Abstract: Sex expression in the striped maple (Acer pensylvanicum) is determined by the environment rather than by genetic instructions.  This descriptive study focuses on the effects of a fungal pathogen, the tarspot (Rhytisma punctatum), on the sexual expression and amount of growth of infected trees.  Severity of infection and morphology are examined on a modular level.  The research is divided into two parts: disease effects between years and disease effects on seed production within a year.  Disease in 1998 correlated to the probability of a module becoming pruned or living  in 1999 as well as the probability of being male or vegetative.  Disease in 1999 was much lower than in previous years: differing by a power of ten.  Perhaps for this reason, unexpected results were found correlating higher disease with greater seed production.  The importance of photosynthetic seed wing area to production was also examined and presents more questions for further study.