Abstract: This study was aimed at testing the hypothesis that lettuce corky root (CR) disease caused by Rhizorhapis suberifaciens was less severe in organic than conventional farms, due to the absence of herbicide and fertilizer, and greater soil microbial activity in organic farms. CR severity and soil quality were assessed in pairs of conventional and organic farms in California. To determine factors contributing to CR, effects of N fertilizer and pronamide herbicide were assessed on CR severity and plant weight in separate field experiments. CR was significantly more severe in conventional than organic farms, and there was a negative exponential relationship between CR severity and microbial activity. Split applications of soluble N fertilizer enhanced susceptibility to CR compared to pre-plant application of slow release N fertilizer. Pronamide increased disease severity on seedlings compared to untreated controls and reduced the dry weights of seedlings and mature heads. Conventional practices, like fertilizer and herbicide use, increase plant susceptibility to and reduce microbial competition or antibiosis against R. suberifaciensin conventional lettuce production farms, potentially leading to enhanced environmental pollution due to a decrease in nutrient use efficiency and an increased need for fertilizer and water for diseased plants. [Ariena H. C. van Bruggen, Isolde M. Francis & Randy Krag (2014). The vicious cycle of lettuce corky root disease: effects of farming system, nitrogen fertilizer and herbicide. Plant and Soil, online 30 Oct] Comment