Technical Abstract: Alternaria cassiae and Colletotrichum truncatum are virulent pathogens of sicklepod (Senna obtusifolia), and hemp sesbania (Sesbania exaltata), respectively, under favorable environmental conditions. In greenhouse experiments, the effects of simulated rainfall on pathogenesis and mortality of these weeds were examined using a rainfall simulator apparatus that reproduced water droplet size, velocity, and kinetic energy analogous to natural rainfall. With A. cassiae little differences occurred between rainfall amounts or timings upon pathogenesis or weed control efficacy of sicklepod, with 85-100% control occurring within 8 d, regardless of dew treatment. With C. truncatum, pathogenesis and mortality of hemp sesbania were significantly greater in plants that received no rainfall followed by an immediate dew treatment. However, regardless of rainfall amounts, disease development and mortality of hemp sesbania were greatly reduced (60%) when dew treatment was delayed by 1 h following inoculation, regardless of rainfall treatments. A 4 h delay in dew treatment was not significantly different than untreated controls. These results indicate that effects of rainfall vary greatly on pathogenesis and weed control between mycoherbicides.