The fungus, Alternaria alternata (Fr.) Keissler Strain 501, has been evaluated as a bioherbicide for control of Eupatorium adenophorum Spreng., but the biology of the pathogen-host interaction and the optimal environmental conditions for disease development and effective weed control are unknown. Disease development of A. alternata Strain 501 mycelia on E. adenophorum was assessed under several factors including pathogen inoculum concentration, plant age, dew period duration, post-dew temperature, storage temperature and duration. The minimum inoculum concentration required to kill E. adenophorum seedlings was 3.2x10(6) mycelial fragment mL-1. E. adenophorum seedlings at the four-leaf-pair stage were more susceptible than the older plants, especially those at the older than seven-leaf-pair stage. With a dew period of at least 14 h, 100% mortality occurred. The optimal post-dew temperature for disease development was 18-25°C. Storage at <4°C maintained the infectivity of A. alternata strain 501 mycelia on E. adenophorum longer. Using light and scanning electron microscopy to examine the infection process of A. alternata Strain 501 mycelia, it was shown that the time from initiation to completion of infection with mycelia was much shorter (14 h) than with conidia (72 h). It was further shown that mycelial infection occurred predominately through direct penetration at intercellular junctions, while conidial infection occurred predominately through stomatal penetration. This suggests that mycelia are more suitable as infection propagules for A. alternata strain 501 in a bioherbicide for the control of E. adenophorum.