Introduction

According to the Forest Health Network (1999), "a healthy forest is one that maintains and sustains desirable ecosystem functions and processes. [...] The condition of forest health is manifested through a spectrum of ecological indicators, including ones related to biodiversity change, resilience, wildlife habitat, aesthetic appeal, and resource sustainability."

Major changes in forest health are caused by two main factors: diseases and insects. Diseases are responsible for one third of the annual productivity losses of Canadian forests, and are more destructive in terms of volume loss than forest fires.

Forest pathology, which is a branch of botany, is the science devoted to studying and controlling diseases that affect trees and forest stands. A forest tree species can be said to be affected by a disease when a living organism (biotic agent) or an environmental condition (abiotic agent) induces changes in its natural growth, its form or its physiology.

Living organisms that can cause a disease are of three types: fungi, bacteria and viruses.

Ascomycete fungus

Basidiomycete fungus

Fungi are the main disease-causing agents in forest trees. They may attack different parts of a tree species indiscriminately, and may even attack dead trees, by causing the wood to rot, for example.

Canker caused by a bacteria

Bacteria are single-celled organisms measuring about 1/1000 centimetre in size. On account of their microscopic size, complex techniques must be employed to identify them. Bacteria, however, cause few of the diseases that occur in forest tree stands.

Viruses are even smaller than bacteria. They reproduce in the host cell, eventually destroying it. Viruses are extremely rare in the forest ecosystem as compared with the agricultural environment. They can nonetheless induce alterations in a tree's metabolism, which may or may not be easy to detect depending on the presence of visible symptoms.

The activities of vertebrates (birds, moles, porcupines, etc.), although generally beneficial for ecosystems, can damage certain trees or tree species.

Ice storm damage

Abnormally early autumn colours

Mineral deficiency

The environmental agents that can cause problems in forest tree species are: climate conditions, soil status, and human activities and their associated effects.

These elements induce disturbances in tree growth or metabolism. The symptoms are seldom typical and are generally the result of a combination of factors acting together which are difficult to identify

Biotic pathogens

Bacteria are single-celled organisms that lack a true cell nucleus and have a single chromosome instead. There are only a few pathogenic bacteria that attack trees.

The majority of forest pathogens are fungi, which generally belong to one of the following divisions: Basidiomycotina, Ascomycotina or Deuteromycotina.

Deuteromycotina reproduce asexually by producing conidia on conidiophores (Figure 1) or within special structures, such as pycnidia. Ascomycotina reproduce sexually by producing 4, 8, 16 or 32 ascospores inside sacs, or asci (Figure 2), within structures called ascomata. These ascomata may be cup-shaped (apothecia), bottle-shaped (perithecia), or balloon-shaped (cleistothecia).

Figure 1
Conidiophores and conidia

Figure 2
Asci and ascospores

Figure 3
Basidia and basidiospores

Basidiomycotina reproduce sexually by forming basidia, which produce four basidiospores (Figure 3) on a structure called ascoma. The basidia develop in gills, pores, teeth or other structures. Rusts (Uredinales) are Basidiomycotina that function as obligate parasites and have a complex life cycle that generally requires an alternate host.

Abiotic pathogens

Abiotic pathogens include weather extremes (drought, early and late frosts, ice storms, flooding), soil problems (inadequate texture, nutrient deficiencies or excesses, toxicity), and human-induced problems (soil compaction, pollutants, acid rain).

There is a range of effects associated with these environmental, or abiotic, agents: increased susceptibility to insects and pathogenic fungi, localized tissue necrosis (twigs, leaves), reduction in growth, tree mortality and stand decline.

Control methods

Forest ecosystems, which cover more than 45% of Canada's land surface, are of paramount importance in terms of preserving biodiversity as well as providing raw materials for the industry. Various measures have been developed over the years to protect this resource against pests such as insects and diseases. The different control methods can be classified as follows:

The purpose of this site is to describe the diseases and insects found in Canada's forest ecosystems. If you would like more information on the use of control methods or chemical products, we recommend that you contact the individuals or organizations listed in the Yellow Pages under "Tree Consultants", "Tree Service", "Horticultural Consultants", "Forestry Consultants" and "Agronomists".

For more information on the registered products used to control a specific insect or disease, please contact the Pest Management Information Service of the Pest Management Regulatory Agency (PMRA), at the following toll-free number: 1-800-267-6315.

Forest sustainability

Forest diseases together with fire and insects are the principal agents driving forest succession. All of these agents have effects on the terrestrial carbon cycle and subsequently on climate. Forest pathologists play a critical role in conducting research to understand the balance between pathogens and forest trees such that disease does not affect sustainable forest practice. Sustainability is a balance between social, environmental, and economic concerns in the forest. Forest diseases can affect all three of these and therefore play a major role in sustainability.