Phosphite shifts physiological and hormonal profile of Monterey pine and delays Fusarium circinatum progression

Fusarium circinatum is the causal agent of pitch canker disease affecting Pinus spp. and Pseudotsuga menziesii worldwide. Under strict quarantine measures, alternative approaches for disease control are necessary. Phosphite (Phi) salts are known for their fungicidal activity and as plant resistance elicitors; however, its potential is yet to be acknowledged in the Pinus-F. circinatum model. The main aim of this study was to assess whether the application of a Phi-based commercial formulation would delay the progression of the pitch canker on Pinus radiata plants, and on the in vitro fungal growth. In vitro assays were performed using different Phi concentrations (1% and 4%) and a non-treated control (0%), and repeated in vivo using inoculated and non-inoculated plants. Plant physiological parameters and hormonal content were evaluated. Phi was effective at inhibiting in vitro mycelial growth in a dose dependent manner. Regardless of fungal inoculation, Phi application induced positive effects on plant performance, despite phytotoxic effects found at 4%. Fusarium circinatum infection led to a reduction in gas exchange and chlorophyll fluorescence (Fv/Fm and 4PSII), while proline and hormone (JA, ABA and SA) levels increased. Phi was effective in delaying disease symptom development in a dose dependent manner, concurrent with in vitro observations: gas exchange and chlorophyll fluorescence (Fv/Fm) were unaffected; proline, MDA and ABA decreased; electrolyte leakage and total soluble sugars increased. This suggests a direct (pathogen growth inhibition) and indirect (host defense priming) action of Phi, showing that Phi represents a potential strategy to control F. circinatum infectionThis research was supported by FEDER through COMPETE (Programa Operacional Fatores de Competitividade), by National Funds through the Portuguese Foundation for Science and Technology (FCT) within the project URGENTpine (PTDC/AGR-FOR/ 2768/2014), and by the COST Action PINESTRENGTH (FP1406), supported by COST (European Cooperation in Science and Technology). FCT/MEC, through national funds, and co-funding by the FEDER (POCI-01-0145-FEDER-016785), within the PT2020 Partnership Agreement and Compete 2020 provide financial support to CESAM (UID/AMB/50017). FCT also supported A. Alves (FCT Investigator Programme e IF/00835/2013), B Correia (PhD grant - SFRH/ BD/86448/2012) and G Pinto (Post doc grant - SFRH/BPD/101669/ 2014)