How to Manage Pests
UC Pest Management Guidelines
Armillaria Root Rot (Oak Root Fungus)
Pathogen: Armillaria mellea
(Reviewed 6/06, updated 2/14)
In this Guideline:
Vines infected with Armillaria root rot become nonproductive and often die within 2to 4 years. Adjacent vines may develop weak, shorter shoots as they are infected by the pathogen. White mycelial mats can be found under the bark at the soil line. Dark, rootlike structures (rhizomorphs) may be seen growing in the soil near infested grapevine roots.
COMMENTS ON THE DISEASE
The fungus survives on diseased wood and roots below ground for many years. Healthy plant roots can become infected when they come in contact with inoculum, including rhizomorphs, from a preceding orchard crop or nearby oak trees. Flood waters sometimes spread infected roots in a vineyard. The fungus is favored by soil that is continually damp. Although the pathogen produces mushrooms, they are not considered significant in disease spread.
Because there are no known Armillaria-tolerant grape rootstocks, preplant chemical fumigation of the soil is the only control for oak root fungus. Treatment is best undertaken in September to November when the soil is still dry. Several preparatory steps are involved:
Before planting or replanting in affected soil, remove, pile, and burn all diseased vines, tree stumps, and roots greater than 1.5 inch in diameter.
In treating portions of an existing vineyard, healthy appearing vines adjacent to those showing symptoms are often also infected and should be removed. If removed, include the area in the fumigation treatment.
Before fumigation with methyl bromide or sodium tetrathiocarbonate, dry out the soil as much as possible. The drier the soil, the deeper the chemical will penetrate and the more effective the treatment will be. Do this by withholding water during summer and by using cover crops (such as sudangrass or safflower) to further deplete soil moisture. Finally, deep-till the dried area, being careful not to spread any diseased roots.
UC IPM Pest Management Guidelines:
W. D. Gubler, Plant Pathology, UC Davis