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Tree Diseases -                                         Causal Agents of Tree Diseases

Causal Agents of Tree Diseases

The factors that cause diseases in trees are often referred to as agents. These agents can be living organisms, such as fungi, bacteria, and viruses, which can cause actual infection of the tree. There are also nonliving agents, which cause injuries at sites that may later serve as points where infection can be initiated. In some cases, the injuries caused by both living and nonliving agents may be so severe that the affected plants die. 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 do not are called "noninfectious." Infectious agents produce some form that enables them to spread and infect other hosts.

Man, animals, and weather are all noninfectious agents. We selected the most important or common agents that can cause damage for individual coverage in the text.

Infectious diseases of trees are primarily caused by fungi. Fungi are simple plants that lack chlorophyll and that colonize their substrate by the development of microscopic, thread-like structures called hyphae or hyphal strands. Because fungi have no chlorophyll, photosynthesis cannot occur; hence, fungi obtain food by enzymatic action from the material upon or in which they are growing.

Fungi reproduce by spores, which function much like seeds but are so small they can be seen only with the aid of a microscope. Spores are primarily spread by wind, rain splash, insects, and movement of soil and infected plant material. When a spore lands on a suitable site, it germinates, producing a germ tube that develops into a hyphal strand as the fungus develops. These strands grow, branch, and, in favorable conditions, spread rapidly.

Many fungi are beneficial. Some function to break down dead organic matter; some (yeasts) are used in baking and brewing; some (molds) are used in cheese and antibiotic production; and others (mushrooms) produce edible fruiting bodies.

There are also a number of fungi that attack and damage living plants. Infectious fungi or the diseases they cause can be categorized as fluctuating or nonfluctuating. Those that fluctuate vary in their presence and severity from year to year. Leaf spots are good examples of fluctuating diseases. Nonfluctuating diseases are those that persist once they are established. Cankers on stems or branches are good examples: they persist until the host or infected part perishes. Some fungi even remain active on a dead host.

The diseases we have selected for coverage can be grouped into the following categories:

needle casts


The rust diseases are a very interesting group, because many of them require two different living hosts to complete a rather complex life cycle. 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. A complete life cycle has five separate fruiting stages. Many rust fungi do not go through all five of the stages, and some are able to complete their life cycle on a single host. Rust fungi derive their nourishment from living plant cells, so that they die if their hosts die. Tree rusts have representatives in both the fluctuating and non-fluctuating disease groups.

The wood 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 the roots, and others are confined primarily to the stem. Once the decay fungus produces a fruiting structure, indicating its presence in the tree, the decay may be well advanced.

There are two major groups of decay fungi: white rots decompose all of the wood components, and brown rots decompose just the cellulose, leaving the lignin. It is estimated that there are about 1700 decay fungi, of which about 6% are brown rots.

Wilt diseases are caused by fungi that invade the vascular system of the host. They interfere with the translocation of fluids within the tree, resulting in a reduced flow of water to the leaves and subsequent wilting. The mechanism by which the fungus causes the interruption of fluid movement is not completely understood, but it probably involves physical obstruction and toxin production by the fungus and the development of structures by the host that tend to plug water-carrying vessels. Trees infected by wilt fungi often have a solid or dotted pattern of color in the outer sapwood when seen in cross section. Wilt diseases are non-fluctuating, eventually resulting in mortality.

Canker fungi cause distortions of the trunk or branches of infected trees and are found on both hardwoods and conifers. 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 frequently invade their hosts through branch stubs or wounds. Once established, the fungus kills the bark, often resulting in a characteristic pattern or color as the host responds to the invasion. Canker fungi fruit on the host, and their spores can be liberated whenever temperature and moisture requirements are met.

Needle cast fungi are common on conifer needles, and many are capable of causing premature defoliation. Infection usually occurs on needles of the current year, and the reproductive stage of the fungus may occur at the end of that season or up to 2 years later. The reproductive structure is usually black and may be circular, oval, or elongate. It may cover up to the entire length of the needle. The fruiting structures of the needle cast fungi 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. Needle cast is a fluctuating disease.

Anthracnose is a disease of hardwood foliage caused by fungi that spend the winter on fallen infected leaves or in the twigs. In the 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. Anthracnose is a fluctuating disease and is not likely to be fatal.

Not all of the organisms covered in this text fit into the above groups. For example, fire blight is caused by a bacterium. Bacteria are very small, single-celled organisms that are responsible for a lot of soft rot and can kill tissue of living hosts. They are often disseminated by insects. Dwarf mistletoe is a parasitic seed plant that infects some conifer species. It is disseminated by a forceful ejection of its seeds and possible incidental transport by birds and small mammals.

Tree Diseases of Eastern Canada- Casual Agents of Tree Diseases, Natural Resources Canada, Canadian Forest Service.  

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