Plants Resistant or Susceptible to Armillaria mellea,
The Oak Root Fungus

Robert D. Raabe

Department of Environmental Science and Management
University of California, Berkeley

Introduction to Armillaria mellea
Information About This List

Part A: Plants which have been tested
Resistant Plants
Moderately Resistant Plants
Susceptible Plants
Fruit and Nut Resistance

Part B: Plants which have not been formally tested
Resistant Plants
Moderately Resistant Plants
Susceptible Plants

Armillaria mellea is a common disease producing fungus found in much of California. It commonly occurs naturally in roots of oaks but does not damage them unless they are weakened by other factors. When oaks are cut down, the fungus moves through the dead wood more rapidly than through living wood and can exist in old roots for many years. It also does this in roots of other infected trees. Infection takes place by roots of susceptible plants coming in contact with roots in which the fungus is active. Some plants are naturally susceptible to being invaded by the fungus. Many plants are resistant to the fungus and though the fungus may infect them, little damage occurs. Such plants, however, if they are weakened in any way may become susceptible and the fungus may kill them.

The plants listed here are divided into three groups. Those listed as resistant are rarely damaged by the fungus. Those listed as moderately resistant frequently become infected but rarely are killed by the fungus. Those listed as susceptible are severely infected and usually are killed by the fungus.

The fungus is variable in its ability to infect plants and to damage them. Thus in some areas where the fungus occurs, more plant species may be killed than in areas where other strains of the fungus occur.

Information about this list

The list is composed of two parts. In Part A, the plants were tested in two ways. Most were grown in an old orchard area known to be infested by the fungus at the University of California Deciduous Fruit Station in Santa Clara, California. Information about early procedures is not available. However, all plants tested since 1952 were planted in the infested soil. In addition, two six to eight inch pieces of woody branch material colonized by the fungus in the laboratory were put next to the root system at the time of planting. Isolates used in these pieces were selected for their ability to be highly virulent forms. Ten plants of each species were tested. Following planting, plants were monitored periodically and dead plants, particularly those with the oak root fungus were recorded. After ten years, all remaining plants were pulled and their root systems were rated as to the amount of infection found. In addition, some plants were tested in greenhouse experiments in which ten plants each were exposed up to one year to two strains of the fungus as listed above except this was done in unglazed ceramic pots. This project was supported by a grant from the Elvin J Slosson Fund and plants so tested are marked with a double asterisk**.

In the Fruits and Nuts section, many plants are included even though they may not be used in the production of crops. They are included here so that researchers may want to include them in their breeding experiments. If any of these plants have been known to be used as ornamental plants, they are included in that section.

Although when tested, pears have been found to be resistant, recently pears in Lake County are becoming infected. Research will determine the reasons for this. Also, the fungus in this area grows through the soil and can spread this way. This is the only known area where this occurs.

Plants listed with a double asterisk** were tested in pots in a research greenhouse. This research was funded by a grant from the Elvenia Slosson Fund.

Plants listed in Part B were not tested for resistance or susceptibility. Information presented is based on finding plants infected by the fungus, or finding plants that are not infected. Additional information was obtained from the California Plant Disease Host Index, published by the California Department of Food and Agriculture and the list of plant diseases published by the American Phytopathological Society, Fungi on Plants and Plant Products in the United States. Plants marked with an asterisk * are from information supplied by the staff of Quarryhill Botanical Garden, Glen Ellen.