Backyard Grapes Identify & treat grapevine problems
Fungal high severity

Armillaria root rot

Armillaria mellea

A soilborne fungus that rots roots and crown, killing vines in a slowly widening patch.

Diagnostic Armillaria mycelial fans beneath the bark of a grapevine root crown Photo: Melodie Putnam, 2010 Pacific Northwest Plant Disease Management Handbook (Oregon State University)
Classic Armillaria mycelial fan just under the bark of affected grapevine tissue Photo: OSU Plant Clinic collection, 2012 Pacific Northwest Plant Disease Management Handbook (Oregon State University)
Underground symptoms of Armillaria root rot in grapevine: rotting wood and whitish mycelium at the collar (a) and a mycelial fan on detached collar bark (b) Photo: Federico Calamita, Hafiz Ali Imran, Loris Vescovo, Mohamed Lamine Mekhalfi and Nicola La Porta (Remote Sensing, 2021) MDPI Remote Sensing CC BY 4.0

Description

Armillaria root rot (Armillaria mellea, the “honey fungus”) lives on woody roots in the soil and can persist for years on the buried roots of trees long after they’re felled. That’s why it so often shows up when a vineyard or garden is planted on recently cleared woodland or an old orchard — the fungus is already there, waiting in the old roots.

The tell-tale signs are underground or at the base: peel back bark at the crown and you may find a white, mushroom-smelling fan of fungus, with black shoestring rhizomorphs on the roots and honey-coloured toadstools in autumn. Because it spreads root-to-root underground, affected vines appear in a growing patch. Prevention — clean, well-drained ground and removing old woody roots before planting — is far more effective than anything you can do once vines are infected.

What to look for

  • Overall decline — stunted growth, yellowing, wilting, sometimes sudden summer collapse.
  • White, fan-shaped fungal mats under the bark of the crown and larger roots, smelling of mushroom.
  • Dark, shoestring-like strands (rhizomorphs) on the root surface.
  • Clusters of honey-coloured mushrooms at the vine base in autumn.
  • Dead vines appear in an expanding patch as it spreads to neighbours.

Treatment options

Cultural & prevention

Start here — habits that stop problems before they spread.

  • Avoid planting where woodland or an old orchard was recently cleared; remove large roots and stumps first.
  • Improve drainage; the fungus thrives in wet soil.
  • Dig out and destroy infected root systems, including of dead vines, to slow spread.
  • Exposing the crown (root-collar excavation) can slow decline on a valued vine.

Organic / low-toxicity

Gentler sprays and biologicals for active outbreaks.

  • There is no cure — site selection, drainage, and removing infected wood are the real controls.

Conventional fungicide / pesticide

For severe cases. Always follow the product label.

  • Pre-plant soil fumigation is a commercial option; it is rarely practical for home growers.

Sources

This entry is based on the references below. Read them for full detail and for spray timings and rates valid where you live.

  1. Armillaria Root Rot (Oak Root Fungus) / Grape / Agriculture: Pest Management Guidelines — UC Statewide IPM Program (UC ANR)
  2. Grape (Vitis spp.) - Armillaria Root Rot — Pacific Northwest Pest Management Handbooks (Oregon State, Washington State, Univ. of Idaho)