COS 76-4 - Warming increases severity and host range for a suite of plant pathogens

Abstract: Fungal pathogens play a key role in productivity of both agricultural and wild plant ecosystems. In agricultural systems, soilborne fungal pathogens reduce crop production; in wild systems, soilborne pathogens can govern plant diversity and productivity via differential effects on co-occurring plant species. Both global food security and ecosystem function rely on our understanding of how fungal pathogens and their hosts will respond to climate change. We know that pathogen abundance and thus disease risk increase at warmer temperatures; however, the impacts on host survival are unclear. Moreover, if host species become more susceptible at warmer temperatures, increased severity of disease caused by generalist pathogens may be accompanied by expansions in pathogen host breadth. In this study, we asked: does increasing temperature alter disease severity and host breadth of soil-borne pathogens that attack seeds?

We cultured fungi from seeds buried for one year in an experimentally fragmented landscape. Following sequencing (ITS1F-ITS4) we selected 10 frequently-occurring putative pathogens and assayed pathogenicity against 12 potential host plant species at two temperatures. For replication, we used 4-12 unique fungal isolates from different locations across the landscape. We compared seed germination success under two temperature conditions, hereafter “cool”: 20℃ average (day 22℃, night 16℃) and “warm”: 25℃ average (day 27℃, night 21℃).

Under cool conditions, five of the 10 fungi were pathogenic, causing statistically significant reductions in seed germination (relative to controls) for 3-5 plant species (p< 0.05, binomial glm). Under warm conditions, all 10 fungi were pathogenic on at least one plant species (p< 0.05, binomial glm). All fungi that were pathogenic at cooler temperatures expanded their host breadth to parasitize more seed species. On average, seed species parasitized at both temperatures suffered more severe losses to disease at warmer temperatures. Five of the 10 fungal taxa grew faster at warmer temperatures (p< 0.05, paired t-tests), and at warm temperatures, fungal growth rate caused more severe disease across host species (t=-3.70, p< 0.01, R2=0.58,). Some seeds germinated more rapidly at higher temperatures, but neither germination timing nor seed size was related to disease susceptibility in either temperature condition (p >0.05, multiple linear regression).

Overall, both host range and disease severity increased when temperatures increased. Our findings suggest that as global temperatures rise, fungal pathogens will not only be more abundant, but also could have more severe and widespread impacts across plant species.