PROHEXADIONE-CA INDUCES REDUCTION IN BACTERIAL BLIGHT SEVERITY AND ALTERATION IN PHENOLIC CONTENT IN WALNUTS

Abstract

Prohexadione-Ca (ProCa) was applied in an adult walnut orchard of cv. Franquette to test its ability to reduce severity of Xanthomonas arboricola pv. juglandis (Xaj) infection, and the effect on the contents of phenolic compounds in walnut fruits. ProCa was used three times over at 8-10 day interval, starting before the stigmas emergence at 0.125 mg l-1 concentration (treatment R1), and at the stage of brown stigmas at 0.250 mg l-1 concentration (treatment R2). Blight severity was assessed within the orchard at the same time as the husk tissues from symptomatic fruits were sampled for phenolic analyses, which were conducted using HPLC with PDA and MS detection. ProCa caused a significant reduction in Xaj severity regardless of the concentration and the first application date. The induced defence reaction was maintained throughout the season. Within a 15-day interval after the ProCa treatments, the walnut husks showed up to a 14 fold higher content of different phenolics than before fruit maturation. The treated fruits had a higher total phenolic content (TPC), more hydroxycinnamic acids (HC), and flavanols, whilst the contents of flavonols and gallic acid had decreased due to ProCa. The expressed potential of ProCa for triggering the defense mechanisms of walnut through the regulation of phenolic synthesis should be confirmed by additional experiments with the final aim of including this product in a sustainable control against Xaj.