"Tree health" as a discipline refers to the study of all factors (biotic and abiotic) that affect the vigour and productivity of a tree, as expressed by different symptoms and types of damage. The health of a tree can be expressed qualitatively by describing the symptoms or damage, or quantitatively through assessments of crown condition.
A few comments are required on the meaning of "pest". FAO defines it as any species, strain or biotype of plant, animal or pathogenic agent injurious to plants or plant products. For this guide, the term "pest" refers to detrimental insects, fungi, animals, weeds, viruses, mites, parasitic plants and phytoplasmas.
In a symptom-based approach to tree health it is useful to have a term that refers to a reduction in tree health without reference to the specific cause. "Ill health" is used increasingly to describe a deviation from the normal, healthy state. It encompasses the effects of disease, insect damage, declines, diebacks and disorders, and other harmful influences that affect the appearance and health of the tree.
Clear definitions assist in providing accurate descriptions of tree health problems. Since reports of "new" diseases often describe symptoms only, it is essential that these be communicated in a comprehensive and accurate manner. It is vital that the terms used to describe the symptoms of a particular problem be clearly understood and consistently applied. The use of scientific names is recommended because they are internationally recognized; common names, on the other hand, may create confusion because they vary from place to place. Short definitions and descriptions of key terms as used in this book are given in Annex 1. These may differ from other published definitions.
Tree health problems may be divided into diseases caused by pathogens, damage caused by insect pests and other animals, disorders linked to abiotic influences and other miscellaneous problems described in various ways, e.g. declines and diebacks. "Disorder" is not specifically defined, although it is often associated with nutrient imbalances.
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.
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.