Unravelling in vitro Biological Control Mechanisms of Trichoderma capillare against Alternaria porri Causing Purple Blotch of Onion

Abstract

Purple blotch is an important fungal disease affecting onion production and is caused by Alternaria porri. The increasing dependence on chemical fungicides, together with the risks of pathogen resistance and environmental contamination, highlights the need for effective and sustainable biological control strategies. In the present study, the antagonistic potential and underlying inhibition mechanisms of Trichoderma capillare OGRDT3 were evaluated against A. porri OGRDAP2, the causal agent of onion purple blotch. The identity of T. capillare OGRDT3 was confirmed based on morphological characteristics and molecular analysis using the translation elongation factor 1-alpha (tef1α) gene, with the sequence deposited under accession number OR102870. The pathogen A. porri OGRDAP2 was molecularly characterized using the internal transcribed spacer (ITS) region, and its sequence was deposited under accession number PV577314. In the dual culture assay, T. capillare OGRDT3 inhibited the mycelial growth of A. porri by 75.55%. The antagonistic activity of OGRDT3 was further investigated using assays for hydrolytic enzymes, hydrogen cyanide (HCN), mycoparasitism and volatile organic compounds (VOCs). OGRDT3 exhibited chitinase and cellulase activities and tested positive for HCN production. In the double-plate assay, VOCs produced by OGRDT3 inhibited the mycelial growth of A. porri by 66.58%. Microscopic observations further demonstrated direct mycoparasitic interactions, including hyphal coiling, attachment, and hyphal lysis of A. porri by T. capillare. Overall, the results indicate that T. capillare OGRDT3 suppresses A. porri through multiple complementary mechanisms involving competition, hydrolytic enzyme activity, volatile-mediated antibiosis, and mycoparasitism. The isolate therefore represents a promising candidate for further evaluation and development as a biological control agent for the sustainable and integrated management of onion purple blotch.