The spread of Phytophthora spp. through irrigation systems and natural waterways can have a significant impact on plant health and requires mitigation. Pressurized carbon dioxide (CO2) can inactivate Phytophthora nicotianae zoospores but its effectiveness at low pressure and on other species was unknown. This study evaluated the effect of injected CO2 at 63 to 4,000 ppm in irrigation water on zoospore survival of four Phytophthora spp. and infectivity of P. nicotianae zoospores. Zoospore survival of P. nicotianae, P. tropicalis, and P. pini was reduced by over 90% at 4,000 ppm and was reduced by 40% at 125 to 2,000 ppm after a 2-h exposure. Survival of P. megasperma was less affected by injected CO2, with a reduction of 37.1% at ≤4,000 ppm. CO2 treatments at 4,000 ppm for 30 or 120 min of water infested with P. nicotianae at zoospore concentrations of 1,000 and 5,000 ml−1 reduced disease incidence of annual vinca (Catharanthus roseus) by 92 and 75%. Comparable efficacy was observed in the CO2 treatment at 2,000 ppm. The CO2 treatments at <2,000 ppm also significantly reduced disease caused by water infested at 1,000 zoospores ml−1. These results indicate that CO2 may have potential as a safe and effective water disinfestant for Phytophthora spp.