Armillaria root rot, caused by fungi in the genus Armillaria, affects trees, shrubs, vines and even herbaceous plants throughout the world, in both tropical and temperate climates. These endemic fungi typically act opportunistically, killing weakened trees, and are frequently a sign of natural stand aging and not a serious concern.  However, Armillaria fungi can also infect and kill healthy trees, or make them more susceptible to other pathogens and insect pests.  Armillaria root rot is most problematic when combined with other stressors and thus changing climate, land use changes, and increasing invasion by pathogens and insects, increases its potential to harm forest trees.

What Is It ? 

Armilaria is native fungal pathogen that is a natural occurrence in the roots of trees. When this fungus attacks the host, it can be hard to detect unless fungal fruiting bodies are present at the base of the trunk. Infection signs can cause upper branching to yellow and die. In conifers infection leads to bleeding of resin from the trunk and gum exudates in broadleaf trees. 

What does it do?

The fungus produces Rhizomorphs that form between the bark and wood, which can result in formation of cankers and eventually girdling. Results of infection cause butt rot of the inner wood. This type of decay only spans into the lower stem of the host tree. In cases of armilaria, the tree can either outgrow the infection, or succumb to girdling death.