|Abstract or Summary
- Numerous isolates of Armillaria mellea were obtained within
a 250 by 450 foot Study Area in a ponderosa pine plantation in central
Oregon. These isolates were recovered from roots of armillaria-killed
young-growth pine, from large stumps of the former pine
overstory, including roots away from the stump proper, and from
roots of living shrubs.
Characteristic interactions between compatible and non-compatible
mycelia in paired culture clearly distinguished group
affiliations among the isolates. Three groups (A, B, and C), unequally
distributed numerically among the hosts from which they
were isolated, were recognized in the Study Area. Members of
the same groups and of other groups were found among isolates
from roots of shrubs and pine in plots surrounding the Study Area.
Isolates of group A were recovered seven-eighths of a mile apart.
Only members of groups A and B were recovered from
armillaria-attacked young-growth pine, while C group members
were predominately recovered from rhizomorphs on roots of living
Inoculation of potted conifer and shrub seedlings with A and
C group isolates revealed the presence of physiological strains of
A. mellea. Rhizomorphs were found on the roots and root crowns
of all plant species tested, but only members of group A were pathogenic
on conifer seedlings. Shrub seedlings were not killed by either
A or C group members although rhizomorphs of members of both
groups were in close physical contact with living root tissue. These
results agree with the field occurrence of groups A and C in the
Roots of living and armillaria-attacked sapling pines were
excavated in the Study Area. Infections of killing potential were
found to occur only at the root crown; lesions on lateral roots were
common, but did not contribute to the death of the trees examined.
There was no evidence in this research of the fungal spread through
root contact reported elsewhere in the literature.
Armillaria attack stimulated the host to secrete large amounts
of resin at the attack site. Attacks in the root crown region, which
ultimately were lethal, caused excessive resin production. The
fungus developed very slowly in host tissue during the period of resin secretion, but thereafter A. mellea quickly ramified the cambium
of the now moribund or dead roots killing the tree.
Cold-water extracts were made from foliage, roots, and litter
of ponderosa pine and shrubs on Pringle Butte, and from soil under
these plants. Extracts of foliage (1:40 and 1:61 dilution) stimulated
mycelial growth, and rhizomorph production and elongation over that
of the control. Extract dilutions of 1:168 and 1:1600 did not greatly
stimulate fungal growth. Extracts of roots, litter, and soil had little
effect on growth of A. mellea in culture.