Phytophthora infection in flooded citrus trees reduces root hydraulic conductance more than under non-flooded condition

• Water relations were studied in Phytophthora-inoculated or healthy citrus trees under flooding or well-watered conditions.
• Phytophthora reduced root hydraulic conductance in citrus trees.
• Flooding reduced stomatal conductance and increased tree mortality more than Phytophthora.

Phythophthora infections in citrus trees may result in poor tree health. Infections are very common under wet conditions. However, the interaction of Phytophthora and flooding conditions has yet to be studied in citrus. In this study, Phytophthora was inoculated into the soil of potted grapefruit trees, and trees were either well-watered or flooded for six weeks; then, these trees were compared with trees without Phytophthora inoculation under the same irrigation regimes. Phytophthora reduced root hydraulic conductance in citrus even though the propagule count was very low when root hydraulic conductance was measured (at the end of the experiment). In the presence of flooding, Phytophthora significantly reduced root hydraulic conductance compared to non-flooded citrus trees inoculated with Phytophthora alone. Flooding reduced stomatal conductance and increased tree mortality more than Phytophthora, but Phytophthora decreased the number of leaves per tree and tended to decrease tree growth more than flooding.