Armillaria root rot

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Armillaria root rot
Armillaria luteobubalina 67510.jpg
Armillaria luteobubalina , widespread in Australia, is a primary cause of Eucalyptus tree death and forest dieback resulting from Armillaria root rot.
Causal agentsSeveral species of the genus Armillaria
HostsSeveral tree species

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.

Contents

Hosts and symptoms

Because this disease is caused by multiple species within the genus Armillaria, it has an extremely broad host range. Hundreds of trees and shrubs are susceptible to root rot to varying degrees. In fact, the only two genera 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.

While Armillaria is a significant and damaging pathogen of tree hosts, it also has many agronomic hosts such as grapevines, berries, roses, stone fruits, rhododendron, and rosaceous plants, although the fungus is primarily native to areas where it can use forest trees as a host. On hosts such as these, infection causes death of the cambium and further decay of the xylem. Vines or stems from these plants, once infected, may remain a potential source for inoculum for up to ten years and can infect neighboring plants, often via rhizomorphs. [1]

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. [2] 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. Rhizomorphs are black, shoe-string like growth structure that can grow out from the host and grow in the soil to infect nearby hosts. Clusters of mushrooms will also form at the base of the infected tree, indicating an infection. However, fruiting is not consistent year-to-year and the mushrooms frequently resemble other species to the untrained eye. Therefore, the presence of mushrooms should not constitute the only diagnostic indicator when identifying the disease. As the name suggests, the caps of these mushrooms are a honey, or light brown color, and the gills of the mushroom and spore print are white. [3] The stem is typically a mottled white and brown, with a ring on the stem, too. Honey mushrooms are edible, but because of their tendency to look so similar to other species, only the very experienced should collect them. Also, zone lines of melanized fungal cells may be visible within infected wood.

Disease cycle

Diagram of white root rot disease cycle White root disease.jpg
Diagram of white root rot disease cycle
Black root-like rhizomorphs on an unidentified European Armillaria species Rhizomorph Armillaria.jpg
Black root-like rhizomorphs on an unidentified European Armillaria species

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. [4] 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. Armillaria can last up to 50 years in stumps of dead trees, infecting new hosts that come up nearby in that time period. In areas of infection, called a root-disease center, can develop.

Environment

Armillaria infects trees in temperate and tropical regions. Armillaria commonly infects stressed trees that have been weakened by insects, other pathogens 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.

Managements

Plant varieties that are resistant to Armillaria or species are resistant to other environmental or biological stressors. 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 but can be 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. [5]

Cultural practices can also be effective for preventing the spread of Armillaria. High temperatures interfere with the progression of this disease; if soil temperatures reach 79 °F (26 °C), then the growth of Armillaria in the soil will be limited. [6] Using collar excavation to expose primary roots directly below the crown to the continuous cycle of solar heating and air may reduce the colonization from Armillaria in trees that are already infected. [7] This is called root collar excavation (RCE), and while it has been applied to citrus orchards and grapevines, this method has high labor costs and involvement due to the difficulty of carefully exposing these primary roots.

Armillaria often occurs when a host is stressed due to other abiotic or biotic factors. It's often more cost effective to treat the stressor that caused the pathogen rather than treating Armillaria on its own, as these treatments and practices are not practical for everyday use.

Importance

This disease poses a serious threat to the timber industry in that it affects nearly every cultivated species of hardwood 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, [8] Europe, and Asia. [9]

Related Research Articles

<i>Armillaria</i> Genus of fungi

Armillaria is a genus of fungi that includes the A. mellea species known as honey fungi that live on trees and woody shrubs. It includes about 10 species formerly categorized summarily as A. mellea. Armillarias are long-lived and form the largest living fungi in the world. The largest known organism covers more than 3.4 square miles (8.8 km2) in Oregon's Malheur National Forest and is estimated to be 2,500 years old. Some species of Armillaria display bioluminescence, resulting in foxfire.

<span class="mw-page-title-main">Texas root rot</span> Pathogenic fungus

Texas root rot is a disease that is fairly common in Mexico and the southwestern United States resulting in sudden wilt and death of affected plants, usually during the warmer months. It is caused by a soil-borne fungus named Phymatotrichopsis omnivora that attacks the roots of susceptible plants. It was first discovered in 1888 by Pammel and later named by Duggar in 1916.

<i>Armillaria mellea</i> Species of fungus

Armillaria mellea, commonly known as honey fungus, is an edible basidiomycete fungus in the genus Armillaria. It is a plant pathogen and part of a cryptic species complex of closely related and morphologically similar species. It causes Armillaria root rot in many plant species and produces mushrooms around the base of trees it has infected. The symptoms of infection appear in the crowns of infected trees as discoloured foliage, reduced growth, dieback of the branches and death. The mushrooms are edible but some people may be intolerant to them. This species is capable of producing light via bioluminescence in its mycelium.

<span class="mw-page-title-main">Mycelial cord</span>

Mycelial cords are linear aggregations of parallel-oriented hyphae. The mature cords are composed of wide, empty vessel hyphae surrounded by narrower sheathing hyphae. Cords may look similar to plant roots, and also frequently have similar functions; hence they are also called rhizomorphs. As well as growing underground or on the surface of trees and other plants, some fungi make mycelial cords which hang in the air from vegetation.

<i>Armillaria luteobubalina</i> Species of fungus in the family Physalacriaceae.

Armillaria luteobubalina, commonly known as the Australian honey fungus, is a species of mushroom in the family Physalacriaceae. Widely distributed in southern Australia, the fungus is responsible for a disease known as Armillaria root rot, a primary cause of Eucalyptus tree death and forest dieback. It is the most pathogenic and widespread of the six Armillaria species found in Australia. The fungus has also been collected in Argentina and Chile. Fruit bodies have cream- to tan-coloured caps that grow up to 10 cm (4 in) in diameter and stems that measure up to 20 cm (8 in) long by 1.5 cm (1 in) thick. The fruit bodies, which appear at the base of infected trees and other woody plants in autumn (March–April), are edible, but require cooking to remove the bitter taste. The fungus is dispersed through spores produced on gills on the underside of the caps, and also by growing vegetatively through the root systems of host trees. The ability of the fungus to spread vegetatively is facilitated by an aerating system that allows it to efficiently diffuse oxygen through rhizomorphs—rootlike structures made of dense masses of hyphae.

<i>Armillaria tabescens</i> Species of fungus

Armillaria tabescens is a species of fungus in the family Physalacriaceae. It is a plant pathogen. The mycelium of the fungus is bioluminescent.

<i>Ceratocystis fimbriata</i> Species of fungus

Ceratocystis fimbriata is a fungus and a plant pathogen, attacking such diverse plants as the sweet potato and the tapping panels of the Para rubber tree. It is a diverse species that attacks a wide variety of annual and perennial plants. There are several host-specialized strains, some of which, such as Ceratocystis platani that attacks plane trees, are now described as distinct species.

<i>Phytophthora cactorum</i> Species of single-celled organism

Phytophthora cactorum is a fungal-like plant pathogen belonging to the Oomycota phylum. It is the causal agent of root rot on rhododendron and many other species, as well as leather rot of strawberries.

<i>Macrophomina phaseolina</i> Species of fungus

Macrophomina phaseolina is a Botryosphaeriaceae plant pathogen fungus that causes damping off, seedling blight, collar rot, stem rot, charcoal rot, basal stem rot, and root rot on many plant species.

<i>Rhizomorpha subcorticalis</i> Species of fungi

Rhizomorpha subcorticalis is a species name that has been used to characterize certain fungal plant pathogen observations where the pathogen is evident only through mycelial cords ("rhizomorphs"). The species in question very likely also produces reproductive structures which would allow it to be situated in the normal taxonomic tree, especially if DNA analysis is available. A name like R. subcorticalis should only be used where such identification is impossible.

<i>Heterobasidion annosum</i> Species of fungus

Heterobasidion annosum is a basidiomycete fungus in the family Bondarzewiaceae. It is considered to be the most economically important forest pathogen in the Northern Hemisphere. Heterobasidion annosum is widespread in forests in the United States and is responsible for the loss of one billion U.S. dollars annually. This fungus has been known by many different names. First described by Fries in 1821, it was known by the name Polyporus annosum. Later, it was found to be linked to conifer disease by Robert Hartig in 1874, and was renamed Fomes annosus by H. Karsten. Its current name of Heterobasidion annosum was given by Brefeld in 1888. Heterobasidion annosum causes one of the most destructive diseases of conifers. The disease caused by the fungus is named annosus root rot.

Rosellinia bunodes is a plant pathogen infecting several hosts including avocados, bananas, cacao and tea.

<i>Rigidoporus microporus</i> Species of fungus

Rigidoporus microporus is a plant pathogen, known to cause white root rot disease on various tropical crops, such as cacao, cassava, tea, with economical importance on the para rubber tree.

<i>Armillaria fuscipes</i> Species of fungus

Armillaria fuscipes is a plant pathogen that causes Armillaria root rot on Pinus, coffee plants, tea and various hardwood trees. It is common in South Africa. The mycelium of the fungus is bioluminescent.

<i>Laminated root rot</i> Fungal plant disease

Laminated root rot also known as yellow ring rot is caused by the fungal pathogen Phellinus weirii. Laminated root rot is one of the most damaging root disease amongst conifers in northwestern America and true firs, Douglas fir, Mountain hemlock, and Western hemlock are highly susceptible to infection with P. weirii. A few species of plants such as Western white pine and Lodgepole pine are tolerant to the pathogen while Ponderosa pine is resistant to it. Only hardwoods are known to be immune to the pathogen.

<i>Armillaria novae-zelandiae</i> Species of fungus

Armillaria novae-zelandiae is a species of mushroom-forming fungus in the family Physalacriaceae. This plant pathogen species is one of three Armillaria species that have been identified in New Zealand.

<i>Armillaria gallica</i> Species of fungus in the family Physalacriaceae

Armillaria gallica is a species of honey mushroom in the family Physalacriaceae of the order Agaricales. The species is a common and ecologically important wood-decay fungus that can live as a saprobe, or as an opportunistic parasite in weakened tree hosts to cause root or butt rot. It is found in temperate regions of Asia, North America, and Europe. The species forms fruit bodies singly or in groups in soil or rotting wood. The fungus has been inadvertently introduced to South Africa. Armillaria gallica has had a confusing taxonomy, due in part to historical difficulties encountered in distinguishing between similar Armillaria species. The fungus received international attention in the early 1990s when an individual colony living in a Michigan forest was reported to cover an area of 15 hectares, weigh at least 9.5 tonnes, and be 1,500 years old. This individual is popularly known as the "humongous fungus", and is a tourist attraction and inspiration for an annual mushroom-themed festival in Crystal Falls. Recent studies have revised the fungus's age to 2,500 years and its size to about 400 tonnes, four times the original estimate.

Forest pathology is the research of both biotic and abiotic maladies affecting the health of a forest ecosystem, primarily fungal pathogens and their insect vectors. It is a subfield of forestry and plant pathology.

<i>Armillaria ostoyae</i> Species of fungus

Armillaria ostoyae is a species of fungus (mushroom), pathogenic to trees, in the family Physalacriaceae. In the western United States, it is the most common variant of the group of species under the name Armillaria mellea. A. ostoyae is common on both hardwood and conifer wood in forests west of the Cascade Range in Oregon, United States. It has decurrent gills and the stipe has a ring. The mycelium invades the sapwood and is able to disseminate over great distances under the bark or between trees in the form of black rhizomorphs ("shoestrings"). In most areas of North America, Armillaria ostoyae can be separated from other species by its physical features: cream-brown colors, prominent cap scales, and a well-developed stem ring distinguish it from other Armillaria.

Stromatinia cepivora is a fungus in the division Ascomycota. It is the teleomorph of Sclerotium cepivorum, the cause of white rot in onions, garlic, and leeks. The infective sclerotia remain viable in the soil for many years and are stimulated to germinate by the presence of a susceptible crop.

References

  1. ↑ "Armillaria Root Rot in Grapes – Grapes". grapes.extension.org. Retrieved 2020-04-30.
  2. ↑ Williams, R.E.; Shaw III, C.G.; Wargo, P.M.; Sites, W.H. (1989). Armillaria Root Disease. Forest Insect and Disease Leaflet, 78. Washington, D.C.: USDA Forest Service. Archived from the original on 26 October 2010. Retrieved 30 October 2010.
  3. ↑ Worrall, J. (2004). "Armillaria root disease". The Plant Health Instructor. doi:10.1094/PHI-I-2004-0706-01. ISSN   1935-9411.
  4. ↑ Worall, James J. (2010). "Armillaria Root Disease". Forest & Shade Tree Pathology. Archived from the original on 21 November 2010. Retrieved 30 October 2010.
  5. ↑ Department of Crop Sciences, University of Illinois at Urbana-Champaign (2000). Armillaria Root Rot of Trees and Shrubs (PDF). Report on Plant Disease, 602. Urbana, Illinois: University of Illinois Extension. Retrieved 30 October 2010.
  6. ↑ "Armillaria Root Rot". Penn State Extension. Retrieved 2020-04-30.
  7. ↑ "Armillaria root rot | College of Agriculture, Forestry and Life Sciences | Clemson University, South Carolina". www.clemson.edu. Retrieved 2020-04-30.
  8. ↑ Coetzee, Martin P. A.; Wingfield, Brenda D.; Harrington, Thomas C.; Steimel, Joe; Coutinho, Teresa A.; Wingfield, Michael J. (2001). "The root rot fungus Armillaria mellea introduced into South Africa by early Dutch settlers" (PDF). Molecular Ecology. 10 (2): 387–96. doi:10.1046/j.1365-294X.2001.01187.x. PMID   11298953. S2CID   714873 . Retrieved 30 October 2010.
  9. ↑ Davari, M.; Askari, B. (2005). "Armillaria mellea as a cause of oak decline in Hatam-baigh forest of Iran". Communications in Agricultural and Applied Biological Sciences. 70 (3): 295–304. PMID   16637190.
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