A Killer Lurks Below

The Delonix tree above fell over during the rainstorms earlier this month. The culprit?

The culprit is plainly visible looking at the crown of the tree. Notice the whitish almost tan growth on the edge of the broken crown of the tree. That is most probably Armillaria mellea, the honey fungus, whose large clusters of mushroom fruiting bodies are visible throughout the Arboretum after the first, soaking cold rain of the season.

Armillaria fungus on the underground portion of the crown. 

Below you can see the fruiting bodies (mushrooms) of the Armillaria fungus pushing up near a Brachychiton tree in the Australian section. 

Armillaria mushrooms fruiting in the Australia section. 

Besides 'Honey Mushroom' the other common name for this fungus is the 'Oak Root' fungus. It can be found in areas that have or have previously had native oak trees. Most of the Arboretum was, in the past, oak woodland. Two species, Quercus engelmannii the Engelmann or 'Pasadena' oak and Quercus agrifolia, the Coast live oak dominated the landscape here.

Indeed, the Honey fungus seems to have an almost symbiotic relationship with the Coast live oak and the Engelmann oak. Healthy trees seem to thrive despite being infested with the fungus. Indeed the oaks and the fungus might have what you might call a semi-beneficial relationship. The fungus destroys woody trees and shrubs that might compete with the oak, and the oak might even provide water and minimal nutrition for the fungus. In fact, it has been shown that an Armillaria fungus has a symbiotic relationship with the Chinese medicinal orchid Gastrodia elata and has been reported as having a mycorrhizal relationship with at least two oak species. 

Although the oaks relationship with the fungus may be beneficial at times, if an oak tree is under any kind of stress (especially soil compaction and over-watering during its dormant period in the late summer) the fungus will aggressively go after the tree, sometimes killing it in one to three weeks time. 

So how to deal with this underground killer? Unfortunately there is no chemical method of control. The main method of control is watering your landscape correctly to prevent activating the fungus (in particular by encouraging water loving organisms like the root rotting Phytophthora fungus that damage plant roots, in turn causing them to attract Armillaria that is present) and removing any large pieces of wood that are 2" or more in diameter that may be buried in the ground.