Agroecological Screening of Copper Alternatives for the Conservation of Soil Health in Organic Olive Production

The efficacy of soil conditioner (vermicompost tea), fertiliser (potassium silicate), and biological control agents (BCAs) as practical agroecological copper alternatives against olive leaf spot (Spilocaea oleaginea (Cast.) Hughe.) disease was investigated between 2018 and 2021 under organic management in a Mediterranean climate. In total, 9 agroecological alternatives to copper oxychloride (vermicompost tea, potassium silicate, Bacillus subtilis EU 007 WP, Platanus orientalis leaf extract, Mycorrhiza mix, seaweed commercial product, Trichoderma citrinoviride TR1, vermicompost tea+Platanus orientalis mix, Penicillium (Mouldy bread pieces)) were applied to olive trees in a randomised block design with 4 replicationsTotal water soluble phenol compounds (TWSP) were found to be the main bioindicator to assess the alternatives and their potential to phase-out copper application. Results related to TWSP indicated that copper oxychloride (control), potassium silicate and vermicompost tea showed significantly higher content of TWSP as we compared zero application of copper and other treatments. These stimulate the antioxidant capacity in olive fruits and reduce the olive leaf spot disease incidence. The pollution effect of copper was monitored during the trial to identify soil pollution in the organic in-conversion experimental land. The total annual ‘active copper’ application was 4.7 kg.ha−1.year−1 and this is in accordance with the legal organic legislation of Turkey. During the conversion period from conventional to organic management, we determined approximately 50% reduced copper content in the soil 0–30 cm depth samples in 2020 (3.70 mg.kg−1) as it is compared to those initial samples (6.43 mg.kg−1) in 2018. We conclude that alternatives to copper that are easily accessible, e.g., vermicompost tea, have a potential for use in organic olive production to replace copper in mitigating olive leaf spots. Furthermore, we find that reduced copper application in organic management with the aim to decrease copper accumulation in soil, fruits and leaves was not yet enough to reduce copper to satisfactory levels. We conclude that further research with the aim of a total replacement of copper fungicide treatments in organic and non-organic systems is needed.