Armillaria Root Rot

Symptoms of Armillaria root rot on blackberry and raspberry vary depending on how far the disease has progressed. Leaf wilting and cane dieback are common early symptoms early symptoms. In severe cases the plants will die. Infected main roots and crowns often have whitish to cream-colored mycelial mats just under the bark. The mats are fan shaped and about as thick as a piece of paper; they have a characteristic mushroom odor. This is virtually the only fungus that produces these mycelial fans on live or recently alive tissue. If mycelial mats are present under the bark of the roots or in the collar region of live plants, it is a good indication that Armillaria is the cause of the decline.

Groups of plants are usually infected and show symptoms, correlating with the location of the inoculum in the soil. If the disease is not controlled, it eventually spreads through the soil by the growth of rhizomorphs. Rhizomorphs are branched structures about 0.12 to 0.16 inch (3–4 mm) in diameter that look like roots but are dark colored. A single rhizomorph is a white mycelial strand encased in a dark red to black rind. It grows through the soil from the roots of infected plants to the healthy roots of neighboring plants. Because rhizomorphs resemble shoestrings, some of the common names for this pathogen are bootlace or shoestring fungus.

Armillaria sometimes produces mushrooms around the base of infected plants in fall and winter. The mushrooms are relatively large and have a yellowish brown cap as well as a ring around the stem just below the cap.

The characteristics and severity of an Armillaria root rot infection are influenced by the type and age of the host plant as well as the species of Armillaria. Environmental conditions also affect disease severity.

The source of an Armillaria infection is from infected roots in the soil. The disease can occur on land that has been cleared of infected oak trees. The fungus can survive on dead roots and grow through the soil as rhizomorphs. Infection of plants occurs through the contact of healthy roots with mycelia or rhizomorphs. The importance of rhizomorphs in the spread of the disease varies with species, site, and environmental conditions. The fungus can survive years in roots left in the soil after an infected plant has been removed and this can be considered to be primary inoculum for further infection.

An assessment of plant health should be made in conjunction with the diagnosis of Armillaria root rot because the pathogen often infects plants that have been weakened by stress or other agents.

The potential for infection by Armillaria can be reduced by the removal and destruction of infected roots or stumps from the soil. It is very difficult to remove all infected roots from the soil, but the progression of the disease can be slowed by this method. Remove all roots larger than about one half inch in diameter.

While not completely reliable, the installation of barriers to prevent the spread of the rhizomorphs may be used on an area that is adjacent to an infested one. Barriers may consist of vertical buried plastic sheets, ditches, or strips of ground that are regularly cultivated.

Proper site preparation, including the removal of infected roots in the soil and the installation of barriers in the soil to contain the pathogen, are acceptable management tools in an organically certified crop. Formulations of the beneficial fungus Trichoderma have been used with some success for delaying pathogen growth in infected roots. Also, the practice of removing soil from around the plant to a depth of about 6 to 8 inches or to a depth that exposes the upper root system has been shown to stop the pathogens progress and allow plant recovery in pears, peaches, plums, and grapes.

Soil fumigants have been used in conjunction with removal of infected material to reduce inoculum; however, most soil fumigants are highly toxic and should only be applied by professional pesticide applicators.