Plants ofXanthium strumarium L. were inoculated once withPuccinia xanthii Schw. during the 4 leaf stage, others were inoculated 7 times atca. 2 week intervals beginning at the 4 leaf stage, and others were protected from rust infection with regular fungicidal spray applications. Attack byP. xanthii was shown to shorten the plant life cycle and reduce plant growth rate and productivity. Burr production was reduced by up to 59% but percentage seed germination remained unchanged. It was concluded that the overall effects ofP. xanthii, and not reduced burr production alone, may achieve control ofX. strumarium in the field.