Trianthema portulacastrum L. (commonly known as horse purslane) is a noxious weed of several economically important field crops in tropical regions of the world. An intensive work on the screening of the biocontrol agents was accomplished through in vitro epidemic study to control weed populations. The foliar disease symptoms on infected weed plants caused by fungal pathogens represented as round to irregular maroon spots with dark borders and the epidemic was identified as leaf spot disease. The pathogen allied with the infection of horse purslane was isolated from infectious propagules by inoculation of leaf bites on a nutrient medium, potato dextrose agar (PDA). The causal agent of leaf spot was confirmed as Gibbago trianthemae Simmons by Koch's postulates. The mycoherbicide ability of G. trianthemae has been examined through visual (standard area diagram) and statistical methods (analysis of variance using the Microsoft Office Excel-Data Analysis Tool Pack 2007). The results revealed that the pathogen causes significantly (P < 0.05) severe infection on host weed and destructs the weed population by leaf spot diseases. The findings of the research suggested that the isolate G. trianthemae is highly virulent and host-specific, and recommended for further studies as a promising biocontrol agent against horse purslane weed.