COS 243-4 - Thermal tolerance of the anther-smut fungus, Microbotryum lychnidis-dioicae: High temperatures reduce growth, infection, and disease expression

Abstract: Pathogen spread can be modeled with the disease triangle, where host susceptibility, pathogen virulence, and a favorable environment all impact whether a pathogen can spread throughout a population. Warming temperatures therefore have the potential to change host-pathogen interactions, though we need to understand the underlying thermal physiology in order to build predictive models. We investigated the thermal tolerance of the fungus Microbotrum lychnidis-dioicae, which causes a sterilizing anther-smut disease in the herbaceous plant Silene latifolia. Anther smut is more prevalent at high elevations, suggesting a low thermal tolerance of the fungus. While the disease has become a model system in ecology, the basic thermal physiology of the pathogen has yet to be investigated. We investigated the role of temperature on fungal physiology and the expression of disease in three ways: First, we measured the thermal tolerances of three different pathogen life-history traits (germination rate, growth rate, and conjugation rate of spores) in vitro. Next, we measured infection rates following experimental inoculation of anther smut on its host plant Silene latifolia at three different temperatures. Finally, we assessed the correlation between temperature and the expression of disease in 145 infected hosts planted in a common garden. We found that temperatures above 30 °C reduced the ability of anther-smut spores to germinate, grow, and conjugate in vitro. Additionally, high temperatures (30 °C) at or 4-days after the time of inoculation reduced the likelihood of infection by 75%. Finally, we found that mid-summer high temperatures in the field temporarily cured symptoms of infected plants. Overall, our results show that high temperatures limit the ability of the fungus to grow, infect and transmit. These findings help explain the observed pattern of higher prevalence at colder elevations and latitudes and indicate that disease prevalence will likely decline as temperatures increase.