Armillaria root rot, Armillaria mellea, is often correlated with oaks and other hardwoods throughout California. The disease, which is a root fungus, attacks trees that are often found in urban landscapes as a result of excessive moisture in the soil. This is typically caused by overwatering in the landscape induced by excess irrigation and not allowing proper time for the roots to soak up the water and let the soil dry out. When too much moisture is in the soil, an anaerobic (oxygen free) environment is created. As a result, roots become starved to death by not having any oxygen in the soil. The roots eventually are not able to absorb water and will begin to die- off over time.
Some of the symptoms associated with armillaria root rot can be thinning of the canopy, dieback in the branches, oozing of fluid near the base of the trunk and mushrooms (fruiting bodies) at or near the base of the trunk. Thinning of the canopy occurs when leaves begin to fall prematurely at the ends of branches and no new growth occurs. The tree will appear to have a less dense canopy over a period of time. Dieback in the branches occurs when branches begin to experience discoloration and drying; they will have very little (if any) leaves attached to the branch. When oozing occurs at the base of the trunk, a dark thick liquid substance excretes from the bark, which gives the appearance that the tree is bleeding. Mushrooms are fruiting bodies that indicate that there is more than likely decay at or near the location of the mushrooms. They resemble the appearance of culinary fungi mushrooms commercially grown and purchased at the grocery store but are not edible.
Other factors that contribute to the disease can be compaction of the soil that is experienced during construction activity around or near the tree. Equipment that is stored near the tree can compact the soil due to the weight of the machinery. Equipment used for excavation can also damage trees by severing or crushing roots, even if only minimal disruption has occurred. Adding fill, dirt, to existing soil around or at the base of tree is not recommended as well because the added soil can increase weight and decrease oxygen in the soil where the roots lay causing unfavorable growing conditions for roots.
Once the tree is infected with the disease, it is more than likely the tree will decline over a period of time and in some cases lead to death or failure of the tree. Areas prone to high winds may cause the tree to topple over and fall. At this time, there are no cures for this disease; one of the mitigations for this disease is to check the soil and irrigation for prolonged periods of watering. This can be accomplished by reducing the amount of sprinkler run time for the area that waters the tree. When planning a landscape project, it is best advised to not plant oak trees in a lawn. Oak trees do not have the same watering requirements as turf so if possible redesign the landscape to incorporate oak trees outside the turf area.