Enhanced control of postharvest citrus fruit decay by means of the combined use of compatible biocontrol agents

Fourteen Pseudomonas spp. and three Trichoderma spp. strains were tested in vitro for their antagonisticproperties against Penicillium digitatum, the causal agent of citrus green mold. Bacterial and fungal strainsstrongly inhibited the pathogen growth in vitro on PDA and on a citrus-based medium. Inhibitory effectswere increased when Pseudomonas and Trichoderma strains were applied in combination. Inhibitoryeffects were also increased by mixtures of Pseudomonas cells and Trichoderma atroviride P1 culturefiltrates. On the opposite, bacterial and fungal filtrates in mixture never induced an improved efficacy,thus indicating that the presence of living bacterial cells was required for a synergistic effect. Accordingly,incidence and severity of disease on orange cv. Tarocco and lemon cv. Femminello were consistentlyreduced when Pseudomonas and Trichoderma strains were applied 72 h before challenging P. digitatum.Pseudomonas syringae strains were the most effective and several combinations had a biocontrol activityhigher than one of the each antagonistic microorganism applied alone. The treatments comprising six P.syringae in mixture with Trichoderma strains T22, P1 and T34 were the most effective, with 80–100% controlof green mold. The development of green mold decay was also effectively inhibited in small-scaledip-treatments, either by single applications or by their mixtures. These experiments suggest that thecombination of Pseudomonas spp. and Trichoderma spp. strains could be considered as a promising meanfor the control of citrus green mold decay, supporting the concept that an improved disease control isgiven by the combined action of the two agents.