Abstract: Although nitrogen fertilization is known to affect plant susceptibility to certainpathogens, little is known on its possible effect on the efficacy of biological control. In thepresent study we examined the effect of five levels of NO3- nutrition on the efficacy of two biocontrol agents (Trichoderma harzianum and Microdochium dimerum) to protect pruning wounds of tomato against Botrytis cinerea. Plants were grown for two months in a greenhousewith a soil-less drip-irrigation system. Differential nitrogen nutrition was applied for the last fourweeks prior to pruning, treatment of wounds with the biocontrol agents and inoculation with twostrains of B. cinerea. They were then incubated in conditions conducive to disease development.Plant fertilization had a highly significant effect on disease development for both strains testedand it significantly influenced the efficacy of both biocontrol agents. High nitrogen fertilizationgenerally decreased disease severity and also enhanced the efficacy biocontrol.