Development of a new resistance inducer formulation against root rot disease on cork oak

Abstract: At the Iberian Peninsula, Quercus suber woodlands are severely affected by the root
rot caused by the oomycetes Phytophthora cinnamomi and Pythium spiculum. The need for
environmental-friendly disease management makes the use of resistance inducers a suitable
option. The main goal of this study was to develop a new resistant inducer formulation (RIF)
based on phosphorous acid against cork oak root rot. Firstly, the potential phytotoxicity of five resistant inducers formulations (RIFs) compounded by potassium phosphite (PP) at five
different concentrations (2.5, 40, 50, 100 and 200 g/l) neutralized with KOH and including 2.15 g/l ZnCl2 (Zn) and 0.25 g/l CaCl2 (Ca), were tested on 1-year old Q. suber seedlings. Two weeks after treatments, only the seedlings treated with the highest doses (200 and 100 g/l of H3PO3) exhibited some phytotoxicity symptoms. Meanwhile no phytotoxicity was observed to the lower RIFs doses. Based on these results, the RIF with 40 g/l of H3PO3 (RIF40), as well as its compounds individually and combined two by two (PP, Ca, Zn, PP + Ca, PP + Zn, Ca + Zn), were chosen to be tested against root disease development induced by P. cinnamomi and Py. spiculum. For both pathogen species, a significantly lower severity of root symptoms was recorded in seedlings treated with every PP treatment (PP, PP + Zn, PP + Ca and RIF40) in comparison with plants infected and treated with water (controls). Moreover, plants treated with the RIF40 did not differ to the plants growing in uninfested soil. Results suggest that the PP at 40 g/l amended with ZnCl2 and CaCl2 effectively controls root rot disease, improving PP efficacy.