Alfalfa, broccoli, carrot and Sudan grass were used as preceding crops residues to evaluate their effect on onion white rot caused by Sclerotium cepivorum Berk. either alone or in combined treatments with Trichoderma harzianum and with the bio-product Plant guard. The fungicide Folicure was used as check treatment. All treatments decreased percentage of infection, soil scleroial number and increased onion bulb yield. Combined treatments were effective more than single ones in most cases. Under greenhouse conditions, broccoli residues as single or combined with Plant guard strongly reduced disease incidence (20.0% infection with 71.0% efficacy) in comparison with the other tested treatments. Under open field
conditions, however, all treatments decreased the percentage of infection during 2015-2016 and 2016-2017 growing seasons. According to the mean values of the two seasons, broccoli residues combined with T. harzianum recorded the lowest percentage of infection by S. cepivorum (16.56%) with 70.06% efficacy. Sclerotial number was counted under greenhouse and open field conditions before onion transplantation and at the end of each season. Before transplantation, under greenhouse conditions all treatments significantly decreased sclerotial number with an efficacy that ranged between 25.89% with alfalfa and 45.5% with broccoli. Under open field conditions treatments decreased sclerotial number between 15.0% with alfalfa and 40.0% with broccoli. At the end of seasons, broccoli resides gave 3.0 sclerotia/g soil (72.73% efficacy) under greenhouse. Wherever, under open filed conditions broccoli residues combined with T. harzianum gave efficacy reached 73.13% and whereas it gave 70.15 % efficacy with Plant guard. All tested treatments under investigation significantly increased bulb yield compared to non-treated plants.