Oak root fungus (Armillaria mellea) is named for a favorite host, but it attacks a variety of woody plants. It is prevalent in the Southeast and in low-elevation, nondesert regions of California. The first symptoms may be dull, yellowed, or wilted leaves and/or sparse foliage; later, entire branches die. The aboveground symptoms result from root death, which cuts off the plant's water and nutrient supply. To verify the presence of oak root fungus, check under the bark of large roots or the trunk (near ground level) for a mat of whitish fungal tissue. In late autumn or early winter, clumps of tan mushrooms may appear around the bases of infected plants.
You may be able to save lightly infected plants--or at least prolong their lives--by removing soil from their bases to expose the juncture of roots and trunk to air, and by cutting out and destroying all dead and infected tissue.
To avoid the problem, choose resistant plants. Avoid placing susceptible plants in locations where oak root fungus is known to have occurred or where Armillaria fungi are likely to be present (on the site of an old orchard, for example). If you want to plant where a tree or shrub has been killed by Armillaria, remove and destroy all roots in the planting area.
There is no chemical control.