The efficacy of phosphonic acid (phosphorous acid) (Foli-R-Fos 200, 20% wlv potassium phosphonate) to control root rots of subterranean clover and ornamental plant species caused by Phytophthora spp. was evaluated. The fungicide effectively protected subterranean clover plants (cv. Woogenellup) from severe tap-root rot caused by P. clandestina in irrigated pastures and also protected several ornamental species grown in pasteurised potting mixes from severe root rot caused by either P. cinnamomi or P. cryptogea. The fungicide has the potential to restore the productivity of pastures based on the root rot susceptible subterranean clover cultivar Woogenellup in irrigated areas and offers an alternative to currently registered fungicides for the control of Phytophthora root rot of ornamental plant species.