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6 Analysing the cause of symptoms

Interpretation of symptoms

The fundamental challenge is to realize when a tree is unhealthy.

The following stages are suggested for interpreting symptoms and making diagnoses from visual evidence only:

Making a diagnosis

Many stages are involved between the first observations of symptoms and the final conclusion about their cause. The first step is to eliminate factors that are most unlikely to be the cause of the problem. For some symptoms the cause may be easily determined, as with the pustules on leaves produced by a rust fungus, for example (Plate 2.2). Other symptoms may have several possible causes, such as dieback (Plate 8), which could be the result of a mammal feeding on a stem and stripping away the bark (Plates 13.13, 7.8), a fungal root disease (Plate 1.3) or a phytoplasma infection (Plate 4.7). Resolving these problems may involve the identification of potential pest organisms. This can be a long and complicated procedure, hence the value of carrying out a preliminary diagnosis.

A good diagnosis takes into account wide knowledge of the host tree, the causal factors that impair its health and the interactions between the host, the factors and the environment in which the tree grows. Identifying the precise cause of a problem is often difficult, yet much can be achieved by following the simple procedures described above.

Problems in linking symptoms to causes

Some difficulties may be experienced in observing and interpreting the wide range of symptoms illustrated in the colour plates (Table 4). Tree health problems are often assumed to be caused by insects because they are easy to find on trees and also because alternative causes of ill health are often unknown (see Table 2). The supporting evidence should always be examined carefully in interpreting the significance and importance of insects and other external features such as bracket fungi on trunks. The role of atmospheric pollution in tree declines should also be examined carefully by reference to the extensive literature on this topic (see Annex 2).

Table 4. Common difficulties in observing and interpreting symptoms





Symptom type not recognized

Phytoplasma diseases, e.g. gliricidia little leaf disease

General unfamiliarity with phytopla sma and virus diseases on trees

1 [e.g. 1.2, 1.9] 4 [e.g. 4.3]

Incorrect diagnosis of symptoms

Confusion between the effects of insect infestation and other pests

Insects are easy to see; poor awareness about the symptoms associated with other pests

13, 14

Significance of symptoms not fully understood

Root disturbances leading to dieback and loss of vigour in crown

Unfamiliarity with root diseases and their pest causes

8, 14A

Unwarranted alarm about physical evidence of damage to tree

Termite feeding on mature tree trunks

Unawareness that bark feeders cause only superficial damage that has no effect on the health of the tree


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