Tree Diseases: Honey Fungus (Armillaria)
Honey fungus, or Armillaria, comprises a genus of parasitic fungi that infect a variety of plants. It is one of the most destructive fungal diseases in the world. The pathogenicity of Armillaria species ranges from mild to severe. Of the species that cause honey fungus, Armillaria mellea and Armillaria ostoyae are the most aggressive. Armillaria gallica frequently infects plants that are suffering from environmental stress, or other infections. Honey fungus infects and kills the roots of vulnerable plants. Affected plants will eventually succumb to infection. The disease is notable for the distinct clumps of honey-colored toadstools that appear briefly on infected stumps from late summer to early winter.
Distribution & Habitat
Honey fungus occurs globally, wherever susceptible plants are present.
Honey fungus infects a bevy of plants. Apple, ash, beech, birch, black locust, buckeye, butterfly bush, camellia, cedar, cotoneaster, cupressus, currant, dogwood, eucalyptus, forsythia, golden chain, hawthorn, holly, hydrangea, leyland cypress, lilac, magnolia, maple, mountain-ash, oak, paenoia, paperplant, pear, photinia, privet, prunus, rhododendron, rose, viburnum, weigela, willow, and witch-hazel are among the plants most prone to infection.
Honey fungus causes a lower stem and root rot on infected plants. The fungi infect plants through reddish-brown to black root-like structures called rhizomorphs that spread from diseased roots. The rhizomorphs penetrate through the soil, and make direct contact with plant roots, initiating new infections. Rhizomorphs can grow up to 3 ¼ feet per year. They are generally located in the top six inches of the soil, though they may reach depths of at least 18 inches. Infections also occur through root grafting, and root contact between diseased and healthy plants. The Armillaria fungi are saprophytic, meaning they may infect live or dead plant material.
Symptoms of Infection
Honey fungus infections result in the decaying of a plant’s root system. The most characteristic symptom of infection is the white fungal growth that forms between the bark and wood, typically at soil level. These sheets of fungal material are slightly luminescent, and emit a strong, sweet odor. Infected plants gradually experience leaf discoloration and wilting. Infected leaves will assume a malformed appearance. This is followed by the dieback of the upper portion of the plant. Dieback generally occurs over several years. Dieback is hastened by extended periods of drought.
Cracks may form on the bark of infected plants, particularly on conifers. Sap may seep from the cracks, and trickle down the main stem. A profusion of flowers or fruit may precede plant mortality. The rhizomorphs produced by most species of Armillaria dwell too deep in the soil to be easily detected. However, Armillaria gallica establishes large rhizomorphs that are easily visible. Some species of Armillaria, such as Armillaria mellea, are bioluminescent. This causes a phenomenon referred to as foxfire, a bioluminescence that can be observed on decaying wood at night.
Yellow-brown fruiting bodies sometimes develop on infected plants from late summer to fall. These fruiting bodies produce spores, which can infect nearby plants that have incurred bark wounds. Infection through sporulation is rare. Clumps of honey-colored toadstools will often appear briefly on infected stumps from late summer to early winter. The toadstools produced by Armillaria ostoyae develop pinkish caps. These toadstools linger for a few days before collapsing. A lack of toadstools in an infected area indicates that the fungus is inactive in the soil.
Additional Notes on Honey Fungus
One patch of honey fungus caused by Armillaria ostoyae is thought to be the largest organism in the world. The mycelia of this patch extends across over 3.4 square miles in Malheur National Forest, a forest located in Oregon. This patch of honey fungus is estimated to be over 2,400 years old.
- No chemical control is currently available for the eradication of honey fungus. If honey fungus is identified, diseased trees should be culled. Dispose of any infected root and stump material. Avoid using the material as compost, as rhizomorphs may continue to form.
- A physical barrier can be erected in the soil to prevent the spread of the rhizomorphs. Use a deep vertical strip of pond lining, or a heavy-duty plastic sheet. Bury the material in the soil, allowing it to protrude about 1 inch above the soil surface.
- Deep cultivation may be performed to disrupt the rhizomorphs, and limit their spread.
- When planting, select trees and shrubs that exhibit an increased resistance to honey fungus. Some of the most resistant plants include bamboo, black walnut, blueberry, boxelder maple, common box, common hornbeam, flannel bush, gold flower, heath, Japenese quince, jasmine, tamarisk, and tupelo.
- Avoid planting at infected sites for at least twelve months.
- Pruning mildly infected roots may allow for the salvaging of important specimens.
- Soil sterilization with commercial products can control the disease.
- Systemic applications of fungicide into the woody tissue of diseased trees and shrubs is usually ineffective.
- Prevent plants from sustaining mechanical injuries, or damage from pests.
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Photo courtesy of Peter O’ Connor CC-by-2.0