Agriculture: Floriculture and Ornamental Nurseries Pest Management Guidelines

Armillaria Root Rot (Oak Root Fungus)

  • Armillaria mellea and other species
  • Symptoms and Signs

    Infected field-grown plants can be stunted, show poor growth, have smaller than normal leaves, and can have other symptoms associated with diseased roots and crowns. Eventually, foliage will wilt, turn brown, and die. The disease usually results in the complete collapse and death of the plant. Diseased crowns and main roots will have white to cream-colored fan-shaped mycelial mats, growing underneath the outer tissues. Dark brown to black stringy mycelial structures (rhizomorphs) are rarely seen on the root surface.

    Comments on the Disease

    Armillaria mellea is a soilborne pathogen that is only found in field soil. The fungus survives on dead roots left in the ground. Woody crops, planted in areas that were formerly oak woodland, can become infected many years after infested oaks were removed. The fungus can spread from one plant to another through the contact of diseased roots with healthy roots. Wet soil conditions resulting from heavy rainfall or excessive irrigations can exacerbate the disease. Under suitable conditions, the mushroom stage of the fungus can emerge from buried diseased roots. The mushroom is large (cap diameter up to 6 inches wide), has a cap with variable coloration (yellow, tan, honey-colored, brown), and bears typical radiating gills with white spores.

    Management

    For field grown ornamental crops the only treatment is fumigation. Before chemical treatment, remove all infected plants and as many large roots as possible. Complete eradication is rarely achieved, and retreatment 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 may not be successful. Other management options include rotating crops and planting ornamentals that are not hosts to this pathogen.

    Common name Amount to use REI‡ PHI‡
    (Example trade name) (hours) (days)
    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.
    SOIL FUMIGATION
    A. CHLOROPICRIN*§ Label rates See label
      COMMENTS: Inject into soil and cover immediately with plastic tarps. Fumigants are a source of volatile organic compounds (VOCs) but are not reactive with other air contaminants that form ozone.
    B. Sequential application of:
    (Note: Fumigants such as 1,3-dichloropropene and metam products are a source of volatile organic compounds (VOCs) but minimally reactive with other air contaminants that form ozone.
     
    CHLOROPICRIN*§/1,3 DICHLOROPROPENE*§
    (Pic-Clor60) 300–332 lb (shank) See label
    (Pic-Clor60 EC) 200–300 lb See label
    COMMENTS: Very effective for control of soilborne fungal pathogens and insects. Drip irrigation requires an emulsifier. For shank fumigation, using higher rates or plastic mulch, especially virtually impermeable film (VIF), improves weed control. For drip fumigation the use of VIF will improve both nematode and weed control. According to state permit conditions, the maximum application rate of 1,3-dichloropropene is 332 pounds active ingredient per acre. Pic-Clor60: One gallon of weighs 12.1 lb; Pic-Clor60 EC: One gallon of weighs 11.8 lb.
    Following 5-7 days after fumigation:
    METAM SODIUM*§
    (Vapam HL, Sectagon 42) 37.5–75 gal See label
    COMMENTS: Water-soluble liquid that decomposes to a gaseous fumigant (methyl isothiocyanate). Efficacy affected by soil texture, moisture, temperature, and percent organic matter. One gallon of product contains 4.26 lb of metam sodium.
    . . . or . . .
    METAM POTASSIUM*§
    (K-Pam HL, Sectagon–K54) 30–45 gal See label
    COMMENTS: Water-soluble liquid that decomposes to a gaseous fumigant (methyl isothiocyanate). Efficacy affected by soil texture, moisture, temperature, and percent organic matter. One gallon of product contains 5.8 lb of metam potassium.
    . . . or . . .
    DAZOMET*§
    (Basamid) 200 lb See label
    COMMENTS: Powder incorporated into the soil, followed by irrigation or tarping. It decomposes to a gaseous fumigant (methyl isothiocyanate).
    * Permit required from county agricultural commissioner for purchase or use.
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action group number.
    § Do not exceed the maximum rates allowed under the California Code of Regulations Restricted Materials Use Requirements, which may be lower than maximum label rates.
    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.
    Text Updated: 11/20
    Treatment Table Updated: 11/20