Two major limiting factors that have an impact on mycoherbicides are temperature and moisture requirements (Auld and Morin 1995; Makowski 1997; Mortensen 1998). TeBeest et al. (1992) considered temperature to be less important than moisture in most cases because many fungal pathogens will infect plants over a broad range of temperatures. However, there is often an interaction between temperature and moisture that has a greater effect than temperature alone. Free moisture and leaf wetness duration can significantly affect the ability of the fungal pathogen to germinate, produce penetration structures, and ultimately cause plant infection. This requirement of leaf wetness duration often increases when temperatures are in suboptimal ranges. Early evaluations of potential mycoherbicide candidates have often been conducted where dew periods in excess of 12 h are provided to ensure a high rate of infection on the weed (Lawrie et al. 1999; McRae and Auld 1988; Morin et al. 1990a). Green and Bailey (2000a,b) reported that A. cirsinoxia for control of Canada thistle required at least 8 h of continuous leaf wetness. The pathogen infected the weed under a broad range of temperatures (10-30°C) when free water or high relative humidity was present, but long durations of intermittent leaf wetness was detrimental to survival of germlings and, therefore, not conducive to high bioherbicidal activity. Some pathogens such as rust fungi are wind-dispersed and generally require less moisture than water-disseminated pathogens such as Colletotrichum spp. where spores are contained in a mucilaginous matrix (Hasan and Wapshere 1973; TeBeest 1991). Makowski (1997) further reiterated that evaluating the impact of environmental parameters under controlled conditions may provide clues about potential performance of bioherbicide candidates, but further investigations under variable field conditions where these factors are difficult to control are more complicated.
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