Jay W. Pscheidt, Oregon State University
Mildly symptomatic grapevines have shorter canes than healthy grapevines. Severe symptoms not only include shorter canes, but also include dwarfed and chlorotic leaves.
Diagnostic mycelial fans can be seen beneath the bark of the root crown of infected plants. Mycelial fans are thick, white layers of fungus that adhere to the root bark and/or the wood beneath the bark. These structures can often be observed in symptomatic vines by digging down about a foot below the soil line and using a pocketknife to remove thin layers of bark from the root collar.
The Armillaria fungus 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 are obviously made up of fungal mycelium when cut open in cross-section.
The disease negatively affects vine mineral-nutrition status and fruit quality.
Armillaria mellea is a fungus that infects grapevine roots, killing the cambium, and decaying the underlying xylem. Armillaria mellea is native to many areas where it occurs on the roots of 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 (using a microscopic, threadlike structure called mycelium), 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. Armillaria mellea 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.
Infections occur 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.
When clearing a new site of native forest trees and shrubs or infected plants (disease centers) there are several precautions to take. First, girdle large trees before removal to hasten decay of roots. After removing above-ground vegetation, clear the 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. Try to burn all woody debris and leave the ground fallow for at least 1 year.
Trenches lined with vertical plastic sheeting may help to prevent infection if inoculum is coming in from an adjacent stand of infected vegetation. If using drip irrigation, move drip-line emitters away from the trunk and place between vines after the first year of planting.
Once vines are infected, there is little that can be done to control Armillaria root rot. Remove and destroy severely infected vines, being careful to remove as much root material as possible from soil. Permanently removing soil in a 3-ft radius around the crown and main trunk root area has been effective in California. If practical, do not replant where infected vines have been removed. Be sure to keep root collars free of soil, especially in vineyards with high gopher populations.
Pre-plant soil fumigation is most effective if the soil has been thoroughly cleared of woody debris. Methyl bromide fumigation has been found to provide the most effective, albeit limited, control. Methyl bromide is more effective if soil is extremely dry. It works better on fine soils with few rocks. Contrary to soil fumigation for nematodes, soil should be as warm and dry as possible. Fumigation in late summer before any rain is best. Apply fumigant as deeply as possible; some spot fumigation may be necessary a few years after planting.
Methyl bromide is being phased out of use, so other fumigants might be used. Sodium tetrathiocarbanate is registered for control of Armillaria root rot. This alternative fumigant is a liquid that breaks down into carbon disulfide gas. Make applications 1 to 4 weeks before planting when soil moisture is at or near field capacity.
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.
Armillaria Root Rot, Michigan State University
Armillaria Root Rot (Oak Root Fungus), University of California-Davis
Armillaria Root Rot, Shoestring Root Rot, Honey Mushroom, Washington State University
Armillaria Root Rot, The Grape Doctor, Texas A & M University
Reviewed by Michelle M. Moyer, Washington State University and Damon Smith, Oklahoma State University