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FUNGAL ENDOPHYTES: CONTRASTING EFFECTS IN
TREES AND GRASSES
STANLEY H. FAETH
Department of Zoology, Arizona State University, Tempe, Arizona 85287-1501, USA
INTRODUCTION
Fungal endophytes, fungi that live asymptomatically and intercellularly within tissues of most plants, have recently
received increasing attention from ecologists (e.g., Strong 1988). The presence of fungal endophytes in plants, especially
grasses, can increase resistance to herbivores due to the production of alkaloidal mycotoxins, but also may increase resistance
to drought and flooding stress, increase plant competitive abilities, and, for endophytes transmitted vertically via seeds, deter
seed predators or increase seed dispersal (Clay 1990, Knoch et al. 1993). Endophytic fungi are typically considered plant
mutualists because of these potential modes of increased plant fitness (Clay 1990). However, most research on fungal
endophytes has involved introduced, agricultural forage grasses, such as tall rescue or perennial ryegrass. Little is known of
the ecological role of endophytes in either native grasses or woody plants relative to plant-herbivore, plant-plant, or plantseed
predator interactions. Here, I summarize our research to date on endophytes in Quercus emoryi (Emory oak) and their
effects on a major herbivore, the leafminer Cameraria sp. nov. (Lepidoptera:Gracillaridae). Leafminer larvae spend 11
months within leaf tissues and are confined to a single leaf chosen by the ovipositing female. Therefore, endophytes should
alter leafininer performance more than that of mobile, exophytic insects. I contrast these results with endophytes in Festuca
arizonica (Arizona fescue). I predict stronger effects of endophytes on herbivores and seed predators of Arizona fescue due
to differences in mode of fungal transmission and specificity,
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METHODS ::
We have monitored seasonal and spatial patterns of fungal endophyte infections and the leafminer in trees of Emory :i
oak at Oak Flat study area in central Arizona for the past 4 and 10 years (Faeth 1991), respectively. We have isolated at least
12 species of endophytes from Emory oak, but four species, QE1 (Asteromella sp.), QE2 (Ascomycete:Diaporthales), QE7
(Plecophomella sp.), and Y1 (filamentous yeast) make up >95% of all infections. All of these endophytes are transmitted
horizontally via spores, likely carried in rainsplash. In observational studies, we have correlated the presence of living and
dead larval leafminers with the frequency of infection. In manipulative experiments, we have either increased (spore
spraying of leaves or spore injection of individual mines) or decreased (enclosing branches with plastic or application of
fungicides) to test the role of individual fungal endophytes on leafminer developmental and mortality.
We have begun to investigate the role of fungal endophytes in Arizona rescue populations and their relationship to
intensity of grazing andsoil nutrients. Arizona rescue harbors two endophytes, Acremonium starrii and a p-endophyte
(Phialophora-like). Both endophytes are transmitted vertically from maternal to offspring plant via seed, but the p-endophyte
also sporulates and can be transmitted horizontally fromadult plant to adult plant. We have conducted preliminary
experiments testing the role of the Acremonium endophyte in reducing seed predation and increasing seed dispersal by seed
harvesting ants (Pogonomymrex species) by presenting E+ (infected) and E- (uninfected) rescue seeds to ant colonies and
following proportions collected and discarded into refuse piles.
RESULTS AND DISCUSSION
Generally, endophyte infections in oaks increase seasonally, with the peak infection level coinciding with summer
rains in Arizona in July-August. Overall infection levels vary with all spatial scales- between localities, between and within
Mattson, W.J., Niemel_i, p., and Rousi, M., eds. 1996. Dynamics of forest herbivory: quest for pattern and principle. LISDA
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