Agro-industry composts as growing medium for growing baby-leaf lettuces in a floating system – added-value to suppress Pythium irregulare

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Production of small leafy vegetables (baby leaf) in floating system can be affected by different diseases such as 'damping off', which is caused by fungi as Pythium spp. Compost can be used as an alternative to peat, a traditional substrate in floating system. In addition, the use of composts with suppressive effect may help control these pathogens and reduce the use of chemicals, being a more sustainable production system. The main issue to obtain added-value compost is by selecting appropriate, specific type and rate of feedstock, or by microbial inoculation. The objective of this work was to investigate the suppressive capacity of three agroindustry composts (C9, C11 and C14) in the pathosystem Pythium irregulare-lettuce. C9 was composed mainly by pepper sewage sludge, pepper peels, carrot and vineyard pruning material, C11 showed similar feedstock inoculated with Aspergillus niger and C14 was mainly composed by tomato and onion peels plus vineyard pruning material. Two different ratios compost/peat (70/30 and 100/0, v/v) were used to fill the trays and the percentage of seed germination and of plant survival was determined at 8 and 20 days after sowing. The highest percentages of seed germination and plant survival were obtained in C14 for both compost/peat ratios assayed, showing plants with less symptoms of infection. In addition, P. irregulare was measured by RealTime PCR in the growing media and in the roots at 20 days after sowing. C14 also showed the lowest pathogen amount compared to the rest of compost but only in the growing media. In this study, the initial composition of compost was relatively important, that permits to obtain a suppressive added-value compost.