Diseases

INTRODUCTION

Forests are damaged by a number of different natural agents including fire, insects and disease.

• The dramatic effects of forest fire are well known, as television coverage each fire season heightens public awareness of the danger and the damage.
• The results of insect damage become apparent to the public when epidemic proportions are reached or when interventions such as aerial spraying draw attention to the issue.
• The effects of disease are less obvious to the untrained eye, yet 40% of forest damage is as a result of disease.

In the days when forests were thought of as unlimited, losses from disease were tolerated. Today, however, as the size of the working forest shrinks, these losses are gaining importance.

Disease damage can create economic, environmental and social consequences that are of concern. Decline in forest health due to disease can impact the quantity and quality of forest products produced and related employment, recreation and habitat values, and can increase forest fire hazard.

This article will explore how forest diseases are classified, the ecological role of disease agents, the main types of forest disease and the methods of disease control being used today.

DISEASE CLASSIFICATION

Forest pathology is the study of tree diseases. A tree disease is defined a sustained condition that interferes with normal functioning resulting in the partial or complete stoppage of development or causing death.

Tree diseases can be classified as:

1. Non-infectious diseases, also called physiological, are caused by abiotic or nonliving factors. These include factors such as air pollution, nutrient deficiencies or excesses, adverse weather, mechanical injury and increased recreation use. Non-infectious diseases are often considered disorders rather than diseases. Examples of a non-infectious diseases include: cedar flagging- where branchlets die off in hot weather, frost lesions or sun scald. A physiological disease or a wound can create conditions that later enable an infectious disease to gain a hold on the tree.
2. Infectious diseases, also called pathogenic, are caused by disease organisms such as fungus, bacteria, viruses, or higher plants. These agents live in the tree and derive nourishment from the tree causing detrimental effects.

The severity of the damage caused by a disease depends on the resistance of the tree. Tree resistance is influenced by the susceptibility of the tree species, the growing conditions and the reproductive ability of the organism. Infectious diseases can be native or introduced.

Native organisms usually do not kill the tree as the tree has co-evolved with this organism and has some resistance to it. However, native diseases can cause severe losses in the stands that are overmature or in stands that are growing slowly. They can cause reduced tree growth, deformity and lower quality wood for products.

Introduced infectious diseases often kill their hosts as these trees have no evolved defences against these invaders. The disease rapidly multiplies and overwhelms the organisms immune system. Introduced diseases have few natural controls to limit them, therefore they can become epidemic in proportions and threaten the existence of a susceptible tree species through its entire range. Air travel is making it easier for diseased plants or products to find their way to Canada in good condition. Some examples of devastating introduced infectious diseases are chestnut blight, Dutch elm disease and white pine blister rust.

Trees that are overmature, or those under stress from environmental conditions such as drought, air pollution or nutrient deficiencies grow more slowly and this makes them more susceptible to disease. Trees that are planted in the wrong conditions would also fall into this category of increased susceptibility.

FUNGUS ECOLOGICAL ROLE

Most infectious tree diseases in British Columbia are caused by fungi. Fungi do not contain chlorophyll and so are unable to produce their own food. They have to get their food from living or dead organic matter. Fungi are either saprophytes (deriving energy from dead, organic matter) or parasites (deriving energy from another living organism). Most fungi reproduce by means of spores. Fungi have many essential ecological roles in the forest.

Saprophytic fungi are generally beneficial as they act as important decomposers. The digestive enzymes fungi produce help to break down tons of dead and dying trees, leaves and branches into soil each year. They take the nutrients locked in logs and leaves and recycle it into a useable form in the soil. Without decomposers, the cycle of plant regeneration would stop.

Fungi also serve as a food source for many animals. Even people search out and eat fungi delicacies such as mushrooms or truffles.

Some saprophytic fungi, called mycorrhiza play a special role in assisting plants and trees to absorb soil nutrients. These tiny organisms form an association with the roots of some trees. The fungus helps the tree roots absorb water and nitrogen and even helps protect the roots against disease. The tree gives the fungus a place to live and nutrients. This is an important partnership for both the fungus and the tree.

Fungi also form symbiotic associations with alga creating a dual organism called a lichen. The fungus provides protection and moisture for the alga, while the chlorophyll filled alga produces food for the fungus. Lichens can survive harsh conditions that are too exposed for others plants. Lichens have special roles to play in the forest ecosystem as a food source (for deer in winter) and in the biological weathering of rocks.

The effect of lichen on trees can sometimes be slightly detrimental as large amount of lichen on small branches can cause reduction of light and interference with gas exchange leading to branch death.

Those types of fungi that have evolved food habits that derive nutrients from living trees are parasitic and cause disease. These are the focus of this article.

MAIN TYPES OF FOREST DISEASE

There are many types of forest diseases. This article will examine the main forest diseases based on the main part of the tree that is afflicted.

Stem diseases: Decay

Wood decay, or heart rot, is the most common disease of mature forests. Almost half of all disease losses are due to heart rot fungi. They work slowly from the inner heartwood out. The fungi spread long thin filaments throughout the cells, which can discolour the wood and weaken it, leaving the tree susceptible to blow down. These filaments or mycelia can grow from 1 millimeter to 5 meters a year.

Decay fungi do not normally infect live wood, they can only damage a living tree if a wound provides an entrance (fire, bird or insect damage, logging scars or wind breakage). They can also grow through dead branches which are a direct line to the heartwood. Natural pruning and growing over of branch stops this, but this process takes time, just like healing of scar. Decay fungi feed on the substances that compose the cell walls of heartwood. They must have moisture to live, decay does not occur in dry wood. White rot fungi usually attacks lignin, while brown rot fungi usually attacks cellulose. After several years, the fungi grow reproductive structures on the outside of the tree, called conks or brackets.

Heart rot is a particular problem in old growth forests. Management of this disease includes shorter rotations, salvage cutting, and care to prevent trunk damage from logging, road building or prescribed burning.

Stem diseases: Rusts

The mycelium of rust fungi are perennial in bark or wood resulting in malformations such as gall, burls, cankers, witches brooms. They frequently result in death particularly with seedlings and sapling.

White pine blister rust is a particularly damaging rust that was introduced to BC in 1910 in infected nursery stock from France. This fungus almost wiped out the native white pines, as they had no natural resistance to this invader. White pine blister rust affects trees of all ages; the greatest impact occurs in young trees The blister rust fungus enters the tree through its new needles, eventually causing a canker to develop in the bark of the host. Mortality occurs when a blister rust canker girdles the main stem, interrupting the flow of water and nutrients to the rest of the tree. The result is either deformity reducing the commercial value or death.

Attempts to control this disease have focused on searching for the few trees that show a resistance to the infection. These trees are used in a breeding program to produce seedlings that may survive the infection. The goal of the breeding program is not to produce totally resistant populations as this might lead to the blister rust mutating. The plan is to eventually produce about 2/3 resistant seedlings, enough to produce a commercial crop.

Another method of control for blister rust is branch pruning. This removes the threat of the fungus reaching the stem where it can cause mortality. Pruning involves the removal of lower branches. In white pine, pruning has been shown to be effective as long as the disease has not spread too far. Initial infection by white pine blister rust occurs on the current year's needles. Removal of all lower branches eliminates the infection potential and any already existing branch infections. Often two pruning lifts are required. Once the trees are tall enough and the lower limbs are above the range of the fungus, they are out of danger.

Another blister rust control effort is to limit the growth of currents and gooseberries which also play a role in the fungus’s life cycle. Theses species (ribes) are the alternate hosts for blister rust. The fungus need these plants to complete their life cycle. Therefore, in theory, if you control this vegetation you will limit the success of the blister rust.

Stem diseases: Dwarf Mistletoe

Dwarf mistletoe is a parasitic plant which causes serious problems in British Columbia. This plant sends its roots into the stem and branches and consumes the tree’s nutrients and water, slowly starving and weakening the host. It causes reduced growth, deformity and sometimes death. Infected trees have tangled deformed branches called "witches brooms" and swollen misshapen trunks and branches which are vulnerable to wind damage. Hemlock is especially susceptible to this disease.

Mistletoe spreads by discharging its sticky seeds at a very high speed. The seeds can travel up to 25 meters from their source, infecting the young trees growing below the infected tree. Seeds generally penetrate young, thin bark. Squirrels and birds are also involved in spreading these sticky seeds.

Management of this disease has focused on removing the infected trees and burning the understory to prevent seed spread. Foresters then select more resistant species for replanting.

Root diseases

Most fungi that attack the roots are decay fungi. These root rots are hard to detect, spread easily and can live in old stumps for up to a century. Root rot spreads to healthy roots through the growth of thread-like fungal mycelium that stretch out through the soil from the infected roots or by direct root contact.

The fungus moves into the heartwood and butt of the tree where the decay causes a reduction in growth and in cone crops, as well as an increased susceptibility to blowdown. Younger stands are especially vulnerable to root rot.

Management of this disease includes removal of infected trees and replanting with more resistant species. Removing and burning infected stumps can also be helpful, but is costly. Main root rots are phellinus and armillaria.

Foliage diseases

Disease that targets leaves often result in little serious damage. Most fungi either attack only the new leaves or only the mature foliage, rarely both, which allows some functioning foliage for the for the tree’s use. The discoloured leaves can produce cosmetic damage which may be a problem in ornamental and Christmas trees.

Needle blights can be serious when the entire crown is infected. The tree may die from the disease, or from the insect infestation which usually follows. Trees become more resistant to foliage diseases with age, so foliage diseases are often more severe on seedlings and younger trees. The sapling to pole stage is more vulnerable to foliage disease as they have fewer years growth to sustain them. The virulence of these diseases are also connected to climatic and growing conditions. Therefore heavy infections extending over a period of years is exceptional.

Foliage diseases include leaf blights, needle casts, sooty mold and rusts. The various fungal agents grow their thread-like mycelia into the tissue of the leaves to absorb nutrients. The leaves may show spots or change colours. The fungus may not develop spores until the following year. After the spores are released, in some species, the infected needles may be "cast" off or dropped.

Some foliage diseases can be controlled by prescribed burning.

Decline diseases

Decline diseases are a combination of abiotic and biotic factors. The birch dieback and the maple decline of eastern Canada are examples decline diseases.

Pre-disposing factors, such as climate, soil type, age and genetics, can cause a weakness which pre-disposes trees to further damage simply because they are growing in the wrong place.

Physical or biological inciting factors, such as a drought or air pollution, can then cause damage from which recovery can be difficult.

Contributing factors, such as a fungal disease, can then speed up the demise of the already weakened trees.

Deterioration of Forest Products

The deterioration of wood products by fungus has always been a serious problem. Decay even influenced the course of naval history in the days of wooden sailing ships.

Stains and rots are fungal attacks that damage forest products. The staining may start in the forest or the lumber may be attacked in the yard. Some staining fungi do not change the wood’s strength or durability, however the change in appearance decreases the value for the customer.

Spores of staining fungi can be carried by wind, water or bark beetles. Rots usually attack sapwood verses heartwood (ie prefer more moisture and a specific temperature). Warm weather and damp conditions promote fungal growth. Storing logs in ponds or under sprinklers can create environments that are too moist for stain fungi. Chemical treatments and kiln or microwave drying also help prevent molds from spreading.

Decay caused by fungus has a dramatic effect on woods strength and durability. Different species of wood show differing resistance to decay organisms, while different decay organisms cause damage at different rates from a few months to years.

Cedar and yew are highly resistant to decay while wood such as aspen, cottonwood and willow have a very low resistance to decay. Resistance to decay is a combination of innate characteristics of the wood itself as well as the conditions in which it is being used.

Proper lumber storage conditions are important in preventing decay. Storage yards must be well drained, all debris removed, and lumber piles must be well ventilated. Chemical treatment for preventing decay is available.

CONTROLLING DISEASE

The approach to disease prevention is multi-pronged: The first line of defence is using silviculture practices to maintain a healthy vibrant stand. Activities such as thinning, pruning, fertilizing and monitoring help keep the stand’s natural defences strong.

Once disease has been identified, activities such as removal of infected trees, prescribed burning and planting of more resistant species help to control the spread of the disease.

In forest nurseries full-scale control methods including fungicides may be incorporated as the high value of their crops the limited area make control measures economically feasible.

On-going research into control measures continues. The development of resistant strains of vulnerable species offers foresters some planting alternatives. Research into silvicultural practices aimed at disease control may produce some new ideas about future control.

As with insects, there is no single solution. An integrated approach to forest health will include all of these options, set long-term objectives, and put monitoring programs into place. Understanding the forest health implications of various forestry practices is essential in developing an overall forest health strategy.

Resources:

• Canadian Forest Service: Common Tree Diseases of BC
• Canadian Forest Service: Entomology:
• Canadian Forest Service: Pathology
• USDA Forest Service - NorthWest Division
• Forest Insects and Diseases
• Forest Health Web

Forest Pathology Boyce, J.S. 1961. McGraw-Hill, NY. Forest Health in Canada: An overview. Forest Health Network. Canadian Forest Service. 1999. Cdn Cataloguing in publication Data. Managing your woodland. Canadian Forest Service. 1992. Canadian Cataloguing in Publication Data Forum Modules COFI. 1992. Common Tree Diseases of British Columbia. Foster,R.E., Wallis,G.W. 1969. Ministry of Forests, Queens Printer. Introduction to forest and renewable resources. Sharpe, G., Hendee C.W. et al 1992. McGraw Hill, NY. Forestry Handbook UBC Faculty of Forestry. 1971.