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

A requirement for extended periods of free water retention for fungal infection continues to limit the development of many potential bioherbicides. The use of hydrophilic polymers in the formulation of bioherbicides to overcome this limitation was investigated, using the potential bioherbicide Colletotrichum orbiculare and weed Xanthium spinosum as a model system. The relative humidity (RH) requirement of C. orbiculare required for adequate infection was greater than or equal to 95%. Water activity (a(w)) required for growth of C. orbiculare was >0.95 a(w). A range of three natural and four artificial polymers, including gums and polyacrylamides was tested. All polymers were found to be non-toxic to the fungal spores and thus suitable for use in bioherbicide formulations. Once incorporated into the formulation, little improvement compared to the non-polymer controls was observed on treated plants in the absence of dew. Water loss studies of the polymer suspensions were carried out at RHs of 33 and 75%. There were significant differences between the polymers' ability to retain water, although some polymers retained 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 zero. It was concluded that possible reasons for polymers not improving disease incidence might have been insufficient volume of water retained, and/or fungal growth being obstructed by the structure of the polymers as water content and a(w) decreased.