Talaromyces was found to antagonize V. dahliae only
when glucose was present in culture media, and not
when other common sugars were used. A commercial
source of glucose oxidase had the same effect on
hyphae of V. dahliae, but only in the presence of glu-
cose; and these effects could be reproduced by low con-
centrations of H 2 O 2 alone. It seems that Talaromyceshas
several potential mechanisms for antagonizing other
fungi; they might act separately or in combination.
Mycoparasitic Pythiumspecies
Most Pythium species are aggressive plant pathogens that
cause economically damaging seedling diseases and
root rots (Chapter 14). But six Pythiumspecies are non-
pathogenic to plants and instead they are aggressive
parasites of other fungi. In Britain the most common
of these is Pythium oligandrum which has distinctive
spiny-walled oogonia (Fig. 12.13). Like Trichodermaand
Clonostachys, discussed earlier, P. oligandrumcan grow
from soil crumbs sprinkled onto agar plates previously
colonized by other fungi. Using this technique as a
method of detection, P. oligandrumhas been found
in nearly half of the soils sampled in Britain, especially
from agricultural sites. P. oligandrumand similar fungi (e.g.
P. acanthicum and P. periplocum) are equally common
in the USA, continental Europe, and New Zealand.
Mycoparasitism by Pythium oligandrumhas been
studied in detail by videomicroscopy (Fig. 12.14). In the
sequence shown, a hyphal tip of the plant pathogen
Botrytis cinereacontacted a hypha of P. oligandrumand
then lysed within 3 minutes, spilling the contents of
the Botrytishypha. P. oligandrumthen produced many
hyphal branches in the lysate and penetrated the
Botrytishypha. Many similar sequences, involving dif-
ferent host fungi and different mycoparasitic Pythium
species, are described in Laing & Deacon (1991).
Some fungi are not invaded by P. oligandrumuntil
1 or 2 hours after contact, and during this time
the mycoparasite coils round the host hypha from
branches that originate at the initial contact point. P.
oligandrumthen penetrates from beneath the coils.
This is commonly seen in interactions with plant-
pathogenic Pythium spp. and in interactions with
mature hyphae of Rhizoctonia solani(Fig. 12.15).Detection of mycoparasites on precolonized agar
plates
Necrotrophic mycoparasites are easily detected and
isolated from soil using the precolonized plate method
(Fig. 12.13). This reveals an interesting feature, because
different types of mycoparasite that occur naturally in
soil are detected on different types of precolonized agar248 CHAPTER 12
Fig. 12.13(a) A method for detecting Pythium oligandrumor other mycoparasites in soil. Three soil crumbs (white arrow-
heads) were placed on an agar plate previously colonized by a soil fungus, Phialophora. Within 4 days, P. oligandrum
started to grow across the Phialophoracolony, collapsing the aerial hyphae (black arrowheads) and producing spiny-
walled oogonia (top right). (b) Narrow hyphae of P. oligandrumattacking and destroying a Phialophorahypha, leading
to production of a spiny-walled oogonium.
(a) (b)