How to Manage Pests
UC Pest Management Guidelines
Armillaria Root Rot (Oak Root Fungus)
Pathogen: Armillaria mellea
(Reviewed 3/09, updated 3/09)
In this Guideline:
SYMPTOMS AND SIGNS
Roots infected with Armillaria mellea have white to yellowish fan-shaped mycelial mats between the bark and the wood. Dark brown to black rhizomorphs can sometimes be seen on the root surface. Infected trees develop pale foliage with small leaves, a lack of new growth, and a thin canopy, usually followed by sudden death when the first hot weather of early summer arrives.
COMMENTS ON THE DISEASE
The fungus survives on dead roots. It spreads from one tree to another through close contact of diseased roots with healthy roots. All stone fruit rootstocks are susceptible to Armillaria root rot. The plum rootstock Marianna 2624 is the most resistant to the fungus, but it is not immune. Use of this rootstock is the only practical alternative if almonds are to be grown in soils where Armillaria has infected roots and killed trees on other rootstocks. Wet soil conditions resulting from heavy rainfall or excessive irrigations can exacerbate the disease.
The only treatment is fumigation. Before chemical treatment, remove all infected trees, stumps, and as many roots greater than 1 inch in diameter as possible. Healthy-appearing trees adjacent to those showing symptoms are often infected also. Removal of these adjacent trees and inclusion of that ground in the soil fumigation may be advisable. Infected trees, stumps, and roots should be burned at the site or disposed of in areas where flood waters cannot wash them to agricultural lands. Complete eradication is rarely achieved, and re-treatment may be necessary in localized areas. If the soil is wet, or if it has extensive clay layers to the depths reached by the roots, fumigant treatment may not be successful. The greatest opportunity for eradication occurs on shallow soils less than 5 feet in depth. Treat Armillaria from late summer to early fall.
UC IPM Pest Management Guidelines:
J. E. Adaskaveg, Plant Pathology, UC Riverside