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Biodiversity is thought to help regulate the impacts of disease through the dilution effect, where biodiversity among potential host species helps limit the impacts of pathogens. However, our knowledge is fragmentary about the direction and magnitude of the effects of plant species richness on disease impact. Here, we gathered data from 145 comparisons presented in 21 papers to conduct a systematic meta‐analysis on the effect of plant species richness on aboveground plant disease impact. We estimated the effect size using Pearson's correlation coefficient (r) with Fisher's z‐transformation. We evaluated how the magnitude of effect size varies between systems, including ecosystem type (grassland versus forest), pathogen taxon (virus versus fungus), study design (observational versus manipulative), parasite life history (biotroph versus necrotroph) and kinds of symptoms associated with the disease. We also tested whether there was a latitudinal trend of the effect size. We found there was a significant overall dilution effect in plant communities, but the magnitude varied among systems. Studies based on manipulative experiments and those in grassland ecosystems showed a significant dilution effect, as did both viral and fungal pathogens. Furthermore, obligate biotrophic pathogens but not necrotrophs showed a significant dilution effect. Diseases with different kinds of symptom manifestation differed, but not in a consistent pattern to the life history of the pathogens. The dilution effect was notably stronger at lower latitudes in the mid‐temperate region than at higher latitudes. This latitudinal trend existed in forest ecosystems, both observational and manipulative experiments, and necrotrophs. Dilution effects occur prevalently in plant communities, although the magnitude depends on ecosystem type, pathogen life history and kinds of symptoms associated with the disease. In conclusion, this study shows the importance of preserving the biodiversity of plants for maintaining ecosystem health.

This study assessed the levels of damage by leaf pathogens and their variability in terms of host species, space (four mature forest sites) and season of the year (dry and rainy), and the mechanisms of infection in the understory of the Los Tuxtlas tropical rain forest. Sixty-five percent of the species surveyed in the dry season (N = 49) and 64.9% of those surveyed in the rainy season (N = 57) were damaged by fungi. Leaf area damaged per plant, on average, was <1% (range: 0.25-20.52%). There was considerable variation in the degree of infection among species, but not among sites and seasons. The survey showed that 43% of the leaves were damaged by herbivores and pathogens concurrently, 16% showed damage by insect herbivory alone, and only 1.4% of the sampled leaves showed damage by pathogens alone. Pathogenicity assays experimentally confirmed that the predominant mechanism of fungal establishment was wounding, such as that caused by herbivory (or other similar sources), and only rarely did infection occur through direct contact (without wounds). The results revealed the omnipresence of leaf fungal infection, although with low damage per plant, and the importance of herbivorous insects in the facilitation of fungal infection in tropical understory plants.

1106I.1107II.1109III.1109IV.1110V.1111VI.1111VII.1113VIII.1114IX.1114X.11161117References1117 Summary Plant pathogens affect the fitness of their hosts and maintain biodiversity. However, we lack theories to predict the type and intensity of infections in wild plants. Here we demonstrate using fungal pathogens of tropical plants that an examination of the life histories of hosts and pathogens can reveal general patterns in their interactions. Fungal infections were more commonly reported for light‐demanding than for shade‐tolerant species and for evergreen rather than for deciduous hosts. Both patterns are consistent with classical defence theory, which predicts lower resistance in fast‐growing species and suggests that the deciduous habit can reduce enemy populations. In our literature survey, necrotrophs were found mainly to infect shade‐tolerant woody species whereas biotrophs dominated in light‐demanding herbaceous hosts. Far‐red signalling and its inhibitory effects on jasmonic acid signalling are likely to explain this phenomenon. Multiple changes between the necrotrophic and the symptomless endophytic lifestyle at the ecological and evolutionary scale indicate that endophytes should be considered when trying to understand large‐scale patterns in the fungal infections of plants. Combining knowledge about the molecular mechanisms of pathogen resistance with classical defence theory enables the formulation of testable predictions concerning general patterns in the infections of wild plants by fungal pathogens.

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