| The fungus, Armillaria mellea, occurs sporadically in
this region and has been reported to infect over 25
species of ornamental trees and shrubs. The most
distinctive sign of Armillaria infection is the honeycolored
mushroom that grows from the roots and
base of plants. The fungus is especially prevalent on
oak but also affects many different kinds of fruit and
nut trees, ornamentals, and herbaceous plantings. It
is often referred to as oak root rot fungus because it
is commonly found on oaks or in areas where oaks
trees once were grown, such as cleared forest land.
Symptoms and Diagnosis
The symptoms of Armillaria are similar to those of
other root disorders; height growth is reduced,
foliage is sparse, and leaves that remain on plants
are stunted and yellow. An abnormal flow of sap
may be visible on the root collar. When soil is
removed from the base of the trunk, black, root-like
strands are visible and attached to larger roots.
White to dark, fan-shaped mats of fungal strands
develop between the bark and wood in infected root
and trunk tissues. The most positive sign of infection
is the production of clusters of honey-colored
mushrooms at the base of the tree near the soil line.
The mushrooms may have stalks 4 to 6 inches high
with caps 1 inch high and 2 to 4 inches wide.
Life Cycle
Armillaria is found in the soil sporadically
throughout the Midwest. The fungus spreads
primarily by root-to-root contact or by root-like
fungal strands. Root-like fungal strands grow through
the soil and adhere to the host roots or root collar
that it encounters. Successfully attacked trees do not
die until infections girdle the base of the tree. On
healthy, vigorous trees, Armillaria is not lethal but if
present, it may begin to grow when the tree dies of
other causes. Young trees are more likely to be killed
by Armillaria. Trees that are 15–20 years old are
more tolerant to attack.
Integrated Pest Management Strategies
1. Remove and destroy infected material. Remove
as much of the stump as possible. Sterilize tools
with a solution of 1–part bleach with 9–parts
water after use.
2. Provide adequate moisture in a well-drained soil
to maintain vigor and resistance to infection.
Plants suffering from drought are extremely
susceptible to infection. Fertilize trees
appropriately in late winter or early spring.
3. No effective chemical controls are known. Plants
reported by Auburn University to be resistant to
Armillaria root rot include ginkgo, tulip tree, ash,
bald cypress, cherry, Chinese
elm, Chinese pistache, crabapple, cryptomeria,
dawn redwood, eucalyptus, hackberry, holly,
incense cedar, Leyland cypress, maidenhair tree,
maple, privet, smoke tree, sweetgum, tree-ofheaven,
white fir, and wisteria. These should be
considered for replacement of diseased trees. New
soil should be used to amend the planting site. All
diseased material and associated roots, as much as
possible, should be excavated before replanting. |
