roots that support that branch will die naturally.

Species such as Junipers are often heavily styled with significant pruning and deadwood creation. This can mean that during the first repot following a big styling, many Junipers will have dead roots that are naturally rotting away.

This knowledge can help with identifying the cause of root rot in your bonsai. If the tree is healthy and the foliage is strong, then it is more likely to be natural dieback and decay of a root that you are dealing with, rather than an invasive disease.

It's important to note that while rotting roots with non-pathogenic micro-organisms can be a normal part of the bonsai process, it can also indicate health issues with your tree.

If you are overwatering your bonsai, then many roots may die as a result of not getting enough oxygen. Large-scale root dieback can also occur from a missed watering on a hot day.

Your bonsai may not have an infection from any of the bad root rot organisms, but extensive root dieback and rot can be a sign of some misapplied water at some point.

Invasive root rot

Invasive or pathogenic micro-organisms (bacteria or fungi) can infect the roots of your bonsai tree and lead to some serious health issues, often resulting in the death of the bonsai. This is what is colloquially referred to as root rot in bonsai, and it's a serious problem.

In contrast to the normal rotting of dead roots, invasive root rot micro-organisms infect living roots.

Tree roots have vascular tissue that is used to carry water up to the foliage and sugar down to the root tips. When a root is infected with root rot, this vascular tissue becomes blocked.

As infected roots are jammed up and start to die, the foliage that they supply will start to die as well. This can be seen as yellowing, browning, or loss of foliage.

What causes root rot on a bonsai?

The most common species of organism that cause root rot in bonsai are Phytophthora, Verticillium, and Pythium.

These pathogens require specific conditions to promote their growth. They generally thrive when conditions are cold and wet.

Root rot can become a particular problem around Spring when there is still plenty of moisture and rainfall, and trees have been freshly repotted.

The common scenario will be this:

  • A tree is repotted, which always involves some trauma to the roots when they are pruned.
  • The bonsai may have been potted in a very organic soil mix.
  • The tree is left in the rain or overwatered after the repot and not allowed to dry out between water applications.
  • Root rot pathogens enter the roots via unhealed wounds from pruning, and thrive in the wet and cold environment.

The key point in the above scenario is that the tree is allowed to stay wet after a repot. Roots require a wet/dry cycle to encourage callus formation on wounds and the growth of new rootlets. Roots that do not have enough oxygen will not form callus over their wounds, leaving them vulnerable to infection.

Species of bonsai commonly affected by invasive root rot include:

Bonsai root rot symptoms

Visible symptoms of root rot in bonsai include:

  • Dulling or greying of foliage
  • Yellowing of leaves
  • Branches dying back with no obvious cause
  • Discoloration at the base of the trunk

Unfortunately, root rot can not be seen in your bonsai tree until the infection is advanced enough to cause these symptoms in the above-ground portions of the tree. The pathogens themselves are microscopic and also under the soil, so there is no way to know the infection is there until it does enough damage for your tree to start showing.

If you suspect a tree has invasive root rot and decide to inspect the roots, you will see signs of large-scale dieback of the root mass, along with the loss of big structural roots. Large roots rot from the inside and can be mushy and crumble between your fingers.

Invasive root rot can be accompanied by a very foul 'earthy' smell.

Bonsai root rot treatment

We do not have effective chemical treatments we can use to tackle root rot, so treatment relies on a repot. Manual removal of infected tissue is required, along with fixing any adverse conditions in the rootball that would encourage the growth of pathogens.

If you discover root rot on your tree, you need to pull it out of the pot straight away and deal with it immediately. This is one of those rare exceptions to the rule of only repotting in Spring.

After removing the tree from its pot you need to start pruning back all of the infected roots. For structural roots, you need to keep cutting away the mush until you reach healthy tissue.

All the soil and pruned roots need to be kept far away from your other trees and disposed of.

Ideally, the tree will go into a clean pot, but if you don't have a spare one then make sure you sterilize the pot before re-planting the tree.

The tree should be potted up in an aggregate soil mix, avoiding organic components as much as possible. Pumice and lava are your friends here.

With a bit of luck, your bonsai will have enough healthy roots left to survive. By repotting in a non-organic soil mix you make it harder for any fungal spores or bacteria to reproduce and re-infect your tree.

Make sure that you water your bonsai appropriately after the repot. As with other repots, you should water thoroughly straight away. You then need to let the bonsai dry out to stimulate wound healing and the growth of new roots.

After that, use our bonsai watering checklist to make sure you're not overwatering so you can give your tree the best chance of recovery.

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