Two citrus composts (C1: composed of 40% citrus wastes, 20% sludge obtained from a citrus industry waste-water treatment facility and 40% green residues; C2: composed of 60% citrus wastes and 40% green residues, and no sludge) and their water extracts amended with Trichoderma harzianum T-78 (T harzianum T-78) were assayed in order to verify if these composts could act as a partial substitute for peat-based growing media as well as enhance suppressiveness against Fusarium wilt in the production of melon (Cucumis melo L.) seedlings at greenhouse nurseries. Over a 43-day growth cycle of melon seedlings, measurements were taken of the nutriactive effect (the capability of a substrate to express additional and/or synergistic nutritional and biostimulating effects), the pathogen incidence (percentage of fresh weight loss of melon plants grown on treatments infected with Fusarium oxysporum with respect to the same treatment without inoculation of the phytopathogen) and the trend of the T harzianum T-78 population. A nutriactive effect was observed in the tested citrus compost-based growing media (96% and 112% plant weight increase with respect to peat for C1Th and C2Th, respectively). Pathogen incidence was significantly lower in C2Th than peat (12% compared to 33%), while no difference was observed in C1Th. The T. harzianum T-78 population showed a significant decrease at the first sampling time compared to the initial quantity (from 10(6) to 10(5) CFU g(-1)), but later recovered over time. These results demonstrate that the combination of citrus compost and T. harzianum T-78 can be a viable alternative to peat and can minimise the application of chemicals necessary to control Fusarium wilt in greenhouse nurseries for melon seedling production. (C) 2009 Elsevier Ltd. All rights reserved.