Broad host-range pathogens as bioherbicides: managing nontarget plant disease risk

Plant pathogens with a broad host range are commercially more attractive as microbial bioherbicides than strictly host-specific pathogens as a result of the wider market potential of a product capable of controlling multiple species. However, the perceived spatiotemporal disease risk to nontarget plants is a barrier to their adoption for weed control. We consider two approaches to managing this risk. First, we consider safety zones and withholding periods for bioherbicide treatment sites. These must ensure inoculum spreading from, or surviving at the site, exposes nontarget plants to no more inoculum than from natural sources. They can be determined using simple dispersal models. We show that a ratio of added:natural inoculum of 1.0 is biologically reasonable as an ‘acceptable risk’ and a sound basis for safety zones and withholding periods. These would be analogous to the ‘conditions of use’ for synthetic chemical herbicides aimed at minimizing collateral damage to susceptible plants from spray drift and persistent soil residues. Second, weed-specific isolates of broad host-range pathogens may avoid the need for safety zones and withholding periods. Such isolates have been found in many broad host-range pathogen species. Their utilization as bioherbicides may more easily meet the requirements of regulators. Mixtures of different weed-specific isolates of a pathogen could provide bioherbicides with commercially attractive spectrums of weed control activity.