Protection of citrus roots against infection by Phytophthora spp. by hypovirulent P. nicotianae is not related to induction of systemic acquired resistance

Aims Phytophthora root rot of citrus in Florida is caused by Phytophthora nicotianae and P. palmivora. In a field experiment, pre-inoculation of citrus fibrous roots before planting with a naturally occurring isolate of P. nicotianae (Pn117) reduced root rot caused by virulent P. nicotianae in an infested citrus orchard. On potted citrus rootstock seedlings, Pn117 caused less disease, but was not impaired in ability to infect and colonize roots compared to virulent Phytopththora spp. In a competition experiment, pre-inoculation of citrus rootstock seedlings with Pn117 followed by inoculation with virulent isolates of P. nicotianae (Pn198) and P. palmivora (Pp99) resulted in an increase in recovery of Pn117 and a decrease in recovery of virulent isolates compared to the co-inoculation treatment.