In 2020, an experiment with onions (Allium cepa L.) on the ‘Stanfield’ variety was conducted, in which the impact of four biostimulants (Vitanica Si, Vitanica MC, Plantonic and Basfoliar Force) in terms of yield quality and quantity, which depends on fungal infections and pest damage, was studied. The experiment was designed at the Laboratory Field of the Biotechnical Faculty in Ljubljana. The experimental field was divided into three blocks, within which 15 treatments were randomly distributed. Biostimulants were applied in intervals of 10 days. They were sprayed four times during the intensive growth of the aboveground part of the onion. When using the Vitanica Si, the yield was the greatest in comparison to the positive and negative control, reached 2 t/hectare. The experiment showed that spraying onions with biostimulants increases the susceptibility of onionto the downy mildew. Plantonic showed the lowest average index of onion leaf infestation with onion purple blotch. Vitanica MC, Vitanica Si and Basfoliar Force showed an increased susceptibility of onions to damage caused by the onion fly i.e. onion leaf miner. Plantonic and Vitanica MC have been shown to be suitable for reducing the click beetle damage. In the case of leaf damage caused by onion thrips, we did not determine significantly damages among treatments. When Vitanica MC and Vitanica Si were used with a systemic fungicide, we found 22.1% greater mass of onion in comparison to the other treatments.
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