For the past three decades, leaf spots have been generally considered of minor importance in the USA, thus they were given the name "minor leaf spots" in the 1960's. However, leaf spots can substantially reduce cotton yield, fiber, and seed quality in epidemics. By the time a well grown cotton crop reaches the stage of fruit production, it has a dense canopy of large leaf surface area. The dense canopy encourages a humid micro-climate within the crop, creating an environment where large numbers of phylloplane micro-organisms can flourish. Many of the organisms are non-pathogenic or they are secondary invaders of necrotic and insect damaged tissue, but about 20 fungi have been reported as primary leaf spot pathogens of the cotton plant. Only some of these pathogens, in varying combinations, would be found in the crop at any one geographical location, although lesions caused by several fungi and the bacterial blight pathogen may often be present on a single leaf. Which of these fungi can be considered as major or minor pathogens often depends on environmental conditions, management practices, and host genotype.