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a biological control agent can become established. For

example, soil-borne sporangia of the common seedling

pathogen Pythium ultimumcan germinate within 1–2

hours in response to volatile metabolites released from

germinating seeds (probably ethanol but possibly also

acetaldehyde), and the seed tissues can be heavily colo-

nized within 6–12 hours of planting in Pythium-

infested soil. Biocontrol agents are unlikely to act fast

enough to control such pathogens (Nelson 1987, 1992).

Athelia rolfsii

The sclerotium-forming fungus Athelia (Sclerotium)

rolfsii(Basidiomycota) is one of the most devastating

seedling pathogens in the warmer parts of the world

where rainfall is seasonal. The sclerotia of A. rolfsii

(Fig. 14.1) survive in soil during the dry season and are

triggered to germinate after the first rains. Then the

fungus grows on the re-wetted crop residues from the

previous season and uses these residues as a food base

to attack the newly sown crop, typically rotting the basal

stem tissues within 3– 4 weeks. Large areas of cereals,

and other crops, can be destroyed by this fungus.

The key to understanding A. rolfsiilies in the beha-

vior of its sclerotia, as shown by the experiment in

Fig. 14.2 and Table 14.1. If sclerotia are harvested from

laboratory cultures they exhibit hyphal germination,

whereby only a few hyphae emerge and these can infect

plants at only a short distance from the sclerotia

(0.5 cm). But sclerotia that have been air-dried show

eruptive germinationwhen remoistened – all the

sclerotial reserves are used to produce a mass of

hyphae, which are organized into mycelial cords

and can infect a plant at up to 3.5 cm distance. The

sclerotia of this fungus are even more effective in the

presence of freshly decomposing organic matter (e.g.

crop residues from the previous season, which start to

decompose after the first rains). Volatile compounds,

especially methanol, are released from these freshly

decomposing residues, due to the actions of pectic

enzymes that persisted in the residues. These volatile

compounds trigger the sclerotia to germinate erupt-

ively and also serve as nutrients, orientating growth

towards the source of methanol. In these conditions,

the sclerotia can infect seedlings even 6 cm away.

These experiments seem to parallel closely the beha-

vior of the fungus in cropping systems, where A. rolfsii

characteristically colonizes the crop residues and uses

these as a food base to support infection of the basal

stem tissues of young plants. It has an extremely

FUNGI AS PLANT PATHOGENS 281

Fig. 14.1(a) Colony of Athelia rolfsiion an agar plate. The sclerotia (some indicated by arrowheads) develop near the
colony margin. The white mycelium is aggregated into mycelial cords. (b) Young wheat plants rotted by A. rolfsiijust
below soil level.

(a) (b)