Quantification of phosphite ions in treated cork oak trees

Abstract: Since 2013, the use of potassium phosphite products has been prohibited in Spain when registered as fertilizers, so other phosphite-based products but registered as pesticides need to be tested for use in oak-rangelands for Phytophthora cinnamomi management. In autumn 2014, an experimental plot with Quercus suber trees affected by the root rot disease were established in a site in Huelva (southern Spain). Sixty trees were selected in the site and classified according to their defoliation class (0 = < 10% defoliation, 1 = 11-25%, 2 = 26-50%). Ten trees per class were treated with Fosetyl-Al by trunk injections at the manufacturer recommended dose (4% phosphite ion), remaining the other 10 trees as untreated controls. Three months after treatments (spring 2015), leaves and root samples were taken and analysed to assess their phosphite-ion content following the methodology described in Stasikowski et al. (2013). The phosphite concentration in root water extracts was estimated by comparison of the intensity of the grey-black precipitate obtained with a standard scale of rising concentrations of phosphite (0.1-50 mM). Values of phosphite ion detected in cork oak leaves varied between 0.1 and 21.5 mM, raising 5.2 to 22.6 mM in roots. A significant and positive correlation (Pearson test) between concentration in leaves and roots was found. ANOVA analysis showed significant differences in phosphite concentration depending on defoliation class in cork oak leaves, being significantly higher for the highest defoliation class (LSD test, α = 0.05). Phosphite concentration in roots also increased with increasing defoliation class, but differences were not significant. We conclude that the active matter applied (same quantity independently of defoliation class) would be equivalently distributed through roots and foliage present in the tree, less abundant as defoliation is higher. A new sampling and phosphite quantification will reveal whether concentrations and correlations found are maintained some years later. Preventive effect on cork oak disease development will be evaluated in the medium-long term.