Species densities, assembly order, and competence jointly determine the diversity-disease relationship.

Since species vary in abundance and host competence (i.e., ability to get infected and transmit a pathogen), changes in species composition caused by biodiversity loss impacts disease dynamics. Forecasting effects of species composition on disease depends on community (dis)assembly, processes determining how species are added to (or lost from) communities. We simulated community assembly by planting mesocosms, nested along a richness gradient, and tested how relationships between richness, species assembly order, and overall density affect disease risk. Mesocosms with up to six crop species of varying competence were inoculated with a soilborne fungal pathogen, Rhizoctonia solani. Disease was measured as species-level prevalence, community-level prevalence, and total number of diseased plants. Regardless of metric, richness limited disease when species assembly order negatively correlated with competence and total density remained unchanged with richness. When density increased with richness or species assembled randomly, richness primarily correlated positively or weakly with disease. Our results align with theoretical expectations and represent the first empirical study to test the influence of species densities, assembly order, and competence on diversity-disease relationships.

Community-level prevalence of a forest pathogen, not individual-level disease risk, declines with tree diversity.

Understanding why diversity sometimes limits disease is essential for managing outbreaks; however, mechanisms underlying this 'dilution effect' remain poorly understood. Negative diversity-disease relationships have previously been detected in plant communities impacted by an emerging forest disease, sudden oak death. We used this focal system to empirically evaluate whether these relationships were driven by dilution mechanisms that reduce transmission risk for individuals or from the fact that disease was averaged across the host community. We integrated laboratory competence measurements with plant community and symptom data from a large forest monitoring network. Richness increased disease risk for bay laurel trees, dismissing possible dilution mechanisms. Nonetheless, richness was negatively associated with community-level disease prevalence because the disease was aggregated among hosts that vary in disease susceptibility. Aggregating observations (which is surprisingly common in other dilution effect studies) can lead to misinterpretations of dilution mechanisms and bias towards a negative diversity-disease relationship.

Sporulation Potential of Phytophthora ramorum Differs Among Common California Plant Species in the Big Sur Region.

Sudden oak death (SOD), caused by the generalist pathogen Phytophthora ramorum, has profoundly impacted California coastal ecosystems. SOD has largely been treated as a two-host system, with Umbellularia californica as the most transmissive host, Notholithocarpus densiflorus less so, and remaining species as epidemiologically unimportant. However, this understanding of transmission potential primarily stems from observational field studies rather than direct measurements on the diverse assemblage of plant species. Here, we formally quantify the sporulation potential of common plant species inhabiting SOD-endemic ecosystems on the California coast in the Big Sur region. This study allows us to better understand the pathogen's basic biology, trajectory of SOD in a changing environment, and how the entire host community contributes to disease risk. Leaves were inoculated in a controlled laboratory environment and assessed for production of sporangia and chlamydospores, the infectious and resistant propagules, respectively. P. ramorum was capable of infecting every species in our study and almost all species produced spores to some extent. Sporangia production was greatest in N. densiflorus and U. californica and the difference was insignificant. Even though other species produced much less, quantities were nonzero. Thus, additional species may play a previously unrecognized role in local transmission. Chlamydospore production was highest in Acer macrophyllum and Ceanothus oliganthus, raising questions about the role they play in pathogen persistence. Lesion size did not consistently correlate with the production of either sporangia or chlamydospores. Overall, we achieved an empirical foundation to better understand how community composition affects transmission of P. ramorum.

Survey of corticioid fungi in North American pinaceous forests reveals hyperdiversity, underpopulated sequence databases, and species that are potentially ectomycorrhizal.

The corticioid fungi are commonly encountered, highly diverse, ecologically important, and understudied. We collected specimens in 60 pine and spruce forests across North America to survey corticioid fungal frequency and distribution and to compile an internal transcribed spacer (ITS) database for the group. Sanger sequences from the ITS region of vouchered specimens were compared with sequences on GenBank and UNITE, and with high-throughput sequence data from soil and roots taken at the same sites. Out of 425 high-quality Sanger sequences from vouchered specimens, we recovered 223 distinct operational taxonomic units (OTUs), the majority of which could not be assigned to species by matching to the BLAST database. Corticioid fungi were found to be hyperdiverse, as supported by the observations that nearly two-thirds of our OTUs were represented by single collections and species estimator curves showed steep slopes with no plateaus. We estimate that 14.8-24.7% of our voucher-based OTUs are likely to be ectomycorrhizal (EM). Corticioid fungi recovered from the soil formed a different community assemblage, with EM taxa accounting for 40.5-58.6% of OTUs. We compared basidioma sequences with EM root tips from our data, GenBank, or UNITE, and with this approach, we reiterate existing speculations that Trechispora stellulata is EM. We found that corticioid fungi have a significant distance-decay pattern, adding to the literature supporting fungi as having geographically structured communities. This study provides a first view of the diversity of this important group across North American pine forests, but much of the biology and taxonomy of these diverse, important, and widespread fungi remains unknown.