Data from: Effects of microclimate on disease prevalence across an urbanization gradient

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 effects
of shade on 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 the prevalence of disease.