Introduction

The rating for each disease is based on field and laboratory observations. When environmental stresses such as insect outbreaks or atmospheric pollutants are also present, a normally minor pathogen can become more lethal to its host.

Management options have been selected based on available methods that are operational at present. A regional pest specialist should be contacted as to other potential management options, if those included in the handbook are not appropriate for the situation. Silvicultural options have been suggested, but chemical control has also been recommended in some cases. This is an area that is constantly changing, and therefore no specific fungicides have been recommended. Only those fungicides that are currently registered for control of specific diseases should be used.

Before any management option is considered, it is necessary to:

1. Determine what pests and environmental conditions are involved.
2. Determine the extent of and potential damage caused by the pest identified.
3. Determine the appropriate management strategy for the pest and forest resource involved.

The ability to diagnose disease conditions properly will depend on many factors, including the severity of the symptoms, the presence of other pests, and the training and experience of the individual. When field identification is not possible, assistance should be sought through a pest specialist or a diagnostic facility. More detailed analysis and identification of problems are available through the Canadian Forest Service, Great Lakes Forestry Centre, n Sault Ste. Marie, staff of the Ontario Ministry of Natural Resources at the Ontario Forest Research Institute, and diagnostic clinics at some universities and colleges. Before samples are submitted for identification, it is critical that a suitable specimen be obtained. Brief instructions are provided for each pest under the heading of “Additional information” to assist the user in procuring these samples.

Causal Agents of Tree Diseases

Agents that cause tree disease can be organisms, such as fungi, bacteria, and viruses. There are also nonliving agents, which cause injuries at sites that may later serve as points of entry for infection. Possibly the best way to classify these agents is according to their infectiousness: those that can multiply within the host are termed "infectious," and those that cannot are called "noninfectious." Infectious agents produce some form that enables them to spread and infect other hosts.

Humans, animals, and weather are all noninfectious agents; only weather is given brief coverage in this book.

The selected diseases are grouped into the following categories: rusts, decays, wilts, cankers, needle casts, and anthracnose/leaf spots. (2)

The rust diseases usually require two different hosts to complete their life cycles. The host of lesser economic importance is usually called the alternate host. Rust fungi get their common name from the rusty orange color of their spores during at least one of their fruiting stages. They derive their nourishment from living plant cells, so they die if their hosts die.

The woody decay fungi are particularly insidious disease agents, as much of their activity occurs inside their hosts without any obvious external symptoms. Some of these fungi are found to cause decay of roots and butt, and others are confined primarily to the stem. Once the decay fungus produces a fruiting structure, indicating its presence in the tree, the decay is well advanced.

Wilt diseases are caused by fungi that invade the vascular system of the host. They interfere with the translocation of fluids in the tree, resulting in a reduction of water to the leaves and subsequent wilting. Trees infected by wilt fungi often have a solid or dotted pattern of color in the outer sapwood when seen in cross section.

Canker fungi cause distortions of the trunk or branches of infected trees. Damage ranges from volume loss of varying degrees to death. Cankered trees under stress from wind or heavy ice and snow accumulation often break at the point of cankering. Canker fungi often invade their hosts through branch stubs or wounds. Once established, the fungus kills the bark, often resulting in a characteristic pattern of color as the host responds to the invasion. Canker fungi fruit on the host, and their spores can be liberated whenever temperatures and moisture requirements are met.

Needle cast fungi are common on conifer needles, and many are capable of causing premature defoliation. The reproductive structure is usually black, may be circular, oval, or elongate, and may cover the entire length of the needle. These structures can be seen with the unaided eye. Most of the needle cast fungi do not cause a serious problem, but several are capable of causing significant damage to young trees and to trees in forest nurseries.

Anthracnose and leaf spots are diseases of hardwood foliage caused by fungi that spend the winter on fallen infected leaves or twigs. In spring, spores discharged by the fungi infect the new leaves. These fungi are capable of causing considerable destruction of leaf tissue as well as premature defoliation.

Collection of Disease Specimens

Selection of good samples of tree diseases is the key that allows the specialist to identify a causal agent. Information describing many aspects of the site is also important. The recommendations provided here have been prepared with the forester in mind but should serve as guidelines for anyone who has occasion to ship samples of tree diseases for diagnosis.

Collection procedures

The following points are important to consider when collecting specimens of tree pathogens:

1. Samples should be as large as possible and should consist of well-developed, representative host material bearing signs and symptoms of the disease (e.g., fruiting bodies). It should be kept in mind that some fruiting bodies are so small that they are almost invisible to the naked eye.

2. Each collection should contain only one type of damage. A number of collections can be shipped together, but each collection should be kept separate from the others (see No. 3 below).

3. Each collection should be placed in a paper bag or other paper container together with a note containing observations and comments that were completed in the field at the time the collection was made.

4. Samples of twigs, branches, or roots should be 10_15 cm long, and each should include the margin between living and diseased tissue.

5. Collections of leaves or small plants should be pressed between pages of a newspaper or magazine or between pieces of cardboard and shipped, without bending, in a protective container.

6. Large fruiting bodies should be dried in the open air, and soft, fleshy ones should be quickly and completely dried in warm, dry air. The color, odor, and size of fruiting bodies and points of attachment to the host should be noted at time of collection, as many of these characteristics are lost after drying.

7. Samples should not be wrapped in cellophane or plastic wrap, because the humidity that results induces the development of contaminating fungi and bacteria, which makes the detection of the true pathogen difficult.

8. In the case of rust fungi with two different hosts, samples should be collected from both hosts whenever possible.

9. Samples should be shipped by the quickest means possible.

Information to include

Although it is recognized that all the information listed below may not be available, an effort should be made to provide as much of it as possible to increase the probability of a successful identification:

1. The full names of the person(s) to whom the answer should be sent should be included, as well as the precise location where the sample was collected.

2. The tree species should be identified, and the following information provided: diameter at 1.5 m above ground; height (estimate); living or dead; and part of tree affected—foliage, flowers, fruit, twigs, branches, stem, butt, or roots.

3. The status of the disease—decreasing, increasing, or constant—should be given.

4. The number of trees affected should be counted or estimated and provided with observations on distribution of infected trees.

5. Contributing factors should be identified—for example, site, poor drainage, mechanical damage, recent construction, winter salt, etc.

6. Remarks on disease symptoms, such as yellowing, wilting, cankers, callus, flagging, girdling, resinosis, dieback, dying and dead top, should also be included.

(1) Myren, D.T., ed. 1994. Tree diseases of eastern Canada. Nat. Resour. Can., Can. For. Serv., Sci. Sustain. Dev. Dir., Ottawa, ON. 159 p.

(2) Fungal nomenclature follows Myren, D.T.; Davis, C.N.; Britnell, W.E. 1994. Revised and expanded names and authors for fungi at the Great Lakes Forestry Centre. Nat. Resour. Can., Can. For. Serv., Great Lakes For. Cent., Sault Ste. Marie, ON. Inf. Rep. O-X-419. 61 p.