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University of California Cooperative Extension Ventura County
669 County Square Drive, Suite 100
Ventura, CA 93003
Phone: 805.645.1451
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March 29 – Ceaser Chavez Day
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Armillaria on Oaks
California live and valley oaks and Armillaria mellea (oak root fungus) both evolved in a climate of winter rains and summer drought (Mediterranean climate). When there is moisture present, winter and spring, the tree and the fungus can both grow, but the soil is cold so the fungus cannot become too aggressive. In summer and fall, when the soil dries out, the fungus goes dormant, and the oak has a chance to repair any damage using deep soil moisture not available to the fungus. In this manner the oaks and the fungus have developed a relationship in which both survive.
When we landscape around oaks and add summer water, we upset this natural balance. In summer, moist and warm soil allows Armillaria mellea to grow and become an aggressive pathogen, which will in time kill the oak trees and other plants in the overwatered landscape.
To stop the Armillaria invasion, you must radically change your landscape and return it to a Mediterranean climate landscape, using plants that will withstand little or no summer/fall watering. I would suggest removing all water-loving plants at least out to the drip line of your oak trees. Next mulch the area with a leaf litter and wood chip mulch three to four inches deep. Spread this mulch out to the drip line. Since the root system of the trees has had some damage, you may need to water a little, but no more than once a month. Keep the area under the oaks as dry as possible without causing further stress to the trees. When the fall rains begin, shut off the irrigation system.