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This is the third version of a Canadian Forest Service tree disease identification guide for British Columbia. The first, Some common tree diseases of British Columbia, by J. E. Bier, was published in 1949. The next edition, Common tree diseases of British Columbia, by R. E. Foster and G. W. Wallis, was published in 1969 and reprinted in 1974. Since the second edition there have been many changes in the scientific names of the disease organisms and in the economic importance of hosts and diseases. This edition has been updated to provide new information, with more and better photographs, to aid in the identification of tree diseases common on the major commercial tree species in British Columbia. Although the book is primarily aimed at forestry professionals, it will also be useful for arborists, horticulturists, and homeowners as a guide in the diagnosis of disease on a wide range of plants.

Forest diseases, as described in this book, are injurious conditions, often expressed by the abnormal growth or development of trees and caused by agents other than fire or insects. Thus, diseases include disorders that reduce growth, lower wood quality, cause predisposition to attack by other agents, or culminate in the death of the trees. Diseased trees may be detected from symptoms and/or signs. Symptoms are expressed by a loss in health or abnormal development of a tree or its parts (e.g., unnatural colour changes, swellings, dwarfing, or wilting). Signs identify the causal agent of the disease (e.g., the fruiting body of a fungus or aerial parts of dwarf mistletoe plants).

Non-infectious or physiological diseases are caused by non-living agents and encompass a wide range of disturbances to the normal functioning of a tree (e.g., unusually high or low temperatures, excess or deficiency of water or nutrients, or pollution). Susceptibility to damage varies with the species, age, and vigour of the trees. Factors inducing physiological disorders may operate for only a brief period as in the case of frost, may extend over part of a growing season as in the case of prolonged drought, or their effect may be cumulative over a number of years as in the case of some pollutants.

Infectious diseases are caused by living agents such as fungi, bacteria, viruses, and higher plants, which attack trees to obtain nutrients essential to their development. The most important infectious diseases occurring in British Columbia are caused by fungi and dwarf mistletoes. The magnitude of damage depends on the relative susceptibility of the tree (host), the virulence of the causal agent and its life history, and the environmental and other circumstances that influence the resistance of the host and the growth and reproductive ability of the causal agent. Consequently, a disease may vary in importance among tree species in one region or in the same tree species in adjacent regions.

Infectious forest diseases are classified as native or introduced. Native diseases do not usually threaten the existence of a tree species, but some may cause severe losses in some stands. Introduced (extra-regional or foreign) diseases, such as white pine blister rust, may become epidemic and threaten the existence of a susceptible tree species throughout its entire range.

Microscopic examination is required to identify many causal agents of diseases, but most of those included in this handbook may be recognized by their symptoms or signs.