How to Manage Pests
UC Pest Management Guidelines
Armillaria Root Rot (Oak Root Fungus)
Pathogen: Armillaria mellea
(Reviewed 12/14, updated 12/14)
In this Guideline:
Vines infected with Armillaria root rot become nonproductive and often die within 2 to 4 years after the first appearance of symptoms, which typically start as a slight stunting of shoots that progresses each year. Adjacent vines often become infected as well. Eventually, a group of dead and dying vines form a "disease center", the location of which reflects the presence of the fungal pathogen, Armillaria mellea, in residual roots in the soil after infected trees (native species or tree crops) are removed. The diagnostic feature of Armillaria root rot is the white mycelial mat that forms under the bark at or below the soil line. The trunk or root wood below the mat is often visibly rotted, with a soft, spongy consistency and light brown color, as compared to white, dense wood on the portion of the trunk that has no sign of the pathogen. Dark, root-like structures (rhizomorphs) may also be seen on the surface of infected grapevine roots and the below-ground section of the trunk. In red cultivars all leaves on diseased vines may turn red late in the season.
COMMENTS ON THE DISEASE
The pathogen can infect hundreds of woody plants, including the tree crops walnut, peach, and almond, which are much more susceptible than grape. Host plants include broad-leaved trees in oak woodlands and stands of conifers; the pathogen is indigenous in many regions. After infected tree crops, grapevines, or native trees are cleared, the vegetative stage of the fungus (mycelium) survives on infected, decaying roots below ground, potentially for many years. Healthy grapevine roots become infected when they come in contact with such inoculum. The fungus is favored by soil that is continually damp during the growing season. Although the pathogen produces mushrooms, the spores released from these fruiting structures are not considered significant in disease spread either to healthy vineyards or between vines within infected vineyards. Furthermore, mushrooms are not common and are very short-lived; they are not required to confirm the presence of the pathogen.
The best management strategy is to remove residual roots before vineyard establishment. In diseased vineyards or new sites supporting other woody plants, use deep ripping to bring thick, woody roots and root crowns to the surface and then remove. This sanitation measure is much more efficient than fumigation alone. There are no known Armillaria-tolerant grape rootstocks. If possible, avoid planting sites infested with Armillaria.
Saving infected vines
Once symptoms of Armillaria root rot appear in vines, it may be possible to slow or stop spread of the pathogen in the early stages of infection by exposing the crown and upper roots and allowing them to dry out—a practice known as "root collar excavation". In spring, remove soil to a depth of 9 to 12 inches, to the point at which the main roots branch from the base of the trunk (the root collar). Keep the root collar permanently exposed to air. This practice is most effective for vines with moderately-stunted shoots and adjacent healthy-looking vines. It is not effective for vines that are severely stunted.
Be aware that fumigants do not kill the pathogen in residual roots buried deep (> 3 feet) in the soil. The efficacy of soil fumigation can be improved, however, by proper sanitation (i.e., removing residual roots) and soil preparation. See your Cooperative Extension farm advisor for additional advice on soil preparation. Contact the county agricultural commissioner's office for current state and local regulations specific to fumigant use in agriculture lands. Set-backs may be required from property lines and on-site structures. Follow directions and regulations carefully. Fumigation is expensive and needs to be done correctly for the chosen fumigant to receive maximum benefit.
UC IPM Pest Management Guidelines: Grape
W. D. Gubler, Plant Pathology, UC Davis
Acknowledgment for contributions to Diseases:G. M. Leavitt, UC Cooperative Extension, Madera County