1. The densities of highly competent plant hosts (i.e. those that are susceptible to and successfully transmit a pathogen) may shape pathogen community composition and disease severity, altering disease risk and impacts. Life history and evolutionary history can 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.
2. We examined foliar fungal pathogens of two host groups in a California grassland: native perennial and non‐native annual grasses. We first characterized pathogen community composition and disease severity of 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.
3. Native perennial and non‐native annual grasses hosted distinct pathogen communities but shared generalist pathogens. Native perennial grasses experienced 26% higher disease severity than non‐native annuals. Only the observational gradient of native perennial grass density affected disease severity; there were no other significant relationships between host group density and either disease severity or pathogen community composition.
4. Synthesis. The life and evolutionary histories of grasses likely influence their competence for different pathogen species, exemplified by distinct pathogen communities and differences in disease severity. However, there was limited evidence that the density of either host group affected pathogen community composition or disease severity. Therefore, 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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加利福尼亚州草地上的原生多年生和非原生一年生草影响病原体群落组成和疾病严重程度

1. 高能植物宿主的密度（即易受病原体传播的病原体的密度）可能会影响病原体群落的组成和疾病的严重程度，从而改变疾病的风险和影响。生命史和进化史会影响宿主的能力。与寿命较短的物种相比，寿命较长的物种往往得到更好的保护，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。
2. 我们检查了加利福尼亚草地上两个寄主群体的叶真菌病原体：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1m ²地块中病原体群落组成和疾病严重性的影响。
3. 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。原生多年生草的病害严重性比非原生一年生草高26％。仅原生多年生草密度的观测梯度会影响疾病的严重性；宿主群密度与疾病严重程度或病原体群落组成之间没有其他显着关系。
4. 综合。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，例如不同的病原体群落和疾病严重程度的差异。但是，仅有有限的证据表明任一宿主组的密度都会影响病原体的群落组成或疾病的严重程度。因此，对不同病原体的能力可能会影响病原体群落的组成和疾病的严重程度，但可能不会与宿主的密度发生相互作用，从而在小规模上改变疾病的风险和影响。