FIELD APPLICATIONS OF COLLETOTRICHUM-ORBICULARE TO CONTROL XANTHIUM-SPINOSUM

The fungal pathogens Colletotrichum orbiculare is being assessed as the basis for a mycoherbicide for biological control of Xanthium spinosum, Bathurst burr or spiny cockleburr. Following preliminary small plot field applications in summer 1987/88, applications of aqueous spore suspensions of 106 or 107 spores ml-1 were made at four sites in summer 1988/89. The sites included a dryland pasture, an irrigated soybean field and two sites at which the effect of artificial dew was assessed using X. spinosum in pots in the field. Percentage kills ranged from 50 to 100%; the best results (98-100%) being obtained at the dryland grazing site. At one of the transplant sites provision of an artificial dew period of 18 h ensured 100% kill of X. spinosum under subsequent high ambient temperatures compared with 50% kill for plants experiencing the natural dew period (0.3 h). The efficacy of C. orbiculare tended to increase with increasing periods of high humidity. The formulation of a spore suspension into a product with a low evaporation rate could play a major role in achieving consistently effective results with C. orbiculare particularly in the western part of the range of X. spinosum. © 1990.