Agriculture: Grape Pest Management Guidelines

Armillaria Root Rot (Oak Root Fungus)

  • Armillaria mellea
  • Symptoms and Signs

    Vines infected with Armillaria root rot become nonproductive and often die within 2 to 4 years after the first appearance of symptoms, which typically start as a slight stunting of shoots that progresses each year. Adjacent vines often become infected as well. Eventually, a group of dead and dying vines form a "disease center", the location of which reflects the presence of the fungal pathogen, Armillaria mellea, in residual roots in the soil after infected trees (native species or tree crops) are removed. The diagnostic feature of Armillaria root rot is the white mycelial mat that forms under the bark at or below the soil line. The trunk or root wood below the mat is often visibly rotted, with a soft, spongy consistency and light brown color, as compared to white, dense wood on the portion of the trunk that has no sign of the pathogen. Dark, root-like structures (rhizomorphs) may also be seen on the surface of infected grapevine roots and the below-ground section of the trunk. In red cultivars all leaves on diseased vines may turn red late in the season.

    Comments on the Disease

    The pathogen can infect hundreds of woody plants, including the tree crops walnut, peach, and almond, which are much more susceptible than grape. Host plants include broad-leaved trees in oak woodlands and stands of conifers; the pathogen is indigenous in many regions. After infected tree crops, grapevines, or native trees are cleared, the vegetative stage of the fungus (mycelium) survives on infected, decaying roots below ground, potentially for many years. Healthy grapevine roots become infected when they come in contact with such inoculum. The fungus is favored by soil that is continually damp during the growing season. Although the pathogen produces mushrooms, the spores released from these fruiting structures are not considered significant in disease spread either to healthy vineyards or between vines within infected vineyards. Furthermore, mushrooms are not common and are very short-lived; they are not required to confirm the presence of the pathogen.

    Management

    The best management strategy is to remove residual roots before vineyard establishment. In diseased vineyards or new sites supporting other woody plants, use deep ripping to bring thick, woody roots and root crowns to the surface and then remove. This sanitation measure is much more efficient than fumigation alone. There are no known Armillaria-tolerant grape rootstocks. If possible, avoid planting sites infested with Armillaria.

    Saving Infected Vines

    Once symptoms of Armillaria root rot appear in vines, it may be possible to slow or stop spread of the pathogen in the early stages of infection by exposing the crown and upper roots and allowing them to dry out—a practice known as "root collar excavation". In spring, remove soil to a depth of 9 to 12 inches, to the point at which the main roots branch from the base of the trunk (the root collar). Keep the root collar permanently exposed to air. This practice is most effective for vines with moderately-stunted shoots and adjacent healthy-looking vines. It is not effective for vines that are severely stunted.

    Fumigation

    Be aware that fumigants do not kill the pathogen in residual roots buried deep (> 3 feet) in the soil. The efficacy of soil fumigation can be improved, however, by proper sanitation (i.e., removing residual roots) and soil preparation. See your Cooperative Extension farm advisor for additional advice on soil preparation. Contact the county agricultural commissioner's office for current state and local regulations specific to fumigant use in agriculture lands. Set-backs may be required from property lines and on-site structures. Follow directions and regulations carefully. Fumigation is expensive and needs to be done correctly for the chosen fumigant to receive maximum benefit.

    Common name Amount per acre** REI‡
    (Example trade name) (hours)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
    A. METHYL BROMIDE*
    Label rates See label
    COMMENTS: Preplant treatment. May only be used under a Critical Use Exemption. Fumigants such as methyl bromide are a source of volatile organic compounds (VOCs) but are not reactive with other air contaminants that form ozone: methyl bromide depletes ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.
    B. METAM SODIUM*
    (Vapam, etc.) Label rates See label
    COMMENTS: Apply in winter when soil moisture is high. Fumigants such as metam sodium are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.
    C. CHLOROPICRIN*
    Label rates See label
    ** Apply with enough water to provide complete coverage. Maximum rate allowed in CA may be less than what is allowed in other states.
    * Permit required from county agricultural commissioner for purchase or use.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Soil fumigants have an entry restricted period determined by tarping requirements.
    Text Updated: 12/14
    Treatment Table Updated: 07/15