Armillaria root rot
Encyclopedia
Armillaria root rot is a fungal root rot
caused by several different members of the genus Armillaria
. The symptoms are variable depending on the host infected, ranging from stunted leaves to chlorotic needles and dieback
of twigs and branches. However, all infected hosts display symptoms characteristic of being infected by a white rotting fungus
. The most effective ways of management focus on limiting the spread of the fungus, planting resistant species, and removing infected material. This disease poses a threat to the lumber industry as well as affecting recreational areas.
s and shrub
s are susceptible to root rot to varying degrees. In fact, the only two species of tree known to be resistant to Armillaria root rot are larch
and birch
. Further investigation is being conducted for additional species, but at this time there is no further evidence leading to the belief that more exist.
As a result of the multitude of possible hosts, symptoms also range a great deal from one infection to another. All infected hosts do however display symptoms common to white rotting fungi. These include light or bleached wood as a result of the degradation of essential cell wall compounds such as lignin
and hemicellulose
. In addition to these, the host will show above-ground symptoms due to fungal infection of the vasculature. These include chlorosis in the needles as well as dieback of twigs and branches. The extent of these symptoms varies with the degree of infection as well as the susceptibility of the host. In addition to these symptoms, the trunks of conifers will also exude excess resin in a process known as resinopsis which results in a layer of resin, debris, and fungal tissue forming around infected roots. Deciduous trees occasionally develop sunken cankers but most often fail to exhibit these symptoms on the trunk, and will instead simply display the other symptoms.
In addition to these symptoms, signs of the infectious organism are very evident in the host. Upon removal of the bark, white mycelial mats are visible along with rhizomorphs
, a distinctive reproductive structure. Clusters of mushroom
s will also form at the base of the infected tree, indicating an infection. Also, zone lines of melanized fungal cells may be visible within infected wood.
For the most part, this fungus exhibits a life cycle characteristic of basidiomycetes
. It reproduces sexually with the mating of hyphae and produces a basidiocarp
at the base of the infected host. This basidiocarp produces basidiospores that will further infect new hosts. However, Armillaria rarely spreads using this method in nature, possibly as a result of the spores being ineffective. In contrast, it will spread either through rhizomorphs or direct mycelial contact. In the case of mycelial contact, the roots of an infected host grow near enough to a new host that mycelia simply grow onto the new host and infect. Rhizomorphs are string-like masses of hyphae utilized if no new hosts are nearby and spread by probing through the soil towards uninfected roots.
s, other pathogen
s and/or climate stresses. It can also kill healthy trees especially in dry areas, like coniferous forests in the western United States
. This fungus is found worldwide, but prefers cool soils and climates.
s. If the infected area has been cleared of trees, plants that are not vulnerable to the disease should be planted for five or so years until Armillaria is eradicated. Stump removal is also an effective management tool, however it is very expensive. Another way to reduce susceptibility is to maintain plant health by regular fertilization (if needed), watering during droughts, and trying not to create wounds on the plant. Fumigation
can also be used to reduce the amount of inoculum.
and proves very difficult to remove once it has entered an area. The ability to spread using rhizomorphs as well as through mycelial contact allow the fungus to spread over very large areas and between many individuals. In fact, Armillaria is recognized as the largest living organism due to its clonal method of spreading. Infection is especially strong where environmental conditions are optimal and a large amount of possible hosts exist. This essentially is anywhere hardwoods are found growing. It has been found in every state in the United States of America as well as on several other continents including Australia
, Europe, and Asia
.
Root rot
Root rot is a condition found in both indoor and outdoor plants, although more common in indoor plants with poor drainage. As the name states, the roots of the plant rot. Usually, this is a result of overwatering. In houseplants, it is a very common problem, and is slightly less common in outdoor...
caused by several different members of the genus Armillaria
Honey fungus
Honey fungus, or Armillaria or оpenky , is a genus of parasitic fungi that live on trees and woody shrubs. It includes about 10 species formerly lumped together as A. mellea. Armillarias are long lived and form some of the largest living organisms in the world...
. The symptoms are variable depending on the host infected, ranging from stunted leaves to chlorotic needles and dieback
Dieback
Dieback may refer to a number of plant problems and diseases including:* Forest dieback caused by acid rain, heavy metal pollution, or imported pathogens* The death of regions of a plant or similar organism caused by physical damage, such as from pruning...
of twigs and branches. However, all infected hosts display symptoms characteristic of being infected by a white rotting fungus
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
. The most effective ways of management focus on limiting the spread of the fungus, planting resistant species, and removing infected material. This disease poses a threat to the lumber industry as well as affecting recreational areas.
Hosts and Symptoms
Because this disease is caused by multiple species within the genus Armillaria, it has an extremely broad host range. Hundreds of treeTree
A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
s and shrub
Shrub
A shrub or bush is distinguished from a tree by its multiple stems and shorter height, usually under 5–6 m tall. A large number of plants may become either shrubs or trees, depending on the growing conditions they experience...
s are susceptible to root rot to varying degrees. In fact, the only two species of tree known to be resistant to Armillaria root rot are larch
Larch
Larches are conifers in the genus Larix, in the family Pinaceae. Growing from 15 to 50m tall, they are native to much of the cooler temperate northern hemisphere, on lowlands in the north and high on mountains further south...
and birch
Birch
Birch is a tree or shrub of the genus Betula , in the family Betulaceae, closely related to the beech/oak family, Fagaceae. The Betula genus contains 30–60 known taxa...
. Further investigation is being conducted for additional species, but at this time there is no further evidence leading to the belief that more exist.
As a result of the multitude of possible hosts, symptoms also range a great deal from one infection to another. All infected hosts do however display symptoms common to white rotting fungi. These include light or bleached wood as a result of the degradation of essential cell wall compounds such as lignin
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
and hemicellulose
Hemicellulose
A hemicellulose is any of several heteropolymers , such as arabinoxylans, present along with cellulose in almost all plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength...
. In addition to these, the host will show above-ground symptoms due to fungal infection of the vasculature. These include chlorosis in the needles as well as dieback of twigs and branches. The extent of these symptoms varies with the degree of infection as well as the susceptibility of the host. In addition to these symptoms, the trunks of conifers will also exude excess resin in a process known as resinopsis which results in a layer of resin, debris, and fungal tissue forming around infected roots. Deciduous trees occasionally develop sunken cankers but most often fail to exhibit these symptoms on the trunk, and will instead simply display the other symptoms.
In addition to these symptoms, signs of the infectious organism are very evident in the host. Upon removal of the bark, white mycelial mats are visible along with rhizomorphs
Mycelial cord
Mycelial cords are linear aggregations of parallel-oriented hyphae. The mature cords are composed of wide, empty vessel hyphae surrounded by narrower sheathing hyphae...
, a distinctive reproductive structure. Clusters of mushroom
Mushroom
A mushroom is the fleshy, spore-bearing fruiting body of a fungus, typically produced above ground on soil or on its food source. The standard for the name "mushroom" is the cultivated white button mushroom, Agaricus bisporus; hence the word "mushroom" is most often applied to those fungi that...
s will also form at the base of the infected tree, indicating an infection. Also, zone lines of melanized fungal cells may be visible within infected wood.
Disease cycle
Basidiomycota
Basidiomycota is one of two large phyla that, together with the Ascomycota, comprise the subkingdom Dikarya within the Kingdom Fungi...
. It reproduces sexually with the mating of hyphae and produces a basidiocarp
Basidiocarp
In fungi, a basidiocarp, basidiome or basidioma , is the sporocarp of a basidiomycete, the multicellular structure on which the spore-producing hymenium is borne. Basidiocarps are characteristic of the hymenomycetes; rusts and smuts do not produce such structures...
at the base of the infected host. This basidiocarp produces basidiospores that will further infect new hosts. However, Armillaria rarely spreads using this method in nature, possibly as a result of the spores being ineffective. In contrast, it will spread either through rhizomorphs or direct mycelial contact. In the case of mycelial contact, the roots of an infected host grow near enough to a new host that mycelia simply grow onto the new host and infect. Rhizomorphs are string-like masses of hyphae utilized if no new hosts are nearby and spread by probing through the soil towards uninfected roots.
Environment
Armillaria infects trees in temperate and tropical regions. Armillaria commonly infects stressed trees that have been weakened by insectInsect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s, other pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s and/or climate stresses. It can also kill healthy trees especially in dry areas, like coniferous forests in the western United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. This fungus is found worldwide, but prefers cool soils and climates.
Management
Plant varieties that are resistant to Armillaria or species are resistant to other environmental or biological stressorStressor
Stressor is a chemical or biological agent, environmental condition, an external stimulus or an event that causes stress to an organism. An event that triggers the stress response may include for example:...
s. If the infected area has been cleared of trees, plants that are not vulnerable to the disease should be planted for five or so years until Armillaria is eradicated. Stump removal is also an effective management tool, however it is very expensive. Another way to reduce susceptibility is to maintain plant health by regular fertilization (if needed), watering during droughts, and trying not to create wounds on the plant. Fumigation
Fumigation
Fumigation is a method of pest control that completely fills an area with gaseous pesticides—or fumigants—to suffocate or poison the pests within. It is utilized for control of pests in buildings , soil, grain, and produce, and is also used during processing of goods to be imported or...
can also be used to reduce the amount of inoculum.
Importance
This disease poses a serious threat to the timber industry in that it affects nearly every cultivated species of hardwoodHardwood
Hardwood is wood from angiosperm trees . It may also be used for those trees themselves: these are usually broad-leaved; in temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen.Hardwood contrasts with softwood...
and proves very difficult to remove once it has entered an area. The ability to spread using rhizomorphs as well as through mycelial contact allow the fungus to spread over very large areas and between many individuals. In fact, Armillaria is recognized as the largest living organism due to its clonal method of spreading. Infection is especially strong where environmental conditions are optimal and a large amount of possible hosts exist. This essentially is anywhere hardwoods are found growing. It has been found in every state in the United States of America as well as on several other continents including Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, Europe, and Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
.