Hosts: Douglas-fir, spruce, ponderosa pine,
subalpine fir, oak
The most diagnostic trait of Armillaria is the thick,
fan-shaped mat of white mycelium in the cambium of roots and root
crown. This may be accompanied by copious resin flow on bark surfaces,
although this trait is not common in the Southwest. Other signs
of the fungus include rhizomorphs, or black shoestring-like structures,
on the outside of infected roots. The fruiting bodies of this fungus
are commonly produced in clusters but can be found singly. The caps
are yellow brown to dark tan, with small scales on upper surface.
The stem is stout, tapering upward to a ring of tissue just below
the cap. The decay is light yellow, soft and spongy to stringy often
containing numerous black zone lines.
Armillaria root decay spreads primarily by rhizomorphs and
root contacts. Armillaria invades the bark and cambial region of
roots and the root collar, killing roots and trees of all sizes.
Wood decay follows cambial attack, and the wood serves as a source
of energy necessary for infection of new hosts. New infections are
less common, occurring when mushrooms release windborne spores that
germinate and colonize recently killed material. As with other root
disease fungi, Armillaria can persist for decades in decaying wood
Armillaria often acts in conjunction with other secondary pests
and pathogens. For example, Dendroctonus and fir engraver
beetles may attack trees with this root disease.
Armillaria is the most common root disease in the Southwest
and may account for up to 80 percent of the root disease conifer
mortality in the region. There are over 10 species of Armillaria,
some are virulent parasites while others are opportunists that act
selectively on small or weak individuals such as those shaded by
taller plants, defoliated by insects, attacked by other fungi, or
weakened by drought.
Insects and Diseases: Other fungi such as Fomitopsis
pinicola produce white mycelium beneath the bark of infected
stumps, but the mycelium is not thick and fan-shaped like that produced
by Armillaria spp.