Arbuscular mycorrhizal (AM) fungi colonize roots of terrestrial plants and have been hypothesized to reduce susceptibility or to improve the vigor of hosts challenged by root pathogens. Meta-analysis was used to test whether a broad pattern exists in which AM fungi not only enhance plant growth but fundamentally alter plant-pathogen interactions. Data were gathered from studies published between 1970 and early 1998. In these studies nonmycorrhizal or mycorrhizal plants were untreated or challenged with root-infecting fungal pathogens or nematodes. Effect sizes for AM fungus treatment, pathogen treatment, and their interaction were calculated based on measures of average plant growth, pathogen growth, or AM colonization as reported by authors. AM fungus had a very large positive effect, and pathogens had a very large negative effect on plant growth. However, the interaction of AM fungi and pathogen depended upon the class of pathogen. AM fungi tended to decrease the harmful effects of fungal pathogens but to exacerbate the harmful effects of nematodes.Overall, inoculation with AM fungi had a large negative effect on growth of pathogens, but the outcome for nematodes depended upon mode of feeding. AM fungi harmed sedentary endoparasitic nematodes. AM fungi improved growth of migratory endoparasitic nematodes, but this outcome was not significant in an analysis limited to independent experiments. Reduced growth of pathogens in mycorrhizal plants may indicate increased host resistance. Alternatively, reduced AM colonization in pathogen-treated plants may suggest that rootinfecting organisms compete for resources.Some experiments that did not meet criteria for meta-analysis were analyzed with 2 tests by calculating the proportion of experiments in which authors reported a significant pathogen effect in the absence vs. presence of AM fungi. AM fungi did not alter the effect of fungal pathogens but reduced the proportion of experiments in which nematodes depressed plant growth in a test limited to independent observations. Comparison of frequency distributions of effect sizes suggests that the difference in results for nematodes in the meta-analysis and the 2 tests stems from differences in the AM fungus-plant-pathogen systems examined in each approach rather than a statistical artifact.AM fungi reduced pathogen growth in ϳ50% of studies included in 2 tests, and this effect was similar for fungal vs. nematode pathogens and sedentary vs. migratory nematodes. Fungal pathogens reduced AM fungus growth more frequently than did nematodes. Both AM fungi and pathogens suffered reduced growth in 16% of experiments that assessed reciprocal effects. Reciprocal suppression was more common when the pathogen was a fungus, which may indicate that fungal pathogens are more likely than nematodes to compete with AM fungi.Almost all research on the effects of AM fungi on plant-pathogen relations has examined economically important species, in low-phosphorus soil, in greenhouse or microplot culture. Although these analyses revealed genera...

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