Efficacy of copper alternatives applied as stop-sprays against Plasmopara viticola in grapevine

The six products, Armicarb, Vitisan, lime sulphur, Caso, Calce Fiocco and Ulmasud, were tested for their ability to control downy mildew (Plasmopara viticola) on grapevine (Vitis vinifera) in order to find effective alternatives to copper, a heavy metal that is commonly used as a fungicide in organic viticulture. Each product was applied to potted grapevines at (1) 40° h (degree hours) and (2) 80° h after inoculation (ai) of the pathogen. These so-called stop-sprays were tested on plants that received two different P. viticola inoculum rates. The formulated potassium bicarbonate product Armicarb and lime sulphur reduced disease incidence and severity similar to or better than the copper hydroxide treatment. Armicarb and lime sulphur reduced disease incidence by up to 46 and 22 %, respectively, and disease severity by up to 73 and 77 %, respectively. However, observed effects were depending on both inoculum rates and time of application. Overall, the results indicate that the tested products showed higher efficacy when applied 40° h ai and when the pathogen was applied at the high inoculum rate, however, that was not always the case. The unformulated potassium bicarbonate product Vitisan and other products based on calcium chloride, calcium hydroxide and acid clay did not reliably affect disease incidence and severity. Possible roles of inoculum density, wetting agents and sprinkler usage during the infection procedure are discussed.