BSPP Presidential Meeting 1999
Biotic Interactions in Plant-pathogen Associations
Session II - Interactions with
Epichlo grass endophytes and
their interaction with a symbiotic fly.
Geobotanisches Institut ETH, Zollikerstrasse 107,
CH-8008 Zrich, Switzerland.
endophytes (Ascomycota) and related asexual species form systemic associations
with many pooid grasses in temperate regions of the Northern hemisphere. During
sexual reproduction the fungus produces reproductive structures (stromata) on
aborted host inflorescence referred to as choke disease. Host specific fungal
species are self-incompatible (heterothallic) and need to be fertilized by
spermatia of opposite mating type. The vector for transferring spermatia are
specialized flies of genus Botanophila which ingest spermatia and pass
them through their gut. Female flies lay eggs on young fungal stromata
and after fertilization hatching larvae feed on developing perithecia as their
exclusive food source. Thus, Botanophila flies live in a symbiotic
relationship with Epichlo fungi, and dependence on stromata as food
source should promote specialization (as a consequence of sexual incompatibility
among different Epichlo taxa). Investigations were made to examine
whether flies are specialized on host grasses and their Epichlo fungi,
and how flies could recognize different hosts. Distribution of fly genotypes
collected from different hosts did not indicate specific preference. However,
field observations and the absence of mating among different host strains in
experimental field plots clearly suggested specificity of flies. Volatile
compounds collected and analyzed from infected host plants showed different
patterns with each host association. Thus, volatiles could serve as specific
attractants. We hypothesize that flies may be unspecific initially when
hatching, then after first contact with a stroma learn to specifically recognize
other stromata of the same host association.
Influence of virus spread on the
outcome of interspecific hybridisation between fungal pathogens
Clive M. Brasier.
Forest Research Agency, Alice Holt Lodge, Farnham, Surrey GU10 4LH, UK.
Two highly destructive pandemics of Dutch elm disease have occurred across
the Northern Hemisphere this century. Each has been caused by the spread of a
different, introduced ascomycete pathogen. First to appear, in the early 1900s,
was the moderately aggressive Ophiostoma ulmi. This was followed, from
the 1940s onwards, by the spread of the highly aggressive O. novo-ulmi,
which has rapidly replaced the O. ulmi in the process. Interspecific
hybridisation in fungi is considered rare, but several lines of evidence show
that interspecific hybridisation has occurred between O. ulmi and O.
novo-ulmi during the replacement period; although the resulting hybrids are
transient and unfit.
When O. novo-ulmi arrived in Europe, the resident O. ulmi
population was highly heterogenic with regard to vegetative compatibility (vc)
loci and mating type. In contrast, the immigrant O. novo-ulmi spread
initially as single vegetative compatibility type/single mating type clones.
Thereafter, the O. novo-ulmi population rapidly (5-6 yr) became
heterogenic for vc types and mating type and the clones disappeared. In North
America, however, similar O. novo-ulmi clones survived much longer (50
yr); and the rate of appearance of new vc types/mating types has been slow. In
New Zealand, an immigrant vc clone has survived 10 yr without significant
Underlying differences between the European, North American and New Zealand
situations indicate (i) that O. ulmi must be present for the
heterogenenicity to occur in the O. novo-ulmi population; (ii) that
deleterious fungal viruses must be abundant in the O. novo-ulmi clones
for the change to heterogenenicity to be rapid; (iii) that it is the spread of
the viruses that favours the survival of the new vc types over the original vc
Research in progress (M. Paoletti, K. W. Buck and C. M. Brasier, unpublished) is
consistent with a hypothesis that the new vc loci, and possibly also new mating
type loci, are acquired by the O. novo-ulmi clones from O. ulmi
through introgression. Virus pressure, therefore, appears to be a significant
influence on the outcome of a major fungal hybridisation event.
Fungal endophytes and nematodes
of agricultural and amenity grasses
1Roger Cook and 2Graham.C.
1Institute of Grassland and
Environmental Research, Aberystwyth, Ceredigion, SY23 3EB, Wales, UK.
2Institute of Grassland and
Environmental Research, North Wyke, Okehampton, Devon, EX20 2SB, UK.
In grasses, obligate mutualist
fungal endophytes (Neotyphodium spp.) have ecological and economic
significance because of the impact of fungal secondary metabolites on
herbivores. These endophytes infect leaves and stems of healthy plants but have
no marked pathogenic effects. A range of insect herbivores, including sap
sucking aphids as well as biting herbivores, is affected by endophytes in tall
fescue and perennial ryegrass. Endophyte-infected grasses can also cause
toxicoses in grazing livestock. As well as protecting the plant from herbivory,
these endophytes can increase plant yield, enhance root growth and modify water
Evidence for the effects of grass endophytes on root feeding nematodes is
equivocal, perhaps reflecting the variety of interactions. None the less, there
are striking examples of endophyte-infected grasses expressing very effective
resistance to nematodes, not found in the endophyte-free host. These examples
include nematodes for which natural genetic host resistance has proven elusive
or difficult to manipulate. This article reviews relationships between Neotyphodium
grass endophytes and root parasitic and other nematodes associated with
We review Lolium spp. and Festuca arundinacea and F. pratensis all
native to Europe, temperate Asia, and North Africa. The more interesting
phenomena involving endophytes are reported from countries where such grass
species have been introduced, for example, from tall fescue in the United States
of America and from perennial ryegrass, L perenne, in New Zealand. The
review of nematode/endophyte interactions is grouped according to the mode of
parasitism of the nematode concerned. It seems clear that to be affected by
endophytes, nematodes must feed on endophyte-infected plants, perhaps ingesting
toxins. Some nematodes may be tolerant of the toxins or may through their
feeding habits or sites, avoid exposure to translocated toxins. In some case,
the responses of plants to endophyte, such as wall thickening or anti-drought
responses, may indirectly affect their host status for particular nematodes.
The endophyte associations are maintained in USA and New Zealand grasses because
of their beneficial features. In contrast, European bred forage grass cultivars
generally have little or no endophyte infection. The symbiotic interaction is
very complex with a good deal of variability. Future studies of endophyte
impacts on nematodes should take account of the causes of variability in the
interactions. This requires standardisation of grass genotype and fungus strain
as well as biochemical characterisation of the combination. Environmental
conditions need either to be controlled or at least measured to relate to
effects on alkaloid production. It is also important to describe the feeding
site and processes of the nematodes being studied. Control of these aspects will
allow the development of experimental systems to test hypotheses about the
relationship between parasite attributes and susceptibility to endophytes. Such
studies have clear potential economic value, given that some combinations of
grass and fungus provide fully effective resistance to nematodes not readily
controlled either by natural genetic resistance or by other means. The use of
selected endophytes on heterogeneous grass populations may offer durable control
of nematodes to the benefit both of forage and amenity grass and to crops grown
Feeding on plant-pathogenic fungi by invertebrates: relationships to saprophytic and mycorrhizal systems
Terence P. McGonigle & M. Hyakumuchi.
Faculty of Agriculture, Gifu University, Gifu 501-1193, Japan.
What is the significance of
grazing on plant-pathogenic fungi by invertebrates? Such grazing certainly
occurs, but the extent to which it modifies pathogenic interactions is unclear.
Here, we consider feeding on fungi in broad terms by arthropods, nematodes,
annelids, and protozoa. Understanding the impact of feeding is important when
extending laboratory data on plant-pathogen interactions into the field. A
summary of relevant published studies from pathogenic systems will be made.
These studies will then be evaluated in terms of what can be learned from
saprotrophic and mycorrhizal systems, as outlined below. Grazing can affect
ecosystem function and fungal community structure. Effects on saprotrophic and
mycorrhizal systems can be direct, such as by the excretion of nitrogen from the
bodies of the grazers, which contributes to mineralisation. Indirect effects are
possible as by the action of comminution and mixing of substrate, inoculum
dispersal, and changes in vigour of the grazed fungus, all of which can modify
mineralisation or immobilisation by the microbes. These indirect effects on
mineralisation are important for saprotrophic fungi, but they may also be
relevant for certain mycorrhizal fungi with limited saprotrophic capabilities.
Effects of grazing on the collection of nutrients from soil and the transfer of
them to the plant in mycorrhizal systems are mostly negative. However, at low
animal densities a stimulation of mycorrhizal function can occur, which is
perhaps related to a stimulation of fungal activity. Available data for effects
on fungal community structure are mostly for saprotrophic systems. Under
controlled conditions, selective grazing has been shown to modify fungal
communities by either suppression of a fungus that was otherwise dominant, or by
further polarising competitive interactions by grazing on less-dominant fungi.
These processes serve to increase and reduce fungal diversity, respectively.
Further, grazing can act to facilitate the transition of fungal communities from
early to late stages of succession. Conclusions relevant to pathogenic systems
will be drawn, and these will be discussed in terms of the co-occurrence of
pathogenic, saprotrophic, and mycorrhizal fungi through the connected pathway of
soil, root, shoot, and air.