See:
Cherry (Prunus spp.) - Armillaria Crown and Root Rot
Cause Armillaria mellea, a fungus, infects tree roots, killing the cambium and decaying the underlying xylem. Often found on newly cleared land but also commercial orchards, this root pathogen is native to the Pacific Northwest where it occurs on the roots of many woody perennials including native trees and agricultural hosts.
This fungus may form mushrooms at the base of infected trees in fall and winter. Mushrooms produce wind-blown spores, but these spores are not a significant means of infecting healthy trees. The fungus spreads vegetatively, below ground, which leads to the formation of groups of dead and dying plants. The fungus can survive on woody host roots long after the host dies. Its vegetative fungal tissue (mycelium) decomposes root wood for nutrients as it grows. When infected plants are removed, infected roots that remain below ground serve as a source of inoculum for trees planted in the same location.
Infection occurs when roots come in direct contact with partially decayed tree roots and are colonized by mycelium. Infection can also occur when tree roots contact rhizomorphs (black, shoestring-like fungal structures) that grow out from partially decayed roots and through the soil. Once tree roots are infected, whether they are living or dead, they serve as a source of inoculum for neighboring trees.
Symptoms The first indication is usually poor growth of the shoots together with premature dropping of the leaves. One part, side or section of the tree may be affected at first. This corresponds to the side that is supported by the roots first attacked by the fungus. Trees may live for a number of years before finally dying. White mycelial plaques or fans can be seen on the main roots or root crown when the outer bark is removed. Rhizomorphs may also be found clinging to the bark in this area. These appear as dark-brown-to-black, branching, root-like structures.
Cultural control
- When clearing a new site of native trees and shrubs or when clearing a site with infected plants there are several precautions to take.
- Girdle large trees before removal to hasten decay of roots.
- After removing aboveground vegetation, clear soil of stumps and large roots. Deep-rip the soil in more than one direction to bring large roots to the soil surface. If possible, remove all roots greater than 1 inch in diameter from the soil.
- Burn all woody debris.
- Leave this ground fallow at least 1 year.
- If using drip irrigation, move drip-line emitters away from the trunk and in between trees after first year of planting.
- Permanently removing soil in a 3-ft radius around the root crown and main trunk root area has been effective in citrus and other tree fruits grown in California and Australia and may be of benefit for managing infected trees in the Pacific Northwest. Also excavate root collars of healthy-looking trees surrounding disease centers. Be sure to keep root collars free of soil, especially in orchards with high gopher populations. Another twist of this idea was used to protect peach trees at planting in the Southeast United States by planting on raised hills or "Smart pots." The soil is removed once the trees have been established, which exposes the root crown.
- Remove and destroy severely infected trees, being careful to remove as much root material as possible from soil. If practical, do not replant where infected trees have been removed.
Reference Miller, P.W. 1945. Diseases of filbert in the Pacific Northwest and their control. Oregon State College Station Bulletin 428.