Armilliaria mellea is
a complex group of roughly ten very closely related species that used to be
named Armillaria melleus [6,13,14]. The
taxonomy of this group has been difficult, mycologists have been making
corrections to this genus for quite some time. At first they thought to put the
honey colored mushrooms in the Armillaria
genus, but with further research they discovered that over 200 species of them
did not actually belong to that genera [10].
Classification of A.
mellea has been difficult due to the species not being easily identifiable
in the field [7]. Molecular methods such as DNA sequencing are required in
addition to field characteristics to help identify the fungus [7].
General Description:
The word melle means honey, and A. mellea are commonly called honey mushrooms [6,14]. The mushroom
did not get this nickname from its taste, but from the honey colored caps that
are produced [2].The caps of these mushrooms are convex in shape with brown to
black hairs up near the center [14]. The entire cap is close to a yellowish brown
color, but with age the caps will darken [14]. Most of the mushrooms spotted in
the field will have a ring around the upper part of the stalk called a veil,
this is known to disappear on some of them [3,14]. The veil is thick, flaring
outward from the stalk, and is white in color [14].
A. mellea is a
soil-borne fungus that can be found year round [12]. They are most common in
late winter and after rain in the fall [2,10].
Photo of A. mella mushrooms
Photo
© Pam Kaminski [6]
Parasitic:
A. mellea are
plant pathogens meaning that they infect and kill plants [1,2]. It causes root
disease, rot, and wood decay in a broad range of host plants [2,9,15]. The most
susceptible hosts are ones that have been previously weakened or injured, but A. mellea is known to infect healthy
trees as well [4,15]. The fungus attacks the tissue located between the xylem
and phloem of the tree which causes death in some of the main roots located
close to the trunk [1]. A. mellea are
considered very serious pathogens because of how well they can spread and
infect. The fungus can infect neighboring trees by moving through the soil [2].
Mycorrhizal:
Interestingly, A.
mellea can also form mycorrhizal relationships with certain plants. The
fungus forms endotrophic mycorrhiza with orchids located in the tropics [9,14].
The orchid genus Gastrodia actually
requires an infection from A. mellea to
complete its life cycle and grow the structure called protocorm [14]. The
mycelium of A. mellea is now within
the protocom of the orchid and connects to nearby trees forming a parasitic
relationship with the tree [14]. This relationship between A. mellea and the trees allows for the appropriate nutrients to
benefit the orchids’ growth [14].
How to spot an
infected tree:
An infection caused by A.
mellea can be difficult to see unless it is in the sexual form where
mushrooms have developed [15]. If there are not mushrooms present and the tree
has multiple dead branches, yellow colored leaves, or reduced growth, then one
can pull bark off of the tree [4,6,15]. By pulling the bark off the tree, the
mycelium or dark rhizomorphs of the fungi will be visible. The mycelium is
often white and describes as looking fan shaped [4,15]. If a tree is infected
it may die rather quickly, or slowly over a period of time [4,6].
Photo of the dark rhizomorphs growing on a tree.
Photo of the dark rhizomorphs growing on a tree.
 |
Photo © Dave Kelly
http://www.first-nature.com/fungi/armillaria-mellea.php
|
 |
Photo is of the fan shaped mycelial growth. |
Distribution:
A. mellea is commonly
found throughout temperate and tropical regions [5,14,15]. They occur in the
Eastern part of North America ranging from the Great Lakes to the east and gulf
coast [10]. These fungi are native to California
where people refer to them as “oak root fungus” or “shoestring rot” due to the
darkly pigmented rhizomorphs. Rhizomorphs resemble thick string like structures
colored black [1,2].
According to a study completed by Robert Raabe, the following
U.S states have reported A. mellea
findings: California, Washington, New Mexico, Idaho, Colorado, Arizona, Utah,
Montana, New Jersey, Pennsylvania, New York, Maine, Maryland, as well as some
North central states. This study also displays other countries that have found A. mellea including but not subject to:
Germany, Italy, India, Poland, Canada, Switzerland, Denmark, Australia,
Tanganyika, Russia, etc. [13] This particular study gives insight as to how far
spread this particular fungus is.
Interestingly A. mellea
was found and identified in Cape Town South Africa [5]. The fungus was DNA
sequenced which linked them to the species that originated in Europe. A. mellea is believed to have been
introduced to South Africa from Europe in the form of potted citrus or grapes
tracing back to 300+ years ago [5].
Habitat:
A. mellea are
commonly found on stumps and logs of living or dead trees [2,14]. They can also
be found scattered on the ground but if this is the case then they are growing
from roots or wood buried underneath the ground [2,14]. They typically need a
root or piece of stem to survive, and they will survive for many years this way
[4]. This fungus has an extremely large host range, but they are most commonly
found on herbaceous plants, deciduous trees, conifers, and orchards [2,4,8,13,14].
A. mellea has also been recorded on
hardwoods, gardens, forest shrubs, grape and citrus plants [4,5]. In South
Africa the fungus has been found on Pinus
and Eucalyptus species in
addition to oak trees [5].
Look alikes:
It is always best to be cautious when identifying mushrooms
because many will have one or more look alikes. A. mellea has two look alike fungi that are commonly misidentified
[2,8]. Armillaria bulbosa is one of these look alikes [2]. This one is
especially important to beware of because it is a poisonous mushroom [2]. A. bulbosa has a ring on the stalk the
same way A. mellea does, but the cap
is a smoother texture [2]. However the main look alike is Armillaria tabescens. This fungus clusters on wood and has the same
spore print color as A. mellea, though
it has a darker color cap and does not have a veil [2]. This is an edible
fungus [8].
Nutrition:
A. mellea obtains
all of its required nutrition through the roots and lower trunk of their host
tree [2]. They do this using extracellular enzymes to obtain food from the
organic matter they are decaying [6].
How is A. mellea used?
As mentioned earlier, A.
mellea will form a symbiotic relationship with the orchid Gastrodia elata, and this is currently
being used in Chinese medicine [3]. Also, people in subarctic regions have been
known to use this fungus as a way to mark their trails at night [2]. The fungi
will luminesce at night allowing people to mark where they have previously been
on a trail [2].
Toxicity:
A. mellea is an edible fungus when
cooked correctly [2,14]. When preparing these mushrooms to eat they must be
fully cooked and the water they were cooked in has to be drained [2]. Only the
firm caps should be eaten while the stalks are thrown out [2]. This species has
been recorded to substitute shiitake mushrooms, but has also caused some
gastrointestinal discomfort [2,14].
Damage to crops:
Unfortunately A. mellea is the leading killer of gardens,
plantations, and orchards [4]. When an infection occurs in an orchard the
fungus will prevent any new development in those areas [4,8].
Bioluminescence:
The genus Armillaria have the ability to luminesce
during the night [7]. The species mellea is commonly called “foxfire” due to
the glow produced at night [2,6]. The fungi will glow the brightest when they
are still growing in decaying wood [7]. The best time to observe the glow is at
night during the fall months. They can be exposed by kicked up the leaf litter on
the ground [7]. The bioluminescence of fungi has been researched, but
mycologists do not yet have a clear understanding of why they do so [2].
Photo of A. mellea during the night time, commonly referred to as "foxfire".
Picture by Smita Bernadet Kujur
http://strangemushrooms.blogspot.com/2010/10/glowing-armillaria-mellea.html
References:
1.
Agriculture and Natural Resources. 2015. Armillaria root rot- Armillaria mellea. University of
California.
http://www.ipm.ucdavis.edu/PMG/r302100811.html
2.
Arora, D. 1986. Mushrooms Demystified. Berkeley,
CA: Published by Ten Speed Press. 196-197 p.
3.
Barron, G. 1999. Mushrooms of Northeast North
America Midwest to New England. Auburn, WA. Published by Lone Pine Publishing.
246 p.
4.
Basnayake V. 2001. Armillaria mellea. North Caroline State University.
http://www.cals.ncsu.edu/course/pp728/Armillaria/Armillaria.htm
5.
Coetzee M, Wingfield BD, Harrington TC, Steimel
J, Coutinho TA, Wingfield MJ. 2001. The root rot fungus Armillaria mellea introduced into South Africa by early Dutch
settlers. Molecular Ecology.
http://www.public.iastate.edu/~tcharrin/Dutch%20Armillaria.pdf
6.
Emberger, G. 2008. Armillaria mellea. Messiah College.
7.
Huffman DM, Tiffany LH, Knaphus G, Healy RA.
2008. Mushrooms and Other Fungi of the Midcontinental United States. Iowa City,
IA: Published by University of Iowa Press. 128-129 p.
8.
Kerik CD, Harold S, Beckman TG. Armillaria Root and Crown Rot.
Department of Plant Pathology. University of Georgia.
http://www.ent.uga.edu/peach/peachhbk/fungal/armillariacr.pdf
9.
Kirk PM, Cannon PF, Minter DW, Stalpers JA.
2008. Dictionary of the Fungi. Wallingford, UK. Published by Cab International.
49 p.
10.
Kuo, M. (2004, October). Armillaria
mellea. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/armillaria_mellea.html
11.
Muszynska B, Sulkowska-Ziaja K, Wolkowska M,
Ekiert H. Chemical, pharmacological, and biological characterization of the
culinary-medicinal honey mushroom, Armillaria mellea (Vahl) P. Kumm.
(Agaricomycetideae). Europe PubMed Central.
http://www.ncbi.nlm.nih.gov/pubmed/22135893
12.
Oak Woodland Management. 2015. Armillaria
mellea: Native Soil Fungus Causing Root Rot.
University of California.
13.
Raabe R. 1962.
Host list of the root rot fungus,Armillaria mellea. Hilgardia
33(2):25-88.
14.
Webster J, Weber RWS. 2007. Introduction to
Fungi. Cambridge, NY: Published by Cambridge University Press. 1 p.
15.
Williams RE, Shaw CG, Wargo PM, Sites WH. 1989. Armillaria Root Disease. U.S Department
of Agriculture Forest Service.
http://www.na.fs.fed.us/spfo/pubs/fidls/armillaria/armillaria.htm
