Mineral nutrients improve phosphonate effectiveness against cork oak root disease

Abstract

Phosphonate fungicides are the most efficient chemicals for the control of oomycete root diseases which are causing significant economic and ecological losses in Mediterranean climate-type forests and natural ecosystems worldwide. However, a more environmental-friendly disease management demanded by society requires the optimization of chemicals application. Thereby, the main goal of this study was to develop a new resistant inducer (RI) formulation based on potassium phosphonate (PP) combined with mineral nutrients efficient against the root rot caused by Phytophthora cinnamomi and Pythium spiculum on cork oak that improve potassium phosphonate effectiveness. The potential phytotoxicity of five RI formulations composed of PP at five concentrations (25, 40, 50, 100 and 200 g/l) and each including 2.15 g/l ZnCl2 (Zn) and 0.25 g/l CaCl2 (Ca), were tested on 1-year old Quercus suber seedlings. Two weeks after treatments, only seedlings treated with the RIs containing the two highest doses of PP (100 and 200 g/l) showed symptoms of toxicity. RI with 40 g/l of H3PO3 (RI40), as well as its compounds individually and in combination two by two (PP (40 g/l), Ca, Zn, PP + Ca, PP + Zn, Ca + Zn), were tested against root disease development induced by P. cinnamomi and Py. spiculum on cork oak seedlings. For both oomycetes, seedlings submitted to every treatment with PP exhibited a significantly lower severity of root symptoms than plants artificially inoculated and treated with water. Furthermore, plants treated with RI40 did not differ to the plants growing in uninfested soil, improving PP alone effect. These results were validated by a field experiment where 60 mature cork oaks in three different defoliation classes (asymptomatic, slight, and moderate defoliation) were trunk injected with RI40 or water (controls). Three years after treatments, root disease development was significantly lower in cork oaks with slight and moderate initial defoliation level and treated with RI40 in comparison with water-treated controls. No effect was detected over both pathogens’ dynamics in soil. Results strongly support that PP effectiveness in controlling Q. suber root disease caused by P. cinnamomi and Py. spiculum is improved by its combination with Zn and Ca, allowing to reduce the PP application rates according to current society demands and the European Plant Health Legislation.