Armillaria Root and Crown Rot (Oak Root Fungus)

Affected trees usually show a general decline in vigor over many years. Sometimes trees that look healthy will suddenly wilt and die in a matter of weeks. The aboveground symptoms are similar to other root problems, including too much water, Phytopthora root rot, gophers, etc. The key symptom to look for is trees that are declining in infection centers (patches of diseased trees with the earliest infected trees typically in the middle) that potentially can expand annually.

To confirm the presence of Armillaria, dig around the crown of the tree and scrape off the bark on small sections of the crown and main roots. It should be easy to see the dense, felty, creamy-white-colored plaques of mycelium between the bark and the wood. Infected wood will have a strong mushroom smell and feel slightly spongy. Sometimes it may be difficult to locate the mycelial plaques in the earlier stages if the infection has not yet moved up to the crown. Dark brown to black rhizomorphs (structures that resemble shoestrings) sometimes can be seen on the root surface. Sometimes, large, densely packed, honey-colored mushrooms form at the base of infected trees in late fall to early winter after rains. Be sure to check for signs of Armillaria root and crown rot before removing dead or declining trees with a backhoe. There are other wood-rotting fungi that also have dense white mycelium, so submitting samples to a lab may be necessary for positive diagnosis.

Armillaria root and crown rot most often occurs when the soil is cool and moist. The pathogen survives in the soil on the roots of infected hosts from one year to the next. In the absence of a living host, it can survive for several years on old infected roots or other woody material buried in the soil; inoculum in North Coast pear orchards has been estimated to be over 100 years old. In pear orchards, Armillaria rhizomorphs infect healthy trees by penetrating into roots. The mycelium of the fungus grows up and down the root under the bark from the point of infection; it eventually reaches the crown where it girdles and kills the tree. Tree-to-tree infection is less important than previously thought, but is more likely in severely infected and high-density orchards.

The fungus can be spread on infected woody material carried by farm machinery or flood water. However, if an orchard is planted on a site where oak, black walnut, or willow trees once grew, chances are very high that the inoculum already exists in the soil on old decaying roots. When these roots or rhizomorphs come in contact with the roots of a healthy, susceptible host, they infect the healthy roots. Most pear orchards along waterways probably have some level of Armillaria present in the soil. Infected pear trees may tolerate the fungus for years without noticeable symptoms. Most pears are listed as immune or highly resistant to Armillaria, including Pyrus betulaefolia, P. calleryana, French pear, Bartlett-rooted cuttings, and Old Home x Farmingdale. Old Home-rooted cuttings and Winter Nelis are listed as moderately resistant. Native hosts, such as oak, willow, cottonwood, and black walnut are tolerant of Armillaria under normal soil moisture conditions.

Recent infections of Armillaria root rot in pear orchards may have been caused by a switch in irrigation practices from flood or furrow irrigation to permanent undertree sprinklers where water is applied more frequently and the soil layer close to the upper roots and tree crown is kept fairly moist through the entire growing season. Most infected orchards have been found in the North Coast with fewer in the Sacramento Delta. Most affected orchards are on sandy loam to loam soils along streams or rivers originally lined with native oaks.

There is no truly effective control for Armillaria root rot. The effectiveness of soil fumigation depends greatly on soil type and moisture.

Exposing an infected crown and upper root area of a tree infected with Armillaria mellea has been found to stop the development of the fungus into the crown area and allow the tree to regrow. In spring, remove soil from around the base of the tree to a depth of 9 to 12 inches. Leave the trunk exposed for the remainder of the growing season. Do not allow soil and debris to build up around the exposed crown and upper roots or the pathogen will resume growth. During the spring, summer, and fall, keep the upper roots and crown area as dry as possible.

It may be helpful to fumigate the soil before replanting infected areas. Before fumigation, remove all infected trees, stumps, and as many roots greater than 1 inch in diameter as possible. Healthy-appearing trees adjacent to those showing symptoms are often infected also. Removal of these adjacent trees and inclusion of that ground in the soil fumigation may be advisable. Infected trees, stumps, and roots should be burned at the site or disposed of in areas where flood waters cannot wash them to agricultural lands. Complete eradication is rarely achieved, and re-treatment may be necessary in localized areas. If the soil is wet or if it has extensive clay layers to the depths reached by the roots, fumigant treatment with metam sodium is preferable. The greatest opportunity for eradication is on shallow soils less than 5 feet in depth.