Mechanistic insights into Trichoderma asperellum-mediated biocontrol of leaf spot disease and growth promotion in lettuce

Leaf spot disease caused by Alternaria alternata is globally one of the utmost destructive fungal diseases in a diversity of economically important crops, resulting in decreased crop quality and large yield losses. The reliance on chemical fungicides for managing this disease rises environmental besides health concerns, necessitating the development of sustainable alternatives, such as biological control strategies. Our current study evaluates the potential of Trichoderma asperellum as a biocontrol agent against A. alternata RaSh3 infection in lettuce at laboratory and pot experiment levels. In vitro, the dual culture assay showed that T. asperellum displayed strong antagonistic activity, significantly inhibiting the radial growth of A. alternata RaSh3 by 34.48% and 47.71% after 3 and 5 days, respectively. Also, the inhibition rate of volatile metabolites of T. asperellum was 33.33%. The cell-free filtrate of T. asperellum (125, 250, 500, 750, and 1000 µL) demonstrated antagonistic activity at all tested concentrations against A. alternata RaSh3. The T. asperellum’s culture filtrate was extracted with ethyl acetate and examined via Gas Chromatography-Mass Spectrometry which revealed that 9-Octadecenoic acid (Z)- /Oleic Acid (21.77%), Phenol, 2,2’-methylenebis[6-(1,1-dimethylethyl)-4-methyl (6.82%), 9-Octadecenoic acid (Z)-, 2-hydroxy-1-(hydroxy methyl)ethyl ester (6.05%), 2-Hydroxy-3-[(9E) -9-octadecenol oxy]propyl(9E)-9-octadecenoate (4.8%), and Hexadecanoic acid, 2,3-dihydroxypropyl ester (4.63%) were major constituents that are known for their antimicrobial activities. Moreover, to improve the physio-biochemical properties of lettuce with T. asperellum application and ensure its beneficial role under A. alternata RaSh3-stressed conditions, a pot experiment was conducted. T. asperellum was applied as a soil drench, and A. alternata RaSh3 was applied on the surface of healthy leaves 15 days after transplantation. It is obvious that A. alternata RaSh3 exhibited a significant decrease in growth traits, pigments, and protein contents, while increasing stress markers (malondialdehyde [MDA] and H2O2 contents) in infected lettuce plants. However, T. asperellum diverted these changes. The application of T. asperellum could reduce disease severity and its incidence by up to 30% and 80% and increase protection up to 54.5%. Notably, T. asperellum obviously improves all the morphological parameters of lettuce (root as well as shoot lengths, fresh and dry weights, along with leaf area) under controlled and pathogen attack conditions. In addition, T. asperellum augmented ascorbate peroxidase, catalase, besides peroxidase activities, while decreasing MDA and H2O2 contents. Hence, these results underscore the prospective of T. asperellum as an essential constituent of modern agricultural practice as a promising eco-friendly biocontrol and biofertilizer agent.