Information on Grape -- Armillaria Root Rot, An Online Guide to Plant Disease Control, Oregon State University

Pacific Northwest 1998 An Online Guide to Plant Disease Control

Grape (Vitis sp.) -- Armillaria Root Rot

Cause: Armillaria mellea, a fungus, infects vine roots, killing the cambium and decaying the underlying xylem. Often found on newly cleared land, this root pathogen is native to the PNW where it occurs on the roots of many forest tree species including Douglas-fir, madrone, oak, willow, and yellow pine. It also attacks black and red raspberries and trailing berries. Other agronomic hosts include currants, gooseberries, nut trees, roses, strawberries, stone fruits and many rosaceous plants. The host range includes over 500 species of woody plants, making its common name of "oak root fungus" slightly misleading.

This fungus may form mushrooms at the base of infected vines in fall and winter. Mushrooms produce wind-blown spores, but these spores are not a significant means of infecting healthy vines. The fungus spreads vegetatively, below ground, which leads to the formation of groups of dead and dying plants called "disease centers". The fungus can survive on woody host roots long after the host dies. Its vegetative fungal tissue (mycelium) decomposes root wood for nutrients as it grows. When infected plants are removed, infected roots that remain below ground serve as a source of inoculum for vines planted in the same location.

Infection occurs when grape roots come in direct contact with partially decayed tree roots and are colonized by mycelium. Infection can also occur when grape roots contact rhizomorphs (black, shoestring-like fungal structures) that grow out from partially decayed roots and through the soil. Once vine roots are infected, whether they are living or dead, they serve as a source of inoculum for neighboring vines. The infection process takes months to happen. Spread between neighboring vines may take more than 10 years to occur. The disease negatively affects vine mineral nutrition status and fruit quality.

Symptoms: Mildly symptomatic grapevines are distinguishable from healthy grapevines only by having shorter canes. Severely symptomatic grapevines not only have shorter canes, but their leaves are dwarfed and may be chlorotic. Diagnostic mycelial fans can be observed beneath the bark of the root crown of infected plants. Dig down about a foot below the soil line and use a pocketknife to remove thin layers of bark from the root collar. Mycelial fans are thick, white layers of fungus that adhere to the root bark and/or the wood beneath the bark. Armillaria also makes black, shoestring-like structures called rhizomorphs, which are occasionally found within the bark and/or extending into surrounding soil. Rhizomorphs may look like roots on the outside but have an entirely different structure when cut open in cross-section.

After scraping away the dead bark, a few white mycelial plaques can be seen on the lower portion of this vine.

Cultural control:

When clearing a new site of forest trees and shrubs or when clearing a site with infected plants (infection centers) there are several precautions to take.
1. Girdle large trees before removal to hasten decay of roots.
2. After removing aboveground vegetation, clear soil of stumps and large roots. Deep-rip the soil in more than one direction to bring large roots to the soil surface. If possible, remove all roots greater than 1 inch in diameter from the soil.
3. Burn all woody debris.
4. Leave this ground fallow at least 1 year.
Trenches lined with plastic sheeting may help if inoculum is from an adjacent stand.
If using drip irrigation, move drip-line emitters away from the trunk and in between trees after first year of planting.
Permanently removing soil in a 3-ft radius around the crown and main trunk root area has been effective in California and may be of benefit for managing infected vines in the Pacific Northwest. Be sure to keep root collars free of soil, especially in orchards with high gopher populations.
Remove and destroy severely infected vines, being careful to remove as much root material as possible from soil. If practical, do not replant where infected vines have been removed.

Chemical control: Soil fumigation is most effective if preceded by thoroughly clearing the soil of woody debris.

Methyl bromide fumigation has been found to give the best, albeit limited, control. Methyl bromide is more effective if soil is extremely dry. It works better on fine soils with few rocks. Restricted-use pesticide.
1. Contrary to soil fumigation for nematodes, soil should be as warm and dry as possible. Fumigation in late summer before any rain is best.
2. Apply fumigant as deeply as possible.
3. Follow directions for application, sealing, aerating, and waiting period before planting.
4. Spot fumigation may be necessary a few years after planting.
Enzone (Sodium tetrathiocarbanate) is a liquid that breaks down into carbon disulfide gas. It is effective when applied at 60 gal/100 gal water/A before planting. Make application 1 to 4 weeks before planting when soil moisture is at or near field capacity. Oregon and Washington only. 4-day reentry.

References:
Baumgarter, K. 2004. Root collar excavation for postinfection control of Armillaria root disease of grapevine. Plant Disease 88:1235-1240.

Baumgarter, K. and D. M. Rizzo, 2002. Spread of Armillaria Root Disease in a California vineyard. American Journal of Enology and Viticulture 53:197-203.

Content edited by: Kendra Baumgartner on January 1, 2009

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