“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.