- Nursery and greenhouse products are economically important agricultural commodities for Oregon and Washington. Nursery crops including tree seedlings can be damaged or killed by soilborne plant pathogens, which can be challenging to manage because of their persistence in soil and limited options for control. Soil solarization is a pre-planting soil treatment that employs solar radiation to heat soil under a transparent plastic film to achieve temperatures detrimental to certain soilborne pathogens and weed seeds. Soil solarization has also been shown to influence soil microbial communities and crop yields. Changes in soil nutrient status (especially in nitrogen) have been postulated as a possible mechanism for enhanced crop growth following solarization. Nitrate (NO3-) and ammonium (NH4+) are forms of nitrogen in soil that can be used by plants and are referred to collectively as inorganic N. The objectives of this research project were to 1) determine how soil solarization influences certain soilborne plant pathogens, inorganic N and microbial communities in Oregon and Washington tree seedling nurseries; 2) determine how soil moisture and the duration of solarization influence the soil microbiome and inorganic N in soil; and 3) determine the thermotolerance of certain common soilborne or root-infecting phytopathogens: Fusarium oxysporum, Pythium ultimum, Phomopsis sp., and Ilyonectria sp. Replicated field trials with solarized and non-solarized plots were conducted at three tree seedling nurseries in July-August, 2016 and 2017. At one site, a factorial experiment with four levels of soil moisture and three durations of soil solarization (3-, 6-, and 9- weeks) was also conducted in July-September, 2016 and 2017. At each site, nylon mesh sachets containing homogenized composite soil samples from that site were buried at 5 and 15 cm depths. Soil temperatures were monitored, and sachets were retrieved after six weeks of treatment. Naturally occurring populations of Fusarium oxysporum and Pythium spp. in sachet soils were quantified through dilution plating soil onto selective media. To assess solarization impacts on the soil microbiome, PCR was performed with DNA extracted from sachet soils to obtain 16S (bacterial) and ITS1 (fungal and oomycete) amplicons, which were then sequenced with the Illumina Miseq platform. Sequence data were compared to 16S (bacteria) and ITS (fungi and oomycete) reference databases for microbial identification and community analysis. Effects of solarization on soil nutrients were determined at the conclusion of 2017 trials and the following spring (April 2018). Soil collected from the 0-15 cm depth was tested for plant available nitrogen, soil pH, and total C and N. Results from soil dilution plating demonstrated that in solarized plots, Fusarium oxysporum and Pythium spp. were not recovered from the 5 cm depth, and populations were generally reduced at the 15 cm depth compared to non-solarized plots, although the differences between solarized and non-solarized population densities at 15 cm were not always statistically significant. Compared to the non-solarized treatment, solarization resulted in reduced species richness and Simpsons’ diversity at the 5 cm depth but not at the 15 cm depth for all microbial communities (bacteria, fungi, and oomycetes), likely reflecting the greater soil heating near the soil surface. Increasing durations of solarization decreased species richness of bacteria, fungi and oomycetes during both years of the study, which could result in thermal inactivation due to longer exposure to high temperatures. Soil moisture level and duration of solarization both significantly influenced all communities in the first year of the study, however in the second year of the study, soil bacterial communities were not influenced by the duration of solarization. Soil nutrient testing demonstrated that soil solarization significantly influenced plant available nitrogen in both field studies (Chapter 2 and 3), however this effect did not persist until the following spring. Ammonium concentrations were increased by solarization at all sites immediately following solarization, but only at one site in spring 2018. Nitrate concentrations were greater in non-solarized soil than solarized soil at two of the three sites immediately following solarization, but not the following spring. The duration of solarization and soil moisture levels during solarization resulted in significant differences in nitrate and ammonium concentrations immediately following trials, however there was a clearer trend with soil nitrate concentrations, which were also influenced by the interaction between moisture and duration. Nitrate was found in higher concentrations in plots that were solarized for longer periods of time, however this was especially true at lower soil moisture levels. At higher soil moisture levels, the effect of duration was less drastic, likely due to oxygen limitations on aerobic nitrifying bacteria and archaea. It is unlikely that changes in soil nutrients could account for the greater crop growth observed in solarized vs. non-solarized plots, because there were no apparent differences in plant available nitrogen by the time crop seeds germinated the following spring. In the controlled environment studies at constant temperature, estimates of the duration of exposure that would be lethal to 99.9% of propagules of each pathogen at 45, 50, and 55°C indicated that Fusarium oxysporum was the most heat tolerant of the organisms tested, followed by Phomopsis sp., Pythium ultimum, and Ilyonectria sp. For example, the effective time period to inactivate each organism at 55°C was 65.2 hours, 9.7 hours, 81.1 minutes, and 69.0 minutes, respectively. Although the duration of necessary exposure differed between organisms tested, results indicate that soil temperatures encountered during solarization could be lethal to certain soilborne pathogens. Results from controlled environment studies will be incorporated into an existing online grower model to predict pathogen mortality given predicted soil temperatures in different locations. Laboratory and field trial results demonstrate that soil solarization has potential in managing certain soilborne plant pathogens in Oregon and Washington, which are in regions previously described as marginally suited for soil solarization. Incorporating soil solarization into nursery production systems could help reduce dependence on soil fumigants, herbicides, fungicides, and synthetic fertilizers.