This fungus is one of many that help to rot away tree stumps and dead tree roots. The problem comes when it decides to start working on a live tree. Armillaria spreads by structures called rhizomorphs which move out into the surrounding soil from rotting stumps. If they come into contact with the roots of nearby trees, they may penetrate into them and start to rot them.

Trees infected with this type of root rot lose vigor as they lose root mass. Leaves may turn yellow and twigs and branches may die. In severe cases, especially on smaller trees, the entire plant may die in a relatively short period of time. Also, live trees may be more subject to falling over in wind or ice storms.

Other types of root rots and factors may cause trees to have similar symptoms. However, a tree infected with armillaria root rot will have a dense, white mat of fungal tissue called a mycelial fan growing between the bark and the wood of the tree.

There is no chemical spray or drench that will control this rot. The only approach is to try to keep the tree as healthy as possible by proper pruning, occasional fertilization and watering in drought, if possible.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

Some plants resistant to armillaria root rot:
Amelanchier - Serviceberry
Chaenomeles - Flowering Quince
Crataegus - Hawthorn
Malus - Apple, Crabapple
Metasequoia - Dawn Redwood
Morus - Mulberry
Myrica pensylvanica - Bayberry
Nandina  - Heavenly Bamboo
Photinia - Nine Bark
Pinus nigra - Austrian Pine
P. radiata - Monterey pine
P. sylvestris - Scotch pine
P. torreyana -Torrey pine
Platanus - Planetree, Sycamore
Prunus - Flowering Almond, Plum and Cherry
Prunus caroliniana - Cherry Laurel
P. lyonii - Catalina Cherry
Quercus ilex - Holly Oak
Rhus aromatica - Fragrant Sumac
Rosa - Rose
Sambucus - American Elder
Sequoia sempervirens - Coast Redwood
Sophora - Japanese Pagoda Tree
Sorbus - Mountain Ash
Spiraea - Spirea
Taxodium distichum - Bald cypress
Ulmus parvifolia - Chinese elm
Wisteria sinensis - Chinese wisteria
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