PS 43-177 - Sea water intrusion influences Phytophthora abundance in Southern Maryland coastal soils

Abstract: Sea level rise is changing the ecology and disease dynamics in coastal forests due to frequent flooding and the increased presence of salinity in the soil. The oomycete Phytophthora spp is responsible for many plant diseases that have had significant environmental and economic consequences. Recently, invasive P. ramorum has been detected in Maryland, raising concerns about the health of native coastal flora, particularly under climate change. Increased salinity in soil imposes similar physiological stress as drought does in native plants. Therefore, we explored whether seawater intrusion would promote higher abundances of Phytophthora spp in coastal soils of Southern Maryland due to potentially having higher plant infections caused by salinity stress. Soil samples were collected from four different sites across a natural salinity gradient on the Potomac River at two different times of the year. One collection was performed in spring when Phytophthora spp is likely to be more active, and at the end of summer when hurricane season starts on the Atlantic coast and there is likely to be increased seawater intrusion. The abundance of Phytophthora spp was determined by performing serial dilutions of soil samples, plating them on the highly selective PARPH media, and incubating them at 25ºC. Colonies were counted after 4 days of incubation and standardized into CFUs/gram of soil (CFUs/g) based on the dilution factor. We performed a Two-Way ANOVA to test for site, season, and their interaction in the number of CFUs/g after checking for normality and heteroscedasticity. We found that the abundance of Phytophthora spp varied by site depending on the season that the soil was collected (F(3,50)=331.08, P < 0.01). In early spring we saw a more clear pattern on the effects of salinity, where sites with high to mid-salinity had higher abundances (F(4,38)=33.78, P < 0.01). In late summer this pattern was not evident. Instead, the highest CFUs/g were observed in soils collected from sites with the lowest and highest salinity concentrations (F(3,8)=82.5, P < 0.01). Our results suggest that seawater intrusion and seasonality can modify patterns of Phytophthora spp abundances in coastal soils of Southern Maryland. As sea level rise increases, Phytophthora spp could become more abundant, signaling the importance of mitigation efforts to protect the health of native Maryland plant species.