Armillaria mellea, also known as Honey Fungus or Oak Root Fungus, causes Armillaria root rot on Coast Live Oak (Quercus agrifolia) and other trees. In advanced stages, it pushes up mushrooms at or near the base of infected trees. Other symptoms appear as discolored foliage, reduced growth, and dieback of branches in the crown of the tree. Eventually, the tree will die.
According to Wikipedia: “Trees become infected by Armillaria mellea when rhizomorphs growing through the soil encounter uninfected roots. Alternatively, when infected roots come into contact with uninfected ones the fungal mycelium may grow across. The rhizomorphs invade the trunk, growing between the bark and the wood and causing wood decay, growth reduction and mortality. Trees that are already under stress are more likely to be attacked but healthy trees may also be parasitized. The foliage becomes sparse and discoloured, twig growth slows down and branches may die back…A growth of fruiting bodies near the base of the trunk confirms the suspicion of Armillaria root rot.”
Summer watering and watering closer than 6 feet from the trunk are the major causes of Oak Root Fungus. Fortunately, there are a number of Armillaria-resistant native trees and shrubs that can be planted near Oaks, including one of my favorites, Hollyleaf Cherry (Prunus ilicifolia). Here is a handy list of some common garden plants showing whether or not they are resistant or susceptible to Armillaria. (Thanks to Chase Agricultural Consulting for compiling the list.) I hope you find it useful.
NOTE: This fungus is edible and was once classified as Agaricus, the same genus as the common button mushroom found in produce sections of markets.