In vitro and in vivo effects of ammonium chloride on Xylella fastidiosa subsp. pauca infecting olives

Ammonium salts  are ionic compounds, soluble in water and strongly dissociated. Although with a different mode of action, timing and doses, they are widely used as artificial fertilizers, food additives, antimicrobial agents in textile industry, surfactants, disinfectants, antistatic agents and cleansing agents.

This study aims to investigate the impact of ammonium chloride on the growth performance of Xylella fastidiosa strain De Donno and explore its potential use in the field to mitigate the impact of infections in olives.

Three concentrations of ammonium chloride (0.25-0.5-1%) were tested in liquid medium to evaluate the influence on the bacterial viability after three days and six days of growth. Crystal violet assay was used to determine the effect of ammonium chloride on the adhesion of the bacterium and its ability to form biofilm. Gradual growth inhibition was observed with increasing concentration of ammonium chloride in the growth medium. The growth inhibition effect is evident on planktonic cells, but is significantly more pronounced on biofilm-forming cells due to the peculiar adhesiveness of the X. fastidiosa strain De Donno.

Field experiments started only recently (2019-2020) and included applications of ammonium chloride 4 times/year from March to October, in olive groves with different incidence of infections. A general increase of the vegetation mass and an attenuation of the wilting and desiccation phenomena (particularly in the bottom portion of the canopies) was observed in all treated trees, although not supported by statistically significant differences. Quantitative PCR tests on the experimental plants did not reveal any difference in the population size of treated and untreated plants.

In conclusion, ammonium chloride showed in vitro effects on bacterial growth/biofilm formation, and attenuation of symptoms severity was detected, although prolonged experiments and observations are needed.