Armillaria Root Rot (Oak Root Fungus)

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.

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.

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.