Tolerance to a disease is generally defined as the ability to be productive in the presence of disease-causing organisms. This is contrary to resistance, which is defined as the ability to completely evade a pathogen due to specific resistance mechanisms. The question as to what defines an HLB-“tolerant” rootstock was posed in an article by Bill Castle, Jude Grosser, Ed Stover and Kim Bowman in the June 2015 issue of Citrus Industry magazine. The article describes observations from several rootstock field trials conducted in collaboration with commercial growers. Independent surveys of these trials conducted by researchers and growers on tree appearance and crop ratings indicated that no one rootstock appears to be tolerant 100 percent of the time. Rather, tree performance seemed erratic and subject to influences by other factors such as psyllid incidence, cultural practices and soil environment.