Armillaria Mellea: Native Soil Fungus Causing Root Rot
Armillaria mellea is a common soil-borne fungus that lives on a wide range of woody and herbaceous plants. Also known as oak root fungus, mushroom root rot, honey fungus and shoestring fungus, it is found in the soil of temperate regions throughout the world and is native to many areas including California.
Under natural conditions, Armillaria mellea usually inhabits the root system of most native oaks without ill effect. But, when trees become stressed, susceptibility increases. The fungus can then begin rotting the roots. The loss of roots may cause the tree to be unable to assimilate water and nutrients, or to fall over from lack of support.
This fungus is sensitive to drying. It can survive indefinitely in the soil on diseased or dead roots and will develop most rapidly under warm, moist conditions.
Infected plants usually show a decline in vigor, noted by yellowing foliage and reduced leaf size and number. This indirectly results in slow radial growth and callus formation over wounds. Some infected plants deteriorate slowly over a period of years while others may wilt and die abruptly.
The vegetative strands of the A. mellea fungus are called mycellium and are the most common sign of the fungus. They are comprised of fungal threads that grow closely together forming the characteristic flat, fan-shaped, whitish, leathery plaques. When exposed, these are easily visible without magnification. When fresh, they smell like mushrooms.
The mycellium are found between the wood and bark, near ground level and below. Infection may be localized to only one or so roots at first. In advanced stages it may develop into the wood. Infected wood is firm at first, eventually decaying to a soft, watery consistency with the characteristic mushroom smell.
To look for mycellium in a tree, systematically pry back small portions of bark at or below the soil level. Use the smallest practical sharp instrument to avoid undue damage. Discovery of mycellium usually requires excavation of the lower trunk, root crown and some major roots.
Identification of A. mellea by discovery of mycellium is fairly definitive. Laboratory culture sometimes is helpful for identification of the mycellium. Samples taken for the laboratory must be fresh and taken from the margin of diseased and healthy tissues. Keep the samples cool and moist and ship promptly.
Positive identification of A. mellea can be made by the discovery of rhizomorphs. These cord-like black structures may be found under loosened bark and on the surface of infected roots. The name shoestring fungus comes from the appearance of these structures. Rhizomorphs are not always found on diseased plants.
When growing, rhizomorphs contact susceptible pant tissue and directly penetrate the bark and infect the plant. Tissue need not be mechanically wounded for rhizomorphs to enter the host for infection to occur. These structures are the primary means by which the fungus spreads from tree to tree. Direct contact with an infected root is virtually the only way the fungus spreads from one plant to another.
Mushrooms are the fruiting stage of Armillaria mellea. They are short-lived, very obvious and not always present. When mushrooms occur, they are in clusters on the wood of the roots or lower trunk of diseased trees. They are honey-yellow to tan-brown in color, commonly 2-5 inches in diameter, and 4-5 inches tall. They have a distinct fresh mushroom-like odor, gills attached to the stem (adnate), and the stem (stipe) has a ring (annulus).
The time of mushrooms occurrence varies by location, between October and February in California. Positive identification of these mushrooms alone is indicative of Armillaria mellea.
Routine inspection will help find the A. mellea fungus before it becomes lethal. If it is found on a tree, or in the vicinity, the following treatments will reduce the rate of the disease loss by helping the host tree to become stronger:
- Excavate soil from root crown.
- Eliminate all forms of plant stress possible, including insects that suck or chew, grade changes, drought, and poor water regime.
- Avoid overwatering. Do not water established trees adapted to the dry summer regions of California.
- Topical application of fungicides to diseased tissue is of questionable value.
- Soil fumigation sterilizes infected soils (and can kill plants) in treated areas. Before planting susceptible or highly valued plants in infested areas this treatment may be useful.
- Introduce only highly disease-resistant plant species in affected areas.
Original Manuscript by Rob Gross (edited by John M. Harper and John W. LeBlanc)