Effects of Simulated Rainfall on Disease Development and Weed Control of the Bioherbicidal Fungi Alternaria cassiae and Colletotrichum truncatum

Alternaria cassiae and Colletotrichum truncatum are bioherbicidal pathogens of sicklepod, and hemp sesbania, respectively. The effects of simulated rainfall followed by 12 h simulated dew application, immediately or delayed by 1 to 4 h, on disease severity and weed control were studied for each pathogen on its weed host under greenhouse conditions. After each simulated rainfall event, treated plants were placed in a dew chamber for 12 h. Regardless of rainfall amount and/or timing, only slight differences occurred on A. cassiae disease severity and sicklepod control (85 to 100% for both parameters). However, when similar tests were imposed on C. truncatum, disease severity and hemp sesbania control were highly variable, ranging from 5 to 100%. Regardless of rainfall amount, disease development and control of hemp sesbania were greatly reduced (60%) when dew application was delayed by only 1 h following inoculation, regardless of rainfall treatment. Rainfall at 1.27 and 2.58 cm had little effect on disease development and control in hemp sesbania, but the effect of transfer time to dew application exhibited a greater role on these parameters. Thus the time between bioherbicide application and dew application was more important for C. truncatum than for A. cassiae. These results indicate that rainfall amounts and the timing of dew application caused differential effects on disease severity and weed control after application of these bioherbicides to their target weeds.