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A new approach to control Citrus Variegated Chlorosis (CVC) caused by Xylella fastidiosa is the use of N-acetyl-cysteine (NAC). CVC symptoms include reduction in fruit size making it not useful for juice or fresh fruit consumption. To analyse the effect of NAC under field conditions, severe CVC symptomatic plants were treated with NAC during two season harvests (2013–2014 and 2014–2015). A significant increase in fruit diameter was observed in the two seasons in plants treated with NAC-fertiliser, improving its quality for commercialisation. A new field trial was carried out during seasons 2017–2018 and 2018– 2019 with similar results to those observed before. Based on the results of NAC on CVC, some field trials were performed in southern Italy to verify its effect on olive quick decline syndrome (OQDS), a novel olive disease caused by a highly pathogenic strain of X. fastidiosa subsp. pauca. The assessment was made based on the level of symptoms on branches (dieback and dessication) of olive trees selected in plots of a highly susceptible cultivar. In general, treatment with NAC seems to decrease the disease progression, especially using NAC-endotherapy. These results were more evident in the trials set up on the plots with low initial incidence of symptomatic trees, where for three years (from 2016 to 2018) some differences were recorded. But, in 2019 as the pressure of inoculum increases in the environment, an increase in disease progression was observed in all treatments. Taken together, as also observed for CVC, NAC does not completely control the bacterium, but does interfere with the progression of the disease. However, in CVCtreated plants, NAC leads to an improvement in fitness of diseased plants during the time of treatment, which was not observed for olive. Some considerations must be pointed out when comparing CVC and OQDS pathosystems. The disease in olive (and in particular in the highly susceptible cultivar used in our experiments) is more severe than in sweet orange, for which no plant death is observed. This suggests that citrus may have a more efficient plant defence response that, along with the effect of NAC, may improve disease resistance. Similarly, it is worth extending the NAC applications to infected olives of less susceptible/resistant cultivars, in the attempt to strengthen the innate host response with the positive effect of the NAC application. In addition, in commercial citrus orchards the control of vectors helps to prevent new infections and consequently the use of NAC may be more effective. |
