Polymers in bioherbicide formulation: Xanthium spinosum and Colletotrichumorbiculare as a model system

A requirement for extended periods of free water retention for fungal infec tion continues to limit the development of many potential bioherbicides. Th e use of hydrophilic polymers in the formulation of bioherbicides to overco me this limitation was investigated, using the potential bioherbicide Colle totrichum orbiculare and weed Xanthium spinosum as a model system. The rela tive humidity (RH) requirement of C. orbiculare required for adequate infec tion was greater than or equal to 95%. Water activity (a(w)) required for g rowth of C. orbiculare was >0.95 a(w). A range of three natural and four ar tificial polymers, including gums and polyacrylamides was tested. All polym ers were found to be non-toxic to the fungal spores and thus suitable for u se in bioherbicide formulations. Once incorporated into the formulation, li ttle improvement compared to the non-polymer controls was observed on treat ed plants in the absence of dew. Water loss studies of the polymer suspensi ons were carried out at RHs of 33 and 75%. There were significant differenc es between the polymers' ability to retain water, although some polymers re tained up to a maximum of 36% water after 16 h at 75% RH; this was reduced to 3.2% at 33% RH. The a(w) of most polymers remained sufficiently high for growth of C. orbiculare as water content decreased until it approached zer o. It was concluded that possible reasons for polymers not improving diseas e incidence might have been insufficient volume of water retained, and/or f ungal growth being obstructed by the structure of the polymers as water con tent and a(w) decreased.