[Illustration: Mushroom or fruiting bodies of the “honey mushroom”
rot.]
Armillaria root and butt rot is caused by the fungus _Armillaria
mellea_. This disease is common in orchards, vineyards, gardens, parks,
and forests throughout the world. Both coniferous and hardwood forest
trees are attacked. The fungus is especially troublesome in plantations,
particularly in stands recently thinned.
The fungus is spread by spores produced by honey-colored mushrooms,
rhizomorphs, and root contacts between diseased trees or stumps and
healthy trees. Rhizomorphs are visible strands of compacted mycelium
(fungus material) that appear as black or reddish-brown “shoestrings.”
They may be flattened when found between bark and wood or cylindrical
when found in decayed wood or soil. Rhizomorphs increase in length at
their tips and in this manner the fungus may move through the soil from
infected trees to uninfected trees. The honey-colored mushrooms are
produced annually and are fairly short lived; they are subject to
desiccation, and are favored by small mammals. The size of the top
varies from two to five inches in diameter. The top is usually flecked
with dark brown scales. White to light yellow gills are borne
underneath. Crown symptoms of the trees affected are similar to those
caused by any malfunctioning of the roots. A reduction in size and
production of leaves or needles, a general thinning of the crown, branch
dying, and yellowing of foliage may precede death or trees may die
rapidly with a rapid red discoloration of the foliage. Trees often die
in groups, but single-tree kill is also common. Cankers bleeding resin,
gum or other exudate at the tree base are common symptoms.
Control is not attempted for this disease under forest conditions.
Losses may be reduced by following proper planting procedures, and by
salvage cutting.
[Illustration: Shoestrings of armillaria.]