- The coniferous forests of the Pacific Northwest are
affected by a variety of endemic fungal root pathogens. Forest
disease surveys have noted the presence of two or more of
these root pathogens infecting the same stump or root, and it
has been suggested that these fungi may be interacting
synergistically. To test this hypothesis, three separate root
disease centers were studied where the dominant pathogens
appeared to be either Armillaria ostoyae (Rom.) Herink,
Heterobasidion annosum (Fr.) Bref., or Phellinus weirii
(Murr.) Gilb. The spruce-type intersterility group of i.
annosum was found infecting trees and stumps within both the
P. weirii and A. ostoyae centers, and was isolated from some
of the same roots as A. ostoyae. Three experiments were designed to study pairwise
interactions between A. ostoyae, P. weirii, the spruce (S) and
pine (P) type intersterility groups of ff annosum, and
Perenniporia subacida (Pk.) Donk. Hyphal and mycelial
interactions were studied microscopically on agar-filled
culture slides, macroscopically on 2% and enhanced malt agar,
and within wood blocks of an Abies sp. (either A. qrandis
(Dougl.) Lindl., or . concolor (Gord. and Glend.) Lindl.) and
Pinus ponderosa Laws.
Direct interspecific hyphal contact occurred between
fungi on culture slides, resulting in sporadic hyphal
vacuolation. However, no other discernible reactions such as
anastomosis or parasitism were observed. Only one of the
pairings, A. ostoyae vs. H. annosum P-type, formed a definite
band of vacuolated hyphae along their zone of interaction.
Other pairings may have had more vacuolated hyphae in the zone
of interaction, but this was difficult to quantify.
Pairwise interactions were also studied on 2% and
enhanced malt agar. Reactions between fungi included the
formation of zones of inhibition, walling-off, abutting
growth, or overgrowth of one isolate by the other. The
reactions of identical fungal pairings were inconsistent
between the two types of media. Isolations from these plate
pairings revealed that visual observations may not be adequate
to classify the reactions which occur.
Interactions within sterilized wood blocks showed the
formation of either single or parallel barrier lines between opposing fungi, and isolations revealed that the colonizing
fungi remained separated on their respective sides of the
The conclusion from this study is that root pathogenic
fungi grow in proximity to one another in nature in what can
be termed associations. However, experimental results lead to
the conclusion that these fungi do not form complexes, nor do
they interact synergistically in causing decay. Pathogenic
fungi appear together independently, as conditions favorable
for their individual infection biologies coincide with the
presence of an inoculum source and a suitable host.