Evaluation of a new natural adjuvant obtained from locust bean gum to reduce the amount of copper necessary to control downy mildew of grapevine

Abstract

Two field trials in integrated cultivation (2014 and 2016) were carried out with the aim to evaluate the efficacy and the persistence of copper oxychloride to control downy mildew of grapevine in mixture with a new natural adjuvant (PSS®) derived from locust bean (Ceratonia siliqua L.) gum. Copper amount applied in vineyards is the subject of interest for its use in organic farming due to the concern about its ecotoxicological profile in the soil that need to modify its use in rates, strategy and total amount per season. Copper oxychloride was applied every 7 and 14 days mixed or not with locust bean gum. Locust bean gum was also applied alone. To evaluate the distribution of copper on grapevine leaves, observations by scanning electron microscope were made. Locust bean gum alone did not influence the development of disease, while significantly increased efficacy and persistence of copper oxychloride both on leaves and bunches. On bunches, at 14 days interval, efficacy on incidence of copper oxychloride plus locust bean gum was about 60% while efficacy on severity was 60.1 and 65.6% for the year 2014 and 2016, respectively. With no locust bean gum, at the same interval, efficacy was significantly lower. Scanning electron microscopy demonstrated that copper fungicide particles when mixed with locust bean gum were still uniformly distributed on leaves after 14 days from application, but if applied alone already after 7 days became scattered with a lower concentration. To our knowledge, it is the first time that galactomannans-derived compounds have been used as adjuvant in pesticides. The use of locust bean gum reduced the annual amount of copper in integrated cultivation from 7.9 kg ha−1 applying copper oxychloride every 7 days to 4.5 kg ha−1 when applied every 14 days mixed with the new locust bean gum sticker.