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pathogens like Heterobasidion, which can colonize

from basidiospores and then grow down into the

dying roots and infect the adjacent healthy trees

(Fig. 12.9).

The simplest way to avoid this is to kill the stump

surface tissues with phytotoxic chemicals such as urea

or boron-containing compounds, enabling saprotrophs

to rot the stumps and to exclude Heterobasidion. This

is common practice in many forests, but is environ-

mentally undesirable, especially if the forests are in

catchment areas for domestic water supplies. Rishbeth

(1963) developed an alternative control method in

which spores of Phlebiopsis giganteawere applied to

exposed stump surfaces immediately after the trees

had been felled. P. giganteais a weakly parasitic fungus

that rapidly colonizes the stump tissues of pine trees,

but poses no threat to healthy trees. It then grows down

into the major roots and prevents Heterobasidionfrom

becoming established. This type of biological control

is termed pre-emptive (competitive) niche exclusion.

We will see other examples of this on leaf and fruit

surfaces, later in this chapter.

Hyphal interference also seems to be involved in

the control of Heterobasidion, because the hyphae of P.

giganteaantagonize Heterobasidionon contact (Fig. 12.7).

As shown in Fig. 12.9, P. giganteamight have multiple

roles in this biocontrol system, helping to protect

fresh stump surfaces, helping to contain any existing

pockets of infection by Heterobasidionin the root zone,

and potentially preventing the pathogen from grow-

ing up to the stump surface and sporulating there.

In developing this practical biocontrol method,

Rishbeth made use of the fact that P. giganteais one

of the relatively few Basidiomycota that sporulate

readily in laboratory culture. The hyphae fragment

behind the colony margin to produce many brick-

shaped conidia, which can be used as inoculum to apply

to stump surfaces. In the initial biocontrol formulations,

Phlebiopsisspores were packaged in sachets containing

sucrose solution at a water potential that prevented

FUNGAL INTERACTIONS 243

Fig. 12.8Control of pine root rot (Heterobasidion annosum) by Phlebiopsis gigantea. (a) A pine plantation showing a
large disease gap (near the top) caused by H. annosum. (b) Trees within this gap are spindly and dying. (c) Bracket-
shaped fruiting bodies of H. annosumgrowing from the base of a heavily diseased tree. (d) Sachets containing spores
of P. gigantea, which can be diluted with water and inoculated onto freshly exposed stump surfaces. (e) Brick-like spores
of P. giganteaare produced abundantly in laboratory culture by fragmentation of the hyphae. (f) A pine stump 1 year
after treatment with spore suspension of P. gigantea, showing abundant growth of P. giganteaover the stump surface.
(Images courtesy of the late J. Rishbeth.)

(a) (b) (c)

(d) (e) (f)