Our lab studies the ecology and evolution of infectious diseases, with a focus on wild populations of herbaceous plant hosts and their powdery mildew pathogens. Here’s a brief description of two of our current projects.
Project 1: Pathogen overwintering along a latitudinal gradient
What happens to pathogens in winter? The answer to this question depends a lot on
(a) what winter conditions are like where the pathogen lives:
Is it very cold and snowy? Cold but not snowy? Somewhat cold with occasional ice storms? Or almost always above freezing?
(b) the availability of susceptible hosts during winter:
Do hosts die off, go dormant, use different habitats, or migrate south? Is their likelihood of pathogen encounter or susceptibility to infection different in winter vs. other seasons?
and (c) the pathogen’s lifestyle:
Does the pathogen require living host tissue at all times, or can it continue living on a dead host or in the environment? Does the pathogen rely on just one host species, or can it infect multiple species?
Some types of pathogens do very well in winter. In the US, cases of human influenza peak during winter, partly because we spend more time in contact with each other indoors, and partly because flu viruses can linger in the air when humidity is low.
Other pathogens like the anthrax bacterium can persist in frozen permafrost for decades, then become infectious if the soil warms. This has recently been a problem in Siberia, where anthrax frozen in historic ungulate (reindeer, horses, cattle) burial sites has been released by global warming, and now threatens people and livestock living there (see also: this paper).
However, for most diseases, we know very little about the role that cold temperatures and snow play in determining the size of epidemics or diversity of infecting strains. Understanding how winter climate affects the prevalence and evolutionary potential of pathogens is especially important because global warming is changing winter temperature and snowfall patterns. Changes in winter conditions affect food webs through many different mechanisms, and understanding the mechanisms involving hosts and their pathogens is critical for anticipating how climate change will impact health of humans, wildlife, livestock, and the crop plants we rely on.
Our study system
We’re focusing on the role of winter in driving the prevalence and diversity of powdery mildew infections in the widely distributed perennial weeds of the genus Plantago (common name = plantains…but the rosette-forming lawn weeds, not the banana cultivar that is delicious when fried).
We’re studying Plantago because
(a) they’re convenient (easy to find in nature, easy to grow in a greenhouse from seed or by cloning, no one minds if you infect a whole bunch of them in an experiment),
(b) they’re well-studied for their ecology and chemistry (including their historical distribution in the pollen record, their population ecology, and their medicinal properties),
and most of all, (c) they’re ecologically very interesting (they grow in habitats ranging from city sidewalks and parks to agricultural fields and meadows, there are both native and introduced Plantago species here in the US, they are food for butterfly caterpillars, and they get infected by lots of different pathogens).
Powdery mildews are fungal pathogens that can only grow on living plant tissue. Some powdery mildew species can infect many species of plant hosts, and others are highly specific to just one plant species. Powdery mildews produce repeated generations of asexual (clonal) transmission spores (called “conidia”) as long there is susceptible host tissue available and environmental conditions are appropriate. These spores are wind-transmitted, and you may have seen them as white powder covering the leaves of garden plants including gourds, melons, roses, and peonies.
Winter can be a major challenge for obligate parasites like powdery mildews. In places where the host plants die back to rootstock in winter, powdery mildews survive through production of specialized resting structures (called “chasmothecia”). These resting structures can be produced by selfing or outcrossing — thus, they may be a source of genetic variation for the pathogen population. However, production and survival of viable resting structures is highly variable over space and time. By insulating powdery mildew resting structures when temperatures are freezing, snow cover might be one factor determining pathogen overwinter persistence.
We are studying Plantago species and their powdery mildew pathogens along a latitudinal gradient in the central US spanning a huge range in average annual minimum temperature (see map below) as well as snow cover (see Fig. 4 of this paper).
We are surveying natural populations to investigate how the prevalence of disease and diversity of pathogen strains vary along this latitudinal gradient. This observational work is paired with laboratory and field experiments (e.g., at the Tyson Research Center) designed to tease apart how host and pathogen traits, as well as ecological and environmental factors, drive this variation. Statistical and mathematical models are key tools we can use to delineate sources of variation in disease prevalence and diversity, and to assess whether the mechanisms we test experimentally explain larger patterns that we observe in the field.
Project 2: Does climate change affect the interplay between soil microbes and aboveground plant enemies?
Grassland systems are of major importance to human societies, providing many crucial ecosystem services such as the production of food, biofuels, and climate regulation. The provision of such ecosystem services depends on complex interactions among organisms and their environment. Plants are attacked by pathogens and herbivores, both above- and belowground. Those antagonistic interactions can have strong impacts on individual plant growth, survival and health, as well as important consequences at the community and ecosystem level. Our knowledge of the ecological processes underpinning these relationships and the ways in which these interactions will change under ongoing habitat loss and global change is very limited, thereby threatening our ability to manage ecosystems in a sustainable way that secures the retention of valuable ecosystem services.
Our study will address how above- and belowground pathogens interact to influence plant community health under changing precipitation regimes, and how plant diversity will mediate those interactions. We will test 1) whether plant diversity (species richness, phylogenetic diversity), soil biota, and water availability interactively affect plant susceptibility to damage by a wide range of aboveground pathogens and herbivores in a large outdoor mesocosm experiment and 2) how these same variables interactively affect plant health and performance in a controlled inoculation study using a subset of the aboveground fungal pathogens. We will take an important step forward, moving traditional single-plant-single-pathogen studies towards a community level approach. Our results will have important applications for habitat restoration and biofuel production, and offer important pilot data for more ambitious funding requests.