Identification of plant pathogens in high-tunnel tomato production in Minnesota, USA

Located between 43 and 50°N, the state of Minnesota, USA, averages 138.5 days above 0°C per year. Growing degree days range from 1300 to 2700. Growers of warm-season crops like tomato have a greater risk of crop loss due to frequent low temperature events at the beginning and end of the growing season. High tunnels are low-input, plastic-covered structures used to protect crops from damaging temperatures, wind, and rain. Temperature and humidity inside a high tunnel can differ significantly from field conditions, changing the profile of plant-pathogenic microorganisms that persist in the crop. A 2013 survey of 122 commercial high-tunnel growers found that 82% could identify fewer than 50% of the pest problems in their high tunnels without assistance; however, only 8% had ever submitted a plant sample for diagnosis. To determine which pathogens were present in high-tunnel tomato crops, 15 farms throughout the state were scouted by plant pathologists three times during the growing season in 2014 and 2015. Scouting trips were planned to coincide with established seedlings, mature flowering plants, and mature plants at harvest. At each visit, scouts examined the crop and took plant samples for diagnosis. In total, 18 known pathogens and two unknown viruses were recorded over the 2-year survey period using microscopic, serological and molecular methods.