The densities of highly competent plant hosts may shape pathogen community composition and disease severity, altering disease risk and impacts. Life history and evolutionary history influence host competence: longer-lived species tend to be better defended than shorter-lived species and pathogens adapt to infect species with which they have longer evolutionary histories. It is unclear, however, how the densities of species that differ in competence due to life and evolutionary histories affect plant pathogen community composition and disease severity. We examined foliar fungal pathogens on two host groups in a California grassland: native perennial and non-native annual grasses. We first characterized pathogen community composition and disease severity on the two host groups to approximate differences in competence. We then used observational and manipulated gradients of native perennial and non-native annual grass densities to assess the effects of each host group on pathogen community composition and disease severity in 1-m2 plots. Native perennial and non-native annual grasses hosted distinct pathogen communities but shared generalist pathogens. Native perennial grasses hosted pathogens with larger host ranges and experienced greater disease severity (75% higher proportions of leaves with lesions) than non-native annuals. While the relative abundances of three common pathogens tended to shift, and disease severity of native perennial grasses tended to increase with increasing densities of both host groups, these changes were not statistically significant. Synthesis. The life and evolutionary histories of grasses likely influence their competence for different pathogen species, leading to distinct pathogen communities and differences in disease severity. However, there was no evidence that the density of either host group significantly affected pathogen community composition or disease severity. Therefore, variation in competence for different pathogens likely shapes pathogen community composition and disease severity but may not interact with host density to alter disease risk and impacts at small scales.

中文翻译：

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加利福尼亚州草原上的原生多年生草和非原生一年生草影响病原体群落组成和疾病严重程度

能力强的植物寄主的密度可能会影响病原体的群落组成和疾病的严重程度，从而改变疾病的风险和影响。生命史和进化史影响宿主的能力：寿命长的物种比生存时间短的物种受到更好的保护，病原体适应具有较长进化历史的感染物种。但是，尚不清楚由于生命和进化史而导致能力不同的物种的密度如何影响植物病原体群落组成和病害严重程度。我们在加利福尼亚草原的两个寄主种群上检查了叶真菌病原体：原生多年生草和非原生一年生草。我们首先对两个宿主组的病原体群落组成和疾病严重程度进行了表征，以评估能力差异。然后，我们使用观测的和经操纵的多年生和非原生年生草密度梯度来评估每个宿主组对1-m2地块中病原体群落组成和疾病严重性的影响。原生多年生和非原生一年生草有不同的病原体群落，但共有通病菌。与非本地一年生草本植物相比，本地多年生草具有更大的寄主范围病原体，并且病害严重程度更高（有病害的叶子比例高出75％）。尽管三种常见病原体的相对丰度趋于变化，而原生多年生草的病害严重程度则随两个寄主群体密度的增加而趋于增加，但这些变化在统计学上并不显着。合成。草的生活史和进化史可能会影响它们对不同病原体种类的能力，从而导致不同的病原体群落和疾病严重程度的差异。但是，没有证据表明任一宿主组的密度都会显着影响病原体群落组成或疾病严重程度。因此，不同病原体的能力差异可能会影响病原体群落组成和疾病严重程度，但可能不会与宿主密度相互作用，从而在较小规模上改变疾病风险和影响。