Increased temperatures associated with urbanization (the “urban heat island” effect) have been shown to impact a wide range of traits across diverse taxa. At the same time, climatic conditions vary at fine spatial scales within habitats due to factors including shade from shrubs, trees, and built structures. Patches of shade may function as microclimate refugia that allow species to occur in habitats where high temperatures and/or exposure to ultraviolet radiation would otherwise be prohibitive. However, the importance of shaded microhabitats for interactions between species across urbanized landscapes remains poorly understood. Weedy plants and their foliar pathogens are a tractable system for studying how multiple scales of climatic variation influence infection prevalence. Powdery mildew pathogens are particularly well suited to this work, as these fungi can be visibly diagnosed on leaf surfaces. We studied the effects of shaded microclimates on rates of powdery mildew infection on Plantago host species in (1) “pandemic pivot” surveys in which undergraduate students recorded shade and infection status of thousands of plants along road verges in urban and suburban residential neighborhoods, (2) monthly surveys of plant populations in 22 parks along an urbanization gradient, and (3) a manipulative field experiment directly testing the effects of shade on the growth and transmission of powdery mildew. Together, our field survey results show strong positive effects of shade on mildew infection in wild Plantago populations across urban, suburban, and rural habitats. Our experiment suggests that this relationship is causal, where microclimate conditions associated with shade promote pathogen growth. Overall, infection prevalence increased with urbanization despite a negative association between urbanization and tree cover at the landscape scale. These findings highlight the importance of taking microclimate heterogeneity into account when establishing links between macroclimate or land use context and prevalence of disease.

中文翻译：

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小气候对城市化梯度疾病患病率的影响

与城市化相关的气温升高（“城市热岛效应”）已被证明会影响不同类群的广泛特征。与此同时，由于灌木、树木和建筑结构的遮荫等因素，栖息地内的气候条件在精细的空间尺度上发生变化。阴影斑块可能起到微气候避难所的作用，使物种能够在高温和/或暴露于紫外线辐射的栖息地中生存。然而，遮荫微生境对于城市化景观中物种之间相互作用的重要性仍然知之甚少。杂草植物及其叶病原体是一个易于处理的系统，用于研究多种尺度的气候变化如何影响感染流行。白粉病病原体特别适合这项工作，因为这些真菌可以在叶子表面明显地诊断出来。我们研究了遮荫小气候对白粉病感染率的影响车前子(1)“大流行枢纽”调查中的宿主物种，其中本科生记录了城市和郊区住宅区道路边缘数千种植物的遮荫和感染状况，(2)对城市化梯度沿线 22 个公园的植物种群进行每月调查， (3)直接测试遮荫对白粉病生长和传播影响的操作性田间试验。总之，我们的实地调查结果表明，遮荫对野生霉菌感染有强烈的积极影响车前子城市、郊区和农村栖息地的人口。我们的实验表明，这种关系是因果关系，即与遮荫相关的小气候条件促进了病原体的生长。总体而言，尽管城市化与景观尺度上的树木覆盖率呈负相关，但感染率随着城市化而增加。这些发现强调了在建立大气候或土地利用环境与疾病流行之间的联系时考虑小气候异质性的重要性。