Growing media, greenhouse tomato yield and Pythium root rot

Increased environmental pressure is forcing the horticultural industry to look for new growing media in replacement of rockwool and high-quality peat substrates. The objectives of this study were to determine: (1) the effect of different substrates on greenhouse tomato (Lycopersicon esculentum Mill.) yield and root rot caused by Pythium, and (2) the threshold values of some substrate physico-chemical properties for tomato yield and Pythium root rot. Two experiments (fall, spring) were conducted using five substrates. In the fall experiment, yield was related to water availability, as long as aeration was sufficient. In the spring experiment, yield depended on air storage and gas concentrations (O₂, CO₂) in the substrate because of their low aeration levels. The effect of substrate types and their physico-chemical properties on Pythium root rot varied according to the cultural conditions. Under fall cropping conditions, substrates showing wet and anaerobic conditions favoured Pythium root rot. In these experiments, adequate aeration properties for tomato plant productivity were obtained with a maximum of 30% low-quality peat added to a mixture of sawdust and compost.