Use of compost to partially substitute non-renewable growing media and suppress soil-borne pathogens on potted vegetable plants

Composts are expected to suppress plant diseases, according to the type of waste, the composting process, and the chemical and microbiological composition. Suppressive composts are generally applied as soil improvers; while it is necessary to develop specific compost-based growing media for applications on potted plants. The aim of this research was to evaluate the suppressiveness of compost + peat growing medium compared to peat. A growing medium made of 20% v/v compost, peat and pumice was tested. Suppressiveness was tested in greenhouse on potted plants against Pythium ultimum on cucumber, Phytophthora nicotianae, and Fusarium oxysporum f. sp. lycopersici on tomato. Pathogens were mixed into the substrate at 1 g of biomass on wheat kernels L^-1 seven days before seeding or with chlamydospores in talc at 1×104 cfu g^-1 of substrate. Seeds of cucumber and tomato were sown into 2-L pots in greenhouse and five pots were used for each treatment. A commercial peat-based growing medium was used as control. The number of alive plants and weight of above-ground biomass were measured 20-30 days after seeding. Cucumber and tomato biomass significantly increased up to 40-50% with the compost + peat growing medium compared to control. The number of diseased tomato plants in substrates inoculated with P. nicotianae was significantly reduced by 40% and the number of diseased cucumber plants by 30% compared to the peat substrate. Fusarium wilt of tomato was reduced by 60% in plants grown with the compost + peat growing medium. The tested growing medium, thanks to its composition based on high-quality compost, improved plant development and controlled Pythium ultimum on cucumber, Phytophthora nicotianae and F. oxysporum f. sp. lycopersici on tomato. Compost is a valuable component of growing media that can partially substitute non-renewable sources and suppress soilborne pathogens.