| Summary: | The majority of all fungal formulations contain <i>Trichoderma</i> spp., making them effective biological control agents for agriculture. Chitosan, one of the most effective natural biopolymers, was also reported as a plant resistance enhancer and as a biocide against a variety of plant pathogens. An <i>in vitro</i> three-way interaction assay of <i>T. atroviride</i>, chitosan, and important plant pathogens (such as <i>Cercospora beticola</i> and <i>Fusarium oxysporum</i>) revealed a synergistic effect on fungistasis. Furthermore, chitosan coating on <i>Beta vulgaris</i> ssp. <i>vulgaris</i> seeds positively affected the onset and efficiency of germination. We show that priming with <i>T. atroviride</i> spores or chitosan leads to the induced expression of a pathogenesis-related gene (PR-3), but only supplementation of chitosan led to significant upregulation of phytoalexin synthesis (PAL) and oxidative stress-related genes (GST) as a defense response. Repeated foliar application of either agent promoted growth, triggered defense reactions, and reduced incidence of Cercospora leaf spot (CLS) disease in <i>B. vulgaris</i>. Our data suggest that both agents are excellent candidates to replace or assist common fungicides in use. Chitosan triggered the systemic resistance and had a biocidal effect, while <i>T. atroviride</i> mainly induced stress-related defense genes in <i>B. vulgaris</i>. We assume that both agents act synergistically across different signaling pathways, which could be of high relevance for their combinatorial and thus beneficial application on field.
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