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Keeping Bonsai Trees Healthy to Avoid Root Rot
by Aldevaux - 3rd Place
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Root rot

Figure 3. Comparison between
healthy and rotted roots

There are two types of root rot, one is caused by parasites; mostly fungi, bacteria and nematodes. The other is physiological or abiotic, caused by various adverse environmental conditions that cause stress. Both types must be controlled to keep the roots healthy at all times.

Physiological or abiotic root rot

After a tree is established in its environment, any soil treatment that changes the oxygen, water supply and nutrient condition of that environment can be extremely detrimental to the root system. In these situations, the root may turn from brown to black, as the result of asphyxiation of cells (figure 3). Eventually, they die and are decayed by the saprophytic microorganisms which feed on dead cells.

Here are the three major stresses or factors that can cause abiotic root rot :

1 – Lack of oxygen is the most important factor for the survival of root cells, as oxygen is required by cells of all living organisms. This fact cannot be over-emphasized to anyone who wants to prevent root rot and the eventual lost of their bonsai. In fact, roots can grow anywhere, even in water like aquatic animals, as long as the water is supplied with enough oxygen, as in the case of hydroponic plant cultures. Stresses caused by soil compaction and over watering are primarily due to the lack of oxygen.

In the case of soil compaction, which is one of the major causes of stress for roots, it is the crushing of all the air spaces in the soil that causes restriction of oxygen to the roots. Compaction also kills many of the beneficial microorganisms in the rhizosphere. In bonsai soil mixtures, this condition is associated with too many fine soil particles and the poor choice of soil mixture or it’s preparation.

Over-watering, which can occur only in a very slow draining soil mixture, causes the air spaces in the soil to fill with water for too long thereby reducing oxygen availability to the root. Furthermore, since it also stalls the respiration process of the root cells, it prevents root exudates from forming carbonic acid ions that are necessary for the absorption process. When absorption is significantly reduced, the health of the tree suffers and the roots become susceptible to root rot.

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