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Since the increasing prevalence of herbicide-resistant weeds and herbicide bans, the use of biological controls with mycoherbicides become an innovative approach of weed control. In this study, we verified the pathogenicity of Phoma multirostrata TBRC 12769 against the common weed in Thailand, tridax daisy (Tridax procumbens), with its mechanism of infection unveiled by fluorescence microscopy. P. multirostrata directly penetrated through epidermal cells, stomata, and trichomes at 48 h post-inoculation. The hyphae also propagated in the lumen of the trichome, enabling the fungus to grow subcuticular to neighboring weed tissues at the bases of leaf trichomes. The necrotic pattern emerged around the trichome. During necrosis, unicellular chlamydospores were also detected inside the leaf trichomes, suggesting an overwintering stage under stress and nutrient-depleting conditions. Trichomes of weed leaves were found to be key infection sites for pathogenesis. Topical application of conidial suspension on T. procumbens potted plants led to 60–98% and 65 and 87% disease incidence under laboratory and greenhouse conditions, respectively, on days 15–20 post-inoculation. The 16-h dew period incubation results in a sharp increase by 37% in the pathogenicity rate. The greenhouse trials verified that the fungus is non-pathogenic to eight crops. Our LC-MS analysis indicated that norharman, a known bioherbicidal compound, and other compounds were detected in the supernatant fraction of fungal culture, of which resulted in a blight symptom on T. procumbens leaves. This study demonstrated that the P. multirostrata isolate is an effective mycoherbicide for this broadleaf weed. • A Phoma multirostrata isolate is a potential myco-herbicide for controlling Tridax procumbens. • The fungus showed no pathogenicity against eight important crops in Thailand. • The weed pathogenesis is mediated by infection through stomata and trichomes and direct penetration of the epidermis. • Fungal inoculation with a dew period markedly accelerated the infection of the weed. [ABSTRACT FROM AUTHOR]

The tridax daisy (Tridax procumbens) is a predominant weed in Thai plantations. Management of weeds in the country, both past and present, heavily relies on the use of synthetic herbicides. Here, we demonstrate the use of a phytopathogen for controlling this weed. The fungus Colletotrichum siamense isolate Cs1 was isolated from a diseased false daisy (Eclipta prostrata). Topical application of a conidial suspension of this fungus on Tridax procumbens plants led to 78–100% disease incidence on days 14–18 post-inoculation. Similar testing in the laboratory and greenhouse settings showed that C. siamense isolate Cs1 was non-pathogenic to cassava (Manihot esculenta) and eight other crops. Fluorescence microscopy of green fluorescent protein-expressing C. siamense indicated that the conidia germinated and formed appressoria for penetration on days 1–7 post-inoculation. Scanning electron microscopy showed hyphae growing on the leaf surface as well as invading the host through stomata. Trichomes of weed leaves served as important infection sites for this myco-pathogen. Oval conidia and pseudohyphae were detected inside the leaf trichomes, indicating that these cell types could help spread the fungus throughout the weed. C. siamense sporulated at high rates, at 1.7 × 1010 conidia/g, on a rice-oat mixture, suggesting a potential production recipe for commercialization. These data suggest that this weed-pathogenic fungus has potential as a bioherbicide for biological control of this weed. • Colletotrichum siamense is a mycopathogen of the weed tridax daisy. • C. siamense shows no or low pathogenic effect on 10 important crops. • The fungus can infect the weed through leaf trichomes and stomata. • Oval conidia and pseudohyphae help disperse the fungus in planta. • The fungus sporulates at high rates on rice mixed with oat. [ABSTRACT FROM AUTHOR]