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The severe impact of Xylella fastidiosa subsp. pauca, ST53, infections in olives prompted for an intense programme of field testing of different formulations for curative purposes. Under our experimental test conditions several compounds (Fosetyl alluminium; Protein of Harpin; COS-OGA; Acibenzolar S-methyl) proved to be inefficacious to reduce the occurrence and the severity of the desiccation phenomena induced by X. fastidiosa on the susceptible olive cultivars. However, among these, applications of N-acetylcysteine (NAC) known to disrupt the biofilm matrix of Xylella cell aggregates, and ammonium chloride provided some encouraging attenuation of the symptoms. Briefly, NAC was tested in the field in four different trials under different experimental conditions (trees of different ages, with different initial incidence of symptoms and infections) and modes of application (fertirrigation, soil application mixed with organic fertilisers, trunk injections). Ammonium chloride was initially applied by a local grower by spraying the olive canopies in the attempt to save the olive trees, the evident reduction of symptoms observed prompted then in spring 2019 to set ad hoc trials to test different concentrations and number of applications. For NAC, endotherapy applications (one per year) in new plantations (preventive treatments) or in olive groves with only limited initial incidence of the infections, were the only conditions that yielded some reduction in the occurrence of dieback and branch desiccation, even if quantitative PCR on the trees did not show any significant reduction (treated vs non-treated controls) in the bacterial population size. Regardless of the mode of application, the uptake of NAC was confirmed in all cases by HPLC analysis and by the phytotoxicity effects (leaf drop) recorded when the highest doses were used. Similarly, for the olive trees sprayed with ammonium chloride, even if clear symptom reduction was recorded, no differences were recorded on the bacterial populations in the mature tissues of the plants. Along with further observation and tests, the competence of the bacterium to colonise the new growth and its vector-transmissibility from the treated trees (NAC and ammonium chloride) will be assessed. |
