Phylogenetic analysis of pathogenic and non-pathogenicTrichodermaisolates from plants, soil and commercial bio products

Abstract

Fungi of the genus Trichoderma are found worldwide in various types of soil, plant rhizospheres, and plant materials. Several Trichoderma spp. are used in plant health management to promote growth and control plant diseases. Although widely considered beneficial, some members have been reported to be pathogenic to maize, causing a disease called Trichoderma ear rot. Since 2018, T. afroharzianum has caused significant infections of maize cobs in Germany, France and Italy. This study aimed to investigate the pathogenicity and phylogenetic relationships among different Trichoderma strains from diverse sources and geographical origins. Species identification and phylogenetic analysis were performed by sequencing internal transcribed spacer (ITS), translation elongation factor 1-α (TEF1-α) and RNA polymerase II subunit B (RBP2) genes, and pathogenicity was tested by artificially inoculating maize cobs under controlled greenhouse conditions. A total of 131 isolates were analyzed and assigned to 20 Trichoderma species. Among these, 39 isolates from six species were pathogenic, causing symptoms of green spore layers between kernels and husk leaves. While previous studies primarily identified T. afroharzianum as the main species causing Trichoderma ear rot, this study found that isolates of T. asperellum, T. atroviride and T. guizhouense also exhibit pathogenicity on maize cobs. Additionally, Trichoderma strains from commercial biocontrol products displayed unexpected pathogenicity inducing up to 92% disease severity on maize cobs. Most T. afroharzianum strains induced high levels of disease severity, although some isolates of the same species did not cause any disease, indicating a large heterogeneity in pathogenicity within the species. Notably, phylogeny reconstruction based on the TEF1-α and RBP2 genes, did not result in any discernible clustering between pathogenic and non-pathogenic isolates. A further novel finding is the isolation of pathogenic Trichoderma isolates from soil, demonstrating that soil can serve as a reservoir for pathogenic species. This study highlights the need for careful selection and monitoring of Trichoderma strains for agricultural use, considering their beneficial and pathogenic potential.