Efficacy of reduced copper dosages against Plasmopara viticola in organic agriculture

The grapevine is the most widely cultivated and economically important fruit species worldwide. Downy mildew produced by Plasmopara viticola, which occurs throughout the wide, is one of the most destructive of all grapevine diseases. In organic farming, copper is the only product effective against this pathogen accepted under European organic farming regulation EC 889/2008. Recently, due to its high environmental impact, the use of copper in organic farming has been limited by legislation to a yearly maximum of 6 kg/ha and its use will probably be even more restricted in the future. For this reason, the average dose recommended by pesticide producers of about 1 kg/ha per treatment should be revised. The aim of this work was to provide scientific support to effectively use reduced copper concentrations. A leaf disk assay was used to determine the minimum copper dosage that provided effective control. Dosages higher than 5 mg Cu/m2 (of grapevine leaf material) did not prove to be more effective than the dosage of 5 mg Cu/m2 itself. With a post-infection disk assay, copper treatments made 1 h after a simulated infectious rain, were also confirmed to give the same efficacy as preventive copper treatments. The efficacy of preventative low copper doses was validated in field trials. In the field, the rates of 200 and 400 g Cu/ha (equivalent to 5 and 10 mg Cu/m2, respectively) was able to significantly reduce downy mildew (72–89% efficacy), confirming results obtained from leaf disks assays. Adjusting from higher copper spray rates, to those between 200 and 400 g Cu/ha should provide sufficient control depending on the infection pressure. This spraying regime should also enable viticulturists to remain under the current annual limit of 6 kg/ha and likely under lower limits that might be proposed in future regulations