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4 Why trees become unhealthy

Causes of ill health in trees

The many reasons for the disruption of the healthy growth of a tree can be divided into two main categories: living (biotic) and non-living (abiotic) factors. More than one factor can affect the health of a tree at any time. A useful distinction can be made between primary pests, which first and principally affect the health of the tree, and secondary pests, which have a less important influence and usually affect trees already weakened by a predisposing factor. The impact of insect pests is often increased by a previous weakening of the tree's vigour and a lowering of its natural resistance to infestation, for example through waterlogging or nutrient deficiencies. One of the most common predisposing factors is poor nursery management. Trees that become pot-bound as saplings do not develop a healthy root system and therefore grow poorly when planted.

Stress and off-site factors undoubtedly have a major role in determining the health or condition of trees, as do poor soil and drainage. However, undue emphasis on poor sites or adverse climatic events such as drought and frosts as primary causes of observed symptoms and damage to trees may prevent a more careful search for possible biotic influences.

Pest (biotic) influences

Some pest groups are better known than others simply because they are easier to see. Insects are frequently found on trees although many are casual feeders and not serious pests, and some are beneficial (natural enemies). Fungi are frequently seen on dead and decayed organic matter, but they may not necessarily be the primary cause of the symptoms observed. Most fungi in nature are saprobic (living on dead or decaying tissue) and only a very small proportion are pathogenic. Insects and fungi are relatively easy to distinguish by direct observation, while the remaining pest groups are not. Several other living agents occur on trees, including mosses, lichens and epiphytes such as bromeliads, but these have only a superficial impact on tree health (see Plate 16).

Major groups of pests that occur on trees are described in Table 1 and are illustrated in the colour plates.

Table 1. Major groups of pests that infest trees





A living microorganism characterized by a cell wall containing chitin and lacking chlorophyll.a Common cause of disease. Associated with a wide range of symptoms. Diverse group of pest organisms: some with large fruiting bodies visible with the naked eye but many only “visible” when grown in an artificial culture in the laboratory. Fungi also play a secondary role in decays and rots.

Rust fungus on Acacia mangium [2.2]


A living microorganism characterized by cell membranes and cell walls.a Uncommon cause of disease but several species have caused widespread losses in trees. Cannot be detected with the naked eye except en masse in bacterial oozes.

Bacterial ooze of Eucalyptus sp. [9.5] Bacterial wilt of Eucalyptus [9.6]


An ultramicroscopic (one dimension less than 200 µm) organism. Viruses cannot reproduce alone (and thus are not living organisms according to some definitions) but must first infect a living cell and take over its synthetic and reproductive facilities.b More common cause of symptoms than generally perceived. Symptoms may resemble those of other pests and factors. Transfer to new host plants by (insect) vectors, sometimes by manual transmission.

Leaf discolouration and flecking on Gliricidia sepium [10.9]


Extremely small, phloem-limited plant pathogenic bacteria-like prokaryotes that lack a cell wall.c Uncommon cause of disease but more widespread than usually thought, mainly because typical symptoms are not recognized.

Gliricidia little leaf disease [8.1 & 8.2]


Widespread, extremely common cause of damage, and rarely host tree specific (unlike many pathogens). Readily seen but often assumed to be the cause of more damage than is supported by biological evidence. Different insect orders are associated with particular patterns of feeding and breeding on trees.

Leaf miner damage on Pterocarpus indicus [13.6]


Common pests whose feeding results typically in distinct symptoms (e.g. galls); mites are not readily seen with the naked eye.

Leaf galls on Vangueria infausta [2.1]

Parasitic plants

Widely present in many tree species that have been weakened by other factors. Rarely the cause of major losses.

Parasitic plant on Schinus molle [14.13]


Some weeds outcompete trees, especially when young; others grow in the canopy and can strangle trunks and branches.

Bromeliads on cacao [16.1]

Larger animals

Includes large mammals such as elephants, monkeys, deer, smaller rodents and birds that feed on the foliage and bark. Damage and losses can be significant although plants often recover.

Deer damage on Gmelina arborea [13.12]

Non-living (abiotic) influences

Non-living factors that impair the health of trees are summarized in Table 2. The effects of poor soil and generally adverse growing conditions on the health of trees, and their causal association with observed symptoms, needs careful examination. Nutrient disorders produce symptoms similar to those of virus diseases and other pest infestations or infections, and a lack of information often makes it difficult to make even a preliminary diagnosis of a problem.

Table 2 describes abiotic factors affecting tree health and gives corresponding examples in the colour plates. Note that fire may weaken trees and make them more susceptible to insect attack.

When poor growing conditions have had long-term effects on the health of a tree, an examination of growth rings can help to reveal the history of factors such as prolonged periods of drought. Other evidence may be more immediate and visible. For example, a shortening of internode lengths in neem suggests a number of years of poor growth (see Plate 15.2).

Table 2. Abiotic factors that affect the health of trees






TOXIC: pesticides, herbicides POLLUTION : deposition on plant, atmospheric, industrial waste

MISCELLANEOUS : salt, oil poured into soil

The role of atmospheric pollution in tree declines may be overstated.

Leaf blight caused by herbicide drift [7.6]

Mechanical agents

MACHINERY : used in agriculture and for construction

HUMAN : malicious, accidental

Mechanical damage can provide means for pathogens (mostly fungi) to enter.

Damage by grass cutting equipment [15.3] Illegal tapping for sap [ 15.6]

Bad pruning allows fungi to enter and establish heart rot [ 15.10]

Soil conditions


PHYSICAL STRUCTURE : poor drainage, inhibition of root development

Trees respond in different ways to lack of particular nutrients.

Boron deficiency on Pinus patula [15.6]

Poor nursery practice [ 15.9]


TOO MUCH : flooding; waterlogging

NOT ENOUGH : drought

Trees differ in their ability to withstand excess or poor availability of water.

Waterlogging [ 1.4]

Drought [ 6.1]


TEMPERATURE : too low or too high

OTHER: lightning, hail, wind, snow

The effects of climate on tree health are often not immediate.

Frost [ 15.7]

Hail damage [ 15.1]

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