Weeds are inherently variable by nature, with many weed species possessing several biotypes. For this reason, the genetic diversity of weed populations can present several challenges when researchers evaluate specific isolates or strains of fungal pathogens (Auld and Morin 1995; Boyetchko et al. 2002). Such is the case with yellow nutsedge that was found to have extensive genetic variability within and between populations and, therefore, greater variability in susceptibility to the bioherbicide agent Puccinia canaliculata, compared to purple nutsedge populations (Horak et al. 1987; Okoli et al. 1997). The success of Colletotrichum gloeo-sporioides f. sp. aeschynomene as the mycoherbicide Collego® was attributed to the uniform susceptibility of the weed northern jointvetch (Templeton et al. 1984). A better understanding of the target weed population structure will contribute to the effective search for bioherbicide agents with consistent performance on genetically diverse weed species (Boyetchko et al. 2002). Another consideration is the view that all bioherbicide candidates must be specific to one particular weed species. The advantage of a highly host-specific bioherbicide candidate is the assurance that nontarget, beneficial plant species, or crops closely related to the weed will not be affected (Makowski 1997; Mortensen 1998). However, a strict host-range requirement may not be economically feasible because the majority of agroeco-systems are comprised of multispecies weed communities (Frantzen et al. 2001). Plurivorous pathogens may be used safely under certain circumstances when they can be separated sufficiently from nontarget hosts in space and time (De Jong et al. 1999). Plant architecture and morphology have played a role in the success or failure of bioherbicide agents. The majority of weed species selected as targets for biological control have been broad-leaved weeds using foliar fungal pathogens (Charudattan 1991; 2001). Grasses are considered more difficult to control because the meristem is covered by a leaf sheath thereby prohibiting direct attack by the pathogen (Greaves and MacQueen 1992). Grass weeds are also closely related to many crops (e.g., cereals) in which they occur, making selectivity of mycoherbicide agents more challenging (Wapshere 1990). A particularly important factor with perennial weeds is their regeneration via rhizomes and stolons, which makes long-term weed control difficult (Greaves and MacQueen 1992). This often results in the weed out-growing the disease caused by foliar-applied bioherbicide candidates. However, there are many prospective soilborne fungal and bacterial agents that may be used as pre-emergent bioherbicides to control Poaceae and perennial weed species (Boyetchko et al. 2002). Other physical barriers such as leaf hairs and waxy cuticle layers may act as impediments to infection and establishment of fungal pathogens on the phyllosphere (Auld and Morin 1995). Although high inoculum applications have been used to overcome biological constraints resulting from low infection efficiency or virulence of the pathogen, these rates may not be technologically feasible due to plugging of spray equipment or economically viable from a production standpoint.
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