Trichoderma asperellum biocontrol activity and induction of systemic defenses against Sclerotium cepivorum in onion plants under tropical climate conditions

Abstract The biocontrol potential of three native Costa Rican Trichoderma asperellum strains has been evaluated against the necrotrophic ascomycete Sclerotium cepivorum, the causal agent of onion (Allium cepa L.) white rot. In Costa Rica, where climatic conditions enhance the development of this pathogen, white rot reduces onion yields up to 50% of total harvest. In our study, the three T. asperellum strains tested in in vitro assays showed their capacity to antagonize S. cepivorum, with BCC1 displaying percentages of colony growth inhibition of the pathogen of 81 and 90% in dual culture and cellophane membrane assays, respectively. In addition, this Trichoderma strain was able to reduce a 74% the plant mortality compared to untreated plants under greenhouse conditions. In field trials, carried out in two consecutive harvest years and in two different tropical locations, the two tested dosages of T. asperellum BCC1 reduced the incidence of white rot in a 3.41% for the lowest dose and 3.61% for the highest dose when compared to onion plants treated with chemical fungicides. Additionally, a significantly increase of 20.4% in onion bulb yield was recorded for the highest dose of BCC1. The potential of T. asperellum BCC1 to induce systemic defenses in onion plants against S. cepivorum was evaluated for four onion defense marker genes in a 21-day time course study by quantitative real-time PCR and using onion plants grown under greenhouse conditions. The expression profile of AcPR1 and AcPAL1, characterized by being undulating, indicates an initial activation of salicylic acid-dependent defense pathways (1 and 7 days) by T. asperellum BCC1 when either applied alone or in combination with the pathogen, while the up-regulation of AcEIN3 observed in those same treatments at day 21 revealed the activation of ethylene-dependent defense pathways by this Trichoderma strain. T. asperellum BCC1 exerts efficient biocontrol against S. cepivorum and activates onion systemic defenses against this pathogen under greenhouse conditions, while it reduces onion white rot incidence and increases crop yield in field trials performed under Costa Ricas tropical climate conditions.