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Abstract
Twenty-eight microorganisms showing in vitro antagonistic activity against Pythium ultimum were tested for their ability to reduce root rot [P. ultimum] on mature tomato plants grown in a greenhouse under hydroponic conditions. Of those, Penicillium brevicompactum, Penicillium solitum strain 1, Pseudomonas fluorescens subgroup G strain 2, Pseudomonas marginalis, Pseudomonas putida subgroup B strain 1, Pseudomonas syringae strain 1, and Trichoderma atroviride were shown to strongly reduce root rot severity, to improve the anchorage, and to increase the marketable yields of the plants grown in rockwool infested with P. ultimum. Experiments conducted in an organic medium containing peat, pine sawdust, and compost (v / v / v ; 60:30:10) also revealed the capability of most of these microorganisms to reduce root rot severity and to improve the anchorage of the tomato plants. However, Pseudomonas marginalis was the only microorganism that significantly improved fruit production of infected plants grown in organic medium. Moreover, Penicillium brevicompactum, Penicillium solitum strain 1, Pseudomonas fluorescens subgroup G strain 2, Pseudomonas marginalis, Pseudomonas putida subgroup B strain 1, and T. atroviride were shown to stimulate the growth of healthy tomato seedlings, suggesting that they act as PGPR (plant-growth-promoting rhizobacteria) or PGPF (plant-growth-promoting fungi). This study led to the selection of potential biocontrol agents against root rot of tomato caused by P. ultimum in hydroponic systems. This may open the way for new alternatives for the biological control of Pythium diseases in hydroponic systems that not only protect the crop but also have a beneficial effect on the plant growth and development in the absence of pathogens.
