Introduction to Fungi, Third Edition

684 ANAMORPHIC FUNGI
with the cuticle. Immediately within the nematode
body, the penetration hypha swells to
form a globose vesicle, the infection bulb
(see Fig. 25.3b), from which assimilative hyphae
radiate throughout the animal, now dead. The
cytoplasm of traps of most predatory fungi
contains an unusual abundance of dense bodies
(microbodies) identified as peroxisomes. Their
function is currently unknown but may be
related to storage (Veenhuis et al., 1989). Similar
organelles, though of different cytological
origin, are seen in the assimilative hyphae
within the nematode. It is likely that they
are involved in amino acid assimilation and/or
the degradation of lipids during the digestion
of the nematode contents (Dijksterhuis et al.,
1993).
The sequence of infection-related development
in the endoparasitic conidial species
Drechmeria coniospora and Verticillium balanoides
has been described by Dijksterhuis et al. (1990,
1991) and Sjollema et al. (1993). A hypha growing
out through the adhesive pad at the conidial
apex forms an appressorium on the nematode
cuticle; this mediates penetration. There is no
infection bulb, and the nematode may remain
alive while the trophic hyphae proliferate
within its body. As in predatory species, numerous
microbodies and lipid droplets are seen
within the trophic hyphae. The extent of mycelium
produced by endoparasitic nematophagous
species in nematodes is often limited, with
most of the captured biomass being converted
to conidia.
25.1.5 Opportunistic parasites of eggs
and cysts
Numerous saprotrophic soil fungi have been
shown to be associated with the eggs and cysts
of nematodes, especially sedentary species parasitizing
plant roots (Stiles & Glawe, 1989). Even
though some host specialization may occur, all
fungi colonizing nematode eggs and cysts are
currently considered opportunistic parasites
(Siddiqui & Mahmood, 1996). They reproduce as
conidia and/or chlamydospores, like most soil
fungi. Infection is by means of hyphal tips, and
no specialized infection structures are formed
apart from appressoria in some species.
Nematode eggs contain chitin in addition to
collagen, and chitinases as well as serine
proteases have been demonstrated in fungal
egg parasites (Morton et al., 2004). The most
important species as potential biological control
agents are Pochonia chlamydosporia which
produces conidia and multicellular melanized
chlamydospores, and Paecilomyces lilacinus
(Siddiqui & Mahmood, 1996; Kerry & Jaffee,
1997).
25.1.6 Biological control of nematodes by
parasitic fungi
Some nematodes are serious parasites of plants
and animals, and attempts have been made to
use all three ecological groups of nematophagous
fungi described above predatory, endoparasitic
and egg- or cyst-colonizing forms for biological
control. Although the promise held by nematophagous
fungi is high and some success has been
reported, no commercial breakthrough has as
yet been achieved.
Predatory nematophagous fungi tend to
show only limited competitiveness in nonnative
soils (Siddiqui & Mahmood, 1996).
Although this may preclude their use in the
biological control of plant-parasitic nematodes,
the situation is different with animal parasites.
Many trap-forming fungi occur on dung along
with the larval stages of nematodes parasitizing
herbivorous animals. Biological control should
therefore be feasible, but a major obstacle is
the requirement for the spores of such potential
biocontrol agents to survive the stringent
passage through the herbivore gut. This
ability has been demonstrated for Arthrobotrys
eudermata (syn. Duddingtonia flagrans), which is
unusual among predatory fungi in forming
thick-walled chlamydospores in addition to
conidia. There is considerable current interest
in biological control based on feed supplemented
with chlamydospores of A. eudermata
(reviewed by Larsen, 2000).
There are obvious problems in using endoparasitic
nematophagous fungi for biological
control due to practical difficulties in producing
sufficient inoculum of many species, and