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Insect pests and diseases routinely affect the health of trees and have an important role in forest dynamics. Occasionally, insect populations grow rapidly to damaging proportions and major disease outbreaks occur. These events can have catastrophic impacts leading to the complete destruction of large areas of natural and/or planted forests, to loss or reduction of vital forest ecosystem functions and to considerable economic losses. In developing countries and countries in transition, severe pest outbreaks may compromise national economics, undermine local livelihoods and threaten food security.

Despite the significant adverse impacts, and indications that outbreaks of forest insect pests and diseases are on the increase, some countries do not have the personnel or infrastructure to identify and carry out forest protection measures. At the same time there is also an increasing awareness of tree health problems linked to non-living or abiotic influences, particularly the perceived effects of atmospheric pollution.

The effective management of all tree health problems depends on their early detection. All investigations should begin with the initial discovery and recognition of symptoms in the field. To improve awareness of tree health and responses to health-related problems, action must be taken to train people who work directly with trees in the recognition and interpretation of symptoms. A broadening of visual skills in the initial assessment of tree health is urgently needed to improve the early detection and timely management of problems.

Sometimes the knowledge that a problem is not serious will suffice and will avoid expensive and unnecessary treatments. Similarly, the recognition that a symptom is of a previously unknown type will help to identify a new problem at an early stage and may prevent significant losses.

This publication aims to help people make visual assessments of tree health problems and to provide a preliminary diagnosis. It is not an identification guide to insect pests and diseases of trees. It will help readers to recognize symptoms of ill health, to distinguish these from normal events that signal a temporary decline, and to improve their skills in making the vital preliminary diagnosis. This is an important and neglected capability that will often be sufficient for formulating a simple plan to contain a disease or insect pest or for deciding alternatively that no action is necessary.

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