Summary1 Metacommunity and neutral theory have reinvigorated the study of 'niches' and have emphasized the importance of understanding the influences of competition, abiotic factors and regional spatial processes in shaping communities. 2 We conducted a field survey to examine the effects of soil characteristics and distance on arbuscular mycorrhizal (AM) fungal communities of maize ( Zea mays ) in sand and clay soils. To address whether the field distributions of AM fungal species represented their fundamental or realized niches, we grew representative species of the two dominant genera, Glomus and Gigaspora , alone or together on Sorghum bicolor plants in sand, clay or a sand/clay mixture in the glasshouse. 3 In the field, soil characteristics and spatial structure accounted for significant proportions of the variation in community composition among sites, suggesting that both environmental variables and dispersal were important factors shaping AM fungal communities. 4 AM fungi in the family Glomeraceae occurred predominately in clay soils, whereas AM fungi in the family Gigasporaceae dominated in sand soils. Niche space of Glomeraceae was further partitioned by levels of soil organic carbon and nitrogen. 5 In the glasshouse, root colonization by Glomus was high in all three soils when grown in the absence of Gigaspora , indicating a broad fundamental niche . Root colonization by Gigaspora was negatively correlated with percentage clay when grown in the absence of Glomus , consistent with the low abundance of this family in clay soils in the field. When grown together, spore production of both Glomus and Gigaspora was significantly reduced only in the sand soil, indicating that competition could limit niches of both families in certain soil environments. 6 Our results suggest that AM fungal distributions are the product of environment, interspecific competition and regional spatial dynamics, emphasizing the importance of using a metacommunity perspective in community ecology.

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Summary• Ericoid mycorrhizal fungi differ in their abilities to use nitrogen sources and may be integral to maintaining fungal and plant diversity in ecosystems in which Ericaceae occur. In this study, we tested whether the fungal communities differ among three species of co-occurring Ericaceae.• Fungi colonizing Cassiope tetragona, Empetrum nigrum and Vaccinium vitisidaea roots in the Arctic tundra were characterized via culture-dependent and culture-independent techniques. The cultured fungi were tested for their ability to colonize Vaccinium uliginosum in laboratory-based assays.• The pure-cultured Helotiales were grouped into eight clades and dominated by the Phialocephala-Acephala complex. Representatives of these clades, plus an unknown basidiomycete with affinity to the genus Irpex (Polyporales), colonized V. uliginosum intracellularly. The Helotiales detected by direct PCR, cloning and sequencing were assigned to 14 clades and dominated by members of the Rhizoscyphus ericae complex. Ordination analyses indicated that culturedependent and culture-independent assays provided distinct views of root fungal communities, but no evidence for host specificity.• These data suggest that ericaceous roots host diverse fungal communities dominated by the Helotiales. However, these fungal communities are unlikely to be controlled by fungal host preferences. The mechanisms maintaining high diversity in root-symbiotic communities remain to be elucidated.

Sclerotinia sclerotiorum is an important pathogen of numerous crops in the North Central region of the United States. The objective of this study was to examine the genetic diversity of 145 isolates of the pathogen from multiple hosts in the region. Mycelial compatibility groups (MCG) and microsatellite haplotypes were determined and analyzed for standard estimates of population genetic diversity and the importance of host and distance for genetic variation was examined. MCG tests indicated there were 49 different MCGs in the population and 52 unique microsatellite haplotypes were identified. There was an association between MCG and haplotype such that isolates belonging to the same MCG either shared identical haplotypes or differed at no more than 2 of the 12 polymorphic loci. For the majority of isolates, there was a one-to-one correspondence between MCG and haplotype. Eleven MCGs shared haplotypes. A single haplotype was found to be prevalent throughout the region. The majority of genetic variation in the isolate collection was found within rather than among host crops, suggesting little genetic divergence of S. sclerotiorum among hosts. There was only weak evidence of isolation by distance. Pairwise population comparisons among isolates from canola, dry bean, soybean and sunflower suggested that gene flow between host-populations is more common for some crops than others. Analysis of linkage disequilibrium in the isolates from the four major crops indicated primarily clonal reproduction, but also evidence of genetic recombination for isolates from canola and sunflower. Accordingly, genetic diversity was highest for populations from canola and sunflower. Distribution of microsatellite haplotypes across the study region strongly suggest that specific haplotypes of S. sclerotiorum are often found on multiple crops, movement of individual haplotypes among crops is common and host identity is not a barrier to gene flow for S. sclerotiorum in the north central United States.

The extent to which interspecific plants share mycorrhizal fungal communities depends on the specificity of the symbiosis. For tropical forest tree seedlings, colonization by mycorrhizal fungi associated with established vegetation could have important consequences for survival and growth. I used a novel molecular technique to assess the potential for sharing of mycorrhizas in forest and pasture in southern Costa Rica, by identifying arbuscular mycorrhizal (AM) fungi in roots of the forest canopy tree species Terminalia amazonia, pasture grasses Urochloa ruziziensis and U. decumbens, and seedlings of T. amazonia planted into experimental reforestation plots. I tested the hypotheses that experimental seedlings were colonized either by the AM fungal community of the forest T. amazonia (suggesting host specificity) or of Urochloa (suggesting absence of specificity/importance of local environment). After two years, pasture-grown T. amazonia seedlings were colonized by neither community, but rather by a species of Glomus that was rarely observed on the other plants. These results suggest that conspecific seedlings planted into existing vegetation generate a distinct mycorrhizal community that may influence competitive interactions and the relative costs and benefits of the AM fungal symbiosis at early stages in the life cycle of tropical trees.