<p>Overdependence on chemical fungicides for management of soil-borne fungal pathogens not only causes adverse effects on the environment and soil microbial ecology but may also trigger pesticide resistance problems and human health hazards. Therefore, researchers have focused on the importance of native weed plants with antifungal properties as a viable alternative option of synthetic chemicals for eco-friendly management of crop diseases. In the present investigation, attempts have been made for evaluation and screening of weed plants having antifungal activity in volatile form against two important soil-borne phytopathogens, <i>Sclerotium rolfsii</i> and <i>Rhizoctonia solani</i>, phytochemical characterization of the most effective weed, and its efficacy in lentil collar rot disease management through soil bio-fumigation. The macerated weed plant <i>Cleome viscosa</i> was found to be extremely effective (IC₅₀ is 1.01&#xa0;g) in inhibiting the mycelial growth of both fungi in volatile form, achieving 100% inhibition of fungal mycelial growth even with just 2&#xa0;g of crushed plant material. Additionally, eight compounds with antimicrobial properties were identified in the <i>C. viscosa</i> plant through GC-MS/MS analysis, including Diethyl Ester of 1,2-Benzenedicarboxylic Acid (40.21%), 1-(4-Isopropylphenyl)-2-Methylpropyl Acetate (8.47%), and 2-Tert-Butyl-4-(1,1,3,3-Tetramethylbutyl) Phenol (8.04%). Soil bio-fumigation with chopped fresh plant of <i>C. viscosa</i> at a rate of 6 tons per hectare effectively suppressed the incidence of collar rot in lentil by 34.64%, and was statistically comparable to the chemical fungicide Carbendazim 50 WP, without causing any phytotoxicity. Wild weed-derived phytochemicals can serve as a safe and efficient alternative to chemical fungicides when carefully pulverized and mixed with cropped soil.</p>