Correlations between Xanthomonas arboricola pv. juglandis severity and endogenous juglone and phenolic acids in walnut.

Abstract

Endogenous phenolic compounds in walnut fruits were correlated with the severity of walnut blight caused by Xanthomonas arboricola pv. juglandis (Xaj) assessed in the field, to determine the possible role of phenolics in resistance to the disease. Healthy fruits of the cvs Franquette, Cisco, and Sampion with different susceptibilities to infection by Xaj were sampled from diseased trees three times during growth and analysed using HPLC with a PDA detector. An identical phenolic profile, consisting of juglone and six phenolic acids (ellagic, gallic, syringic, p-coumaric, caffeic, and chlorogenic), was detected in the studied cultivars. Juglone was the most abundant, ranging between 373 mg 100 g-1 and 5,074 mg 100 g-1 DW, compared to the least abundant caffeic and p-coumaric acids, which did not exceed 10 mg 100 g-1 DW. A negative correlation between the total amount of phenolics present in the fruit tissues and blight severity was found in all cultivars, indicating the role of these compounds in the fruit-bacteria interactions. As the major phenolic characterized by the strongest seasonal fluctuations, juglone seemed to have the main and negative relation with disease development during the year. Thus, its involvement into the defence mechanism of walnut against bacterial blight is strongly suspected. The same may apply to gallic acid, considering its seasonal variations with respect to disease incidence. Additional studies including in vitro determination of the anti-bacterial activity of some phenolics, and their response to artificial inoculation with Xaj seem desirable to clarify the role of phenolic compounds in walnut resistance against this bacterium.