Disease Spotlight: Armillaria Root Rot
Armillaria invades the roots, causing symptoms of the crown that resemble a host of other problems. Trees that are infected by a less aggressive strain will decline over a period of several years. The canopy will be thin, with stunted growth, off-color foliage, branch dieback, and on conifers, an excessive crop of cones before death. Aggressive forms of armillaria will kill trees within one season.
Armillaria has a tricky adaptation that allows it to become named the “humongous fungus.” (In 1992, an underground mat representing 37 acres of a continuous fungal clone in Michigan was found to be the heaviest in the world–weighing as much as a blue whale. ) Armillaria can live as a parasite, causing disease as described above, or it may live for decades as a saprophyte, happily feeding on coarse woody debris. When the opportunity arises, it can easily transform its metabolic processes from absorbing dead tissue to absorbing living plant tissue. Spread of the disease occurs when vigorous rhizomorphs grow through the soil and happen to contact healthy tree roots. Root-to-root contact or root grafting can also spread armillaria from an infected to an uninfected host.
Like all wood-invading fungi, once it has become established, there is no chemical that can eradicate it. Since armillaria is a root rot, pruning the canopy or removing a portion of the tree will have no effect. Trees that are suspected of light infections can sometimes survive several years with optimal watering and fertilization. Dead trees should be removed. Grinding the stump will prevent armillaria from remaining in the site longer than one to three years.
-Marion Murray, IPM Project Leader