Armillaria root rot (Oak root fungus) — Armillaria mellea
Armillaria root rot, also known as oak root fungus disease or shoestring disease, affects mostly woody plants but also affects certain herbaceous perennials, such as begonia, carnation, daffodil, dahlia, geranium, and peony. Armillaria infects and kills cambial tissue, causing major roots and the trunk near the ground to die.
The first aboveground symptoms are often undersized, discolored, and prematurely dropping leaves. Branches die, often beginning near the tops of plants; on herbaceous hosts, stems become discolored and cankered. Eventually the entire plant can be killed. Armillaria forms characteristic white mycelial plaques that have a mushroomlike odor when fresh. Mycelia grow between the bark and wood on woody hosts and can grow through soft plant tissue and appear on the surface, especially with herbaceous hosts. Clusters of mushrooms may form at the base of infected woody plants. Black or dark reddish brown rootlike structures (rhizomorphs) are frequently attached to the surface of roots or the root crown.
Dematophora root rot also causes white growths that may be confused with Armillaria, but Dematophora tends to occur in smaller patches and grows throughout the wood rather than just under the bark.
Armillaria thrives under moist conditions, for example when irrigated turf is planted around the roots of California native oaks. Plants become infected through root contact with infected plants or rhizomorphs attached to infected roots. Armillaria root rot can develop slowly, and symptoms may not appear until the fungus is well established. The fungus can survive for many years in dead or living tree roots.
Preventing infection of new plants and planting resistant species are the only effective controls for Armillaria. Prepare the site well. Remove old roots and debris from the soil before planting. Use pathogen-free plants and air-dry soil well before planting. Provide plants with appropriate cultural care, especially proper irrigation, and adequate drainage.
For more information, see the Pest Notes: Armillaria Root Rot.
ground symptoms of oak root fungus
Mushrooms at base of infected tree
Mycelial fans between bark and wood